ICRP Publication 135
Diagnostic Reference Levels in Medical Imaging
Recommended citation
ICRP, 2017. Diagnostic reference levels in medical imaging. ICRP Publication 135. Ann. ICRP 46(1).
Authors on behalf of ICRP
E. Vaño, D.L. Miller, C.J. Martin, M.M. Rehani, K. Kang, M. Rosenstein, P. Ortiz-Lopez, S. Mattsson, R. Padovani, A. Rogers
Abstract - The International Commission on Radiological Protection (ICRP) first introduced the term ‘diagnostic reference level’ (DRL) in 1996 in Publication 73. The concept was subsequently developed further, and practical guidance was provided in 2001. The DRL has been proven to be an effective tool that aids in optimisation of protection in the medical exposure of patients for diagnostic and interventional procedures. However, with time, it has become evident that additional advice is needed. There are issues related to definitions of the terms used in previous guidance, determination of the values for DRLs, the appropriate interval for reevaluating and updating these values, appropriate use of DRLs in clinical practice, methods for practical application of DRLs, and application of the DRL concept to newer imaging technologies. This publication is intended as a further source of information and guidance on these issues. Some terminology has been clarified. In addition, this publication recommends quantities for use as DRLs for various imaging modalities, and provides information on the use of DRLs for interventional procedures and in paediatric imaging. It suggests modifications in the conduct of DRL surveys that take advantage of automated reporting of radiation-dose-related uantities, and highlights the importance of including information on DRLs in training programmes for healthcare workers. The target audience for this publication is national, regional, and local authorities; professional societies; and facilities that use ionising radiation for medical purposes, and responsible staff within these facilities. A full set of the Commission’s recommendations is provided.
© 2017 ICRP. Published by SAGE.
Keywords: Diagnostic reference levels; Patient doses; Optimisation.
AUTHORS ON BEHALF OF ICRP E. VAÑÓ, D.L. MILLER, C.J. MARTIN, M.M. REHANI, K. KANG, M. ROSENSTEIN, P. ORTIZ LÓPEZ, S. MATTSSON, R. PADOVANI, A. ROGERS
Key Points
8.1. General
(358) The DRL process should be used to evaluate whether, in routine circumstances, the amount of ionising radiation applied for a medical imaging procedure at a local healthcare facility, when assessed for a representative sample of patients (not individual patients) for a defined clinical task, is too high or too low. The DRL process allows identification of equipment and procedures for which radiation dose levels are high, so that optimisation of protection can be undertaken.
(359) A DRL value is considered to be consistently exceeded when the local median value of the appropriate DRL quantity for a representative sample of patients within an agreed weight range is greater than the local, national, or regional DRL value. Here, ‘consistently’ means ‘in a majority of cases’ and not ‘over a period of time’.
(360) DRLs may be established by authorised bodies. The numerical values of DRLs are advisory. However, an authorised body may require implementation of the DRL concept.
(361) Organisations responsible for different components of the tasks of collating data on DRL quantities and setting national DRLs should be identified in each country or region.
(362) DRL values shall not be used for individual patients or as trigger (alert or alarm) levels for individual patients or individual examinations.
(363) Comparison of local practices with DRL values is not sufficient, by itself, for optimisation of protection. Action is required to identify and address any deficiencies. The highest priority for any diagnostic imaging examination is achieving image quality sufficient for the clinical purpose. Image quality or, more generally, the diagnostic information provided by the examination (including the effects of postprocessing), must be evaluated as part of the DRL process, and methods to achieve optimisation should be implemented.
(364) All individuals who have a role in subjecting a patient to a medical imaging procedure should be familiar with the DRL process as a tool for optimisation of protection.
(365) The concept and proper use of DRLs should be included in the education and training programmes of the health professionals involved in medical imaging with ionising radiation.
8.2. DRL quantities
(366) Quantities used for DRLs should assess the amount of ionising radiation applied to perform a medical imaging task, and should be easily measured or determined. DRL quantities assess the amount of ionising radiation used for a medical imaging procedure, not absorbed dose to a patient or organ. The one exception is mammography, for which DG may be used.
(367) DRL quantities should be appropriate to the imaging modality being evaluated, to the specific study being performed, and to the specific size of the patient.
(368) The Commission stresses that the radiation protection quantity ‘effective dose’ (used for other purposes in the ICRP radiological protection system) should not be used as a DRL quantity. It introduces extraneous factors that are neither necessary nor pertinent for the purpose of a DRL.
(369) For projection radiography, two DRL quantities are recommended – Ka,e (or Ka,i) and PKA – in order to simplify assessment of proper use of collimation, especially in paediatrics.
(370) DRL values developed for advanced digital radiographic techniques (e.g. tomosynthesis, dual-energy subtraction, contrast-enhanced subtraction, cone-beam CT) need to take the ‘multiple image’ aspect of the technique into account, and should distinguish these procedures from more standard ones.
(371) For mammography, the recommended DRL quantity is one or more of Ka,i, Ka,e, and DG, with the choice of quantity depending on local practices and regulatory requirements.
(372) For mammography, a simple approach could be setting DRL values for breasts of 5.0 0.5 cm thickness. Establishing DRL values for different breast thicknesses is a more complex but better approach to refine the DRL process for mammography.
(373) For interventional radiology, all of the following DRL quantities are recommended (if available): PKA, cumulative air kerma at Ka,r, fluoroscopy time, and the number of radiographic images (e.g. cine images in cardiology and digital subtraction angiography images in vascular procedures).
(374) The recommended DRL quantities for CT are CTDIvol and DLP. The number of scan sequences in the examination may be helpful as well. SSDE provide more accurate estimates of paediatric patient doses than CTDIvol or DLP, which are both indices of the dose to standardised phantoms and may be used as an additional aid in optimisation.
(375) The recommended CTDIvol value to be used is the CTDIvol for each sequence. The recommended DLP value is the cumulative DLP for the entire examination. DLP values for individual scan sequences can also be useful, and may be used in addition to the cumulative DLP.
(376) For nuclear medicine, the ideal DRL quantity would be the administered activity per body weight of a specific radionuclide for a specific clinical task and, if relevant, the radiopharmaceutical used. The Commission recommends that weightbased administered activities should be used for children, adolescents, and low-weight patients, and considered for other groups. Setting a fixed maximum administered activity for very obese patients may also be considered. It is recognised that, in many countries, a standard activity is used in clinical practice for adult patients.
(377) Weight-based administered activities may not be appropriate for examinations where the radiopharmaceutical is concentrated predominantly in a single organ (e.g. thyroid scans, lung perfusion scans).
(378) As DRL values for nuclear medicine procedures and CT procedures apply to radiation from very different modalities, and use different DRL quantities, for hybrid imaging procedures (SPECT-CT, PET-CT), it is appropriate to set and present DRL values for each modality independently.
8.3. Use of median values of the national survey distribution
(379) Compliance with DRL values does not indicate that the procedure is performed at an optimised level with regard to the amount of radiation used. The Commission recognises that additional improvement can be obtained by using the median value (the 50th percentile) of the distribution used to set the national DRL value.
(380) This median value of the national distribution can serve as an additional tool to aid in optimisation, may be a desirable goal at which to aim using standard techniques and technologies, and represents a situation closer to the optimum use of the applied radiation.
(381) When the facility’s median value of a DRL quantity is lower than the median value of the national distribution, image quality (or diagnostic information, when multiple images are used) might be affected adversely. Image quality should be considered as a greater priority in the review if additional optimisation efforts are undertaken.
8.4. DRL surveys and registries
(382) The Commission recommends setting local and national DRL values based on DRL quantities for imaging examinations and procedures performed on patients. The use of phantoms is not sufficient in most cases. When phantoms are used, the effects of operator performance, the selected imaging protocol, and patient variability are not taken into account.
(383) The use of phantoms is important in the investigation of x-ray equipment performance, and is important in evaluating the performance of fluoroscopy and CT equipment with respect to the amount of radiation used during the optimisation of protection.
(384) Data on DRL quantities may be collected using surveys, registries, or other automated data collection methods.
(385) Calibrations of all dosimeters, kerma-area product meters, etc., used for patient dosimetry should be performed regularly and should be traceable to a primary or secondary standard laboratory.
(386) The accuracy of DRL quantity data produced by and transferred from x-ray systems should be verified periodically by a medical physicist.
(387) The examinations/procedures included should, in general, represent the most frequent examinations performed in the region for which dose assessment is practicable, with priority given to those that result in the highest patient radiation dose.
(388) DRLs are not intended for use in radiation therapy, but they should be considered for imaging for treatment planning, treatment rehearsal, and patient set-up verification in radiotherapy.
(389) National surveys and registries for setting DRL values should normally include medium- and large-sized healthcare facilities that have a sufficient workload to ensure that data for a representative selection of patients can be obtained. The sample should also cover the range of healthcare providers.
(390) For large countries, a survey of a random selection of a small proportion of all the healthcare facilities in the country can provide a good starting point for setting national DRL values, if no national registry or method for automated data collection exists. Results from 20–30 facilities are likely to be sufficient in the first instance. In a smaller country with fewer than 50 healthcare facilities, an initial survey of 30–50% of them may suffice.
(391) A survey for a particular examination in a facility should normally involve collection of data on DRL quantities for at least 20 patients, and preferably 30 patients for diagnostic fluoroscopy and CT examinations, and 50 patients for mammography. For paediatrics, these figures may need to be decreased for facilities where relatively few children are examined. For registries, all available and appropriate data should be used.
(392) There should be some standardisation of weight for adult patients included in surveys of diagnostic procedures if data are collected from fewer than 50 patients (e.g. patients with weights between 60 and 80 kg for a mean weight of 70 ± 5 kg).
(393) HIS and RIS can provide data for large numbers of patients, but may not include patient weight. As with all DRL surveys, the results rely on the accuracy of data entry.
(394) RIS and associated software may permit data on DRL quantities to be obtained in an automated fashion, either locally or through a national registry. When automated processes are used, the data for all cases of a specific procedure should be obtained and used for optimisation.
8.5. Setting DRL values
(395) The DRL value should be tied to defined clinical and technical requirements for the selected medical imaging task.
(396) The appropriate image quality or diagnostic information needed for the clinical task should be a priority when setting DRL values. DRL values may differ for different clinical tasks, especially for CT where visualisation of differences in the internal structure of tissues or identification of nodules is often important. Different tasks may require use of different image filters with varying exposure levels.
(397) It is important when developing DRL values that all data collected come from similar procedures across all participating facilities. This ensures that comparisons among facilities remain valid and useful.
(398) It may be important to specify, in detail, the views normally included and the clinical task associated with the procedure. This may be required where differing exposure factors or different views (or numbers of views) are employed for different clinical indications.
(399) When two imaging modalities are used for the same procedure (e.g. PET-CT, SPECT-CT), it is appropriate to set and present DRL values for both modalities independently.
(400) DRL values are dependent on the state of practice and the available technology (including postprocessing software) at a particular point in time.
(401) Median values (not mean values) of the distributions of data collected from a representative sample of patients within an agreed weight range should be used for comparison with DRL values. The mean can be affected substantially by a few high or low values.
(402) National DRL values should be set as the 75th percentile of median values obtained in a sample of representative centres.
(403) If regional (multi-national) DRL values are created, they should be set as the median value of the national DRL values (each of which is set at the 75th percentile) for the countries in the region. If the sample of available data is small, other approaches may be used by agreement among the involved countries.
(404) The process to set and to update DRL values should be both flexible and dynamic. Flexibility is necessary for procedures where few data are available (e.g. interventional procedures in paediatric patients), or from only one or a few centres. A dynamic process is necessary to allow initial DRL values to be derived from these data while waiting for a wider survey to be conducted.
(405) When a procedure is not performed on a regular basis in most hospitals, local DRL values may be determined using the data from a single large hospital with a relevant workload of procedures (e.g. a specialised paediatric hospital).
(406) Local DRL values set by a group of radiology departments can play a role, where effort has already been invested in optimisation. The group could set a local DRL value based on more regular surveys of local practice that will normally be lower than any national DRL value. Where the number of facilities or x-ray rooms is small, the median of the distribution of values of the DRL quantity is recommended as a ‘typical value’. Typical values can also be set for newer technologies that enable decreased amounts of radiation to be used in achieving a similar level of image quality.
(407) Published DRL values should be accompanied by a statement of the local group, nation, or region from which the patient data were collected, the size of the ‘standard’ patient on whom the data are based, the details of the specific examination, as appropriate, and the date of the survey.
8.6. DRLs for interventional procedures
(408) The Commission recommends retaining the term ‘diagnostic reference level’ for the DRL process as applied to interventional procedures.
(409) For interventional procedures, complexity of the procedure may be considered in setting DRL values, and a multiplying factor for the DRL value may be appropriate for more complex cases of a procedure.
(410) If possible, the data from all interventional procedures performed (not just from a limited sample) should be collated to derive local and national DRL values.
8.7. Paediatric DRLs
(411) A single ‘representative patient’ should not be used to define DRLs for paediatric imaging, as weight in children can vary by a factor of more than 100 from a premature infant to an obese adolescent.
(412) The amount of radiation used for examinations of children can vary tremendously due to the great variation in patient size and weight, from neonates to adult-sized adolescents. This variation in patient radiation dose is appropriate. Variation in patient radiation dose due to incorrect technique or failure to adapt the imaging protocol from adults to children to account for both paediatric diseases and paediatric patient size is not appropriate.
(413) Weight bands are recommended for establishing paediatric DRL values for examinations of the trunk and should be promoted for paediatrics. Age bands can be used if age is the only available measure.
(414) Age groupings are recommended for establishing DRL values for examinations involving the head.
(415) For CT, the DRL quantities are CTDIvol and DLP, based preferably on calibration with a 32-cm-diameter phantom for body examinations and a 16-cm-diameter phantom for head examinations. Values for these quantities should be obtained from patient examinations. SSDE may be used as an additional source of information for optimisation.
(416) Modern CT scanners permit determination of effective diameter or patient equivalent thickness. This should be considered as an additional refinement for setting paediatric DRLs.
(417) For nuclear medicine imaging, consideration should be given to adjusting administered activities based on agreed factors linked to weight. Adjustments should be made for paediatric examinations.
8.8. Application of DRLs in clinical practice
(418) National and regional DRL values should be revised at regular intervals (3–5 years) or more frequently when substantial changes in technology, new imaging protocols, or postprocessing of images become available.
(419) Median values of the DRL quantity for medical imaging procedures in a representative sample of patients within an agreed weight range for a specific x-ray room, radiology department, or other facility should be compared with local, national, or regional DRL values to identify whether the data for that location are substantially higher or lower than might be anticipated.
(420) If a local or national DRL value for any procedure is consistently exceeded, an investigation should be performed without undue delay, and, if appropriate, corrective action should be taken.
(421) When corrective action is required, it is necessary to keep in mind that DRL values are not dose limits.
(422) Corrective action (optimisation of protection) should include a review of equipment performance, the settings used, and the examination protocols. The factors most likely to be involved are survey methodology, equipment performance, procedure protocol, operator skill, and, for interventional techniques, procedure complexity.
(423) In the optimisation process, account must always be taken of the level of image quality required for the medical imaging task. Image quality must always be adequate to provide the information required for the clinical purpose of the examination and the actual size of the patient irradiated.
(424) When a facility’s median value of a DRL quantity is substantially less than the DRL value, image quality (or diagnostic information, when multiple images are used) might be affected adversely. Image quality should be examined as a priority when the examination protocol is reviewed.
(425) The DRL audit process does not stop after a single assessment. Repeat evaluations are required following any optimisation, and the whole process should be repeated after an appropriate time interval.
(426) Local surveys of DRL quantities should normally be performed as part of the QA programme unless these data are continuously submitted to a registry, in which case, review of the registry data should be performed. A representative selection of examinations for each x-ray unit should be surveyed at intervals of about 3 years, and whenever substantial changes in technology or software have been introduced.
(427) Local surveys of DRL quantities, as part of the QA programme, should be performed annually for CT and interventional procedures, unless these data are continuously submitted to a registry, in which case review of the registry data should be performed at least annually. Annual surveys or review of registry data are also appropriate as part of the QA programme for SPECT-CT and PET-CT.
(428) If continuous collection of data on DRL quantities is possible through registries or automated collation of data from electronic databases, the dose management process may take the form of a regular review of all the data to identify any adverse trends.
(429) The method for managing and achieving optimisation for dental radiography differs from the method for other x-ray applications. Dental DRL values are set in terms of incident air kerma measured during routine tests. Based on test results, recommendations can be made on changes to protocols (equipment settings) and adjustments. The investigator should work with the dentist to optimise protection. Improvement in protection linked to new technology can be realised which otherwise might not be achieved.
Executive Summary
1. Introduction
(a) The Commission first introduced the term ‘diagnostic reference level’ (DRL) in Publication 73 (ICRP, 1996). The concept was subsequently developed further, and practical advice was provided in Supporting Guidance (ICRP, 2001a). This development and the 2001 advice are summarised in Annex A.
(b) As the Commission stated in Publication 103 (ICRP, 2007a), one of the principles of optimisation of protection in medical exposures is implemented through the use of DRLs. The DRL has proven to be an effective tool that aids in optimisation of protection in the medical exposure of patients for diagnostic and interventional procedures. DRLs are not intended for use in radiation therapy, but they should be considered for imaging for treatment planning, treatment rehearsal, and patient set-up verification in radiotherapy.
(c) With time, it has become evident that additional advice is needed. There are issues related to definitions of some of the terms used in previous guidance, determination of the values for DRLs, the appropriate interval for re-evaluating and updating these values, appropriate use of DRLs in clinical practice, methods for practical application of the DRL process, and application of the concept to certain newer imaging technologies [e.g. dual-energy computed tomography (CT), positron emission tomography-CT (PET-CT), single photon emission CT (SPECT-CT), cone beam CT, digital radiography, tomosynthesis], and additional difficulties in paediatric practice, particularly due to the wide range in sizes.
(d) In this publication, the Commission recommends the use of four different DRL terms (defined in the Glossary):
DRL (a form of investigation level used as a tool to aid optimisation of protection in the medical exposure of patients for diagnostic and interventional procedures).
DRL quantity (a commonly and easily measured or determined radiation metric that assesses the amount of ionising radiation used to perform a medical imaging task).
DRL value (an arbitrary notional value of a DRL quantity, set at the 75th percentile of the distribution of the medians of distributions of the DRL quantity obtained from surveys or other means).
DRL process (the cyclical process of establishing DRL values, using them as a tool for optimisation, and then determining updated DRL values as tools for further optimisation).
(e) This publication is intended as a further source of information and guidance on these issues. Some terminology has been clarified. In addition, this publication recommends DRL quantities for various imaging modalities, provides information on the application of DRLs to interventional procedures and in paediatric imaging, highlights common errors in the determination and application of DRL values, suggests modifications in surveys for establishing DRL values that take advantage of automated reporting of radiation-dose-related quantities, and highlights the importance of including information on DRLs in training programmes for healthcare workers and in information for patients.
(f) The target audience for this publication is national, regional, and local authorities; professional societies; and facilities where ionising radiation is used for medical exposures, and responsible staff within these facilities.
(g) A full set of the Commission’s recommendations is provided in Section 8 of this publication. In addition, each section is preceded by a set of main points that summarise the principal concepts in that section. A limited summary of the most important points and recommendations is presented below for the convenience of the reader.
2. Diagnostic reference levels
(h) The principles of justification and optimisation of protection are key and complementary radiological safety tenets. DRL is the Commission’s term for a form of investigation level used to aid in optimisation of protection in the medical exposure of patients for diagnostic and interventional procedures. A DRL value is a selected level of a DRL quantity for broadly defined types of equipment for typical examinations for groups of patients within an agreed weight range or, in certain specific circumstances, a standard phantom. DRLs do not apply to individual patients. They are derived as an arbitrary threshold from radiation metric data obtained locally and collected nationally or regionally. A DRL is a supplement to professional judgement and does not provide a dividing line between good and bad medical practice. All individuals who have a role in subjecting a patient to a medical exposure should be familiar with DRLs as a tool for optimisation of protection.
(i) The application of the DRL process is not sufficient, by itself, for optimisation of protection. Optimisation is generally concerned with maintaining the quality of the diagnostic information provided by the examination commensurate with the medical purpose while, at the same time, seeking to reduce patient exposures to radiation to a level as low as reasonably achievable. Image quality or, more generally, the diagnostic information provided by the examination (including the effects of postprocessing) must also be evaluated. Methods to achieve optimisation that encompass both the DRL process and image quality evaluation should be implemented. In some cases, optimisation may result in an increase in dose.
(j) A dose below a DRL value does not, by itself, indicate that the procedure is performed at an optimised level with regard to the amount of radiation used. Therefore, the Commission recognises that additional improvement can often be obtained by using the median value (the 50th percentile) of the national distribution of values of dose-related quantities to provide additional guidance for further optimisation efforts. If local median values of the DRL quantity are below the national median value, image quality, rather than the amount of radiation used, should be considered as a greater priority in this additional optimisation process. The basis for this recommendation is that if practices at the local facility have already achieved levels of radiation use that are below the national median value, further reduction in the amount of radiation used is not the principal concern. When local practices result in levels of radiation that are below the national median value, ensuring that image quality is adequate should be a priority.
3. DRL quantities and values
(k) DRL quantities should be appropriate to the imaging modality being evaluated, should assess the amount of ionising radiation applied to perform a medical imaging task, and should be easily measured or determined. When two imaging modalities are used for the same procedure (e.g. PET-CT, SPECT-CT), it is appropriate to set and present DRL values for both modalities independently.
(l) An authorised body may require implementation of the DRL process as a tool to promote optimisation, but DRL values should be considered advisory. The numerical value of the DRL should be tied to defined clinical and technical requirements for the selected medical imaging task. The Commission recommends setting DRL values based on surveys of the appropriate DRL quantities for procedures performed on an appropriate sample of patients. The use of phantoms is not sufficient in most cases, as the effects of operator performance are not taken into account when phantoms are used.
(m) DRL values are not static. As optimisation of examinations continues or hardware and software improve, DRL values should be updated on a regular basis. When new imaging techniques are introduced, an effort should be made to measure suitable DRL quantities and set DRL values as soon as is practicable.
(n) For interventional procedures, the complexity of the procedure may be considered in setting DRL values, and a multiplying factor for the DRL value may be appropriate for more complex cases of a procedure.
4. Local, national, and regional DRLs
(o) Organisations responsible for different components of the tasks of collating data on DRL quantities used for patient examinations and setting DRL values should be identified in each country or region. The process to set and update DRLs should be both flexible and dynamic. Flexibility is necessary for procedures where few data are available (e.g. interventional procedures in paediatric patients), or where data are available from only one or a few centres. A dynamic process is necessary to allow initial DRLs to be derived from these data while waiting for a wider survey to be conducted.
(p) Data for determining national DRL values for x-ray procedures are obtained from surveys or registries. Values of appropriate DRL quantities from patient examinations are collected from several different health facilities. The 75th percentile value of the distribution of median values of a DRL quantity at healthcare facilities throughout a country is used as the ‘national DRL’.
(q) When national DRL values exist for many or most countries within a region (e.g. the European Union), regional DRL values may be determined by using the median value of the available national values. These may provide guidance on the need for optimisation or protection for neighbouring countries without their own DRL surveys or registries, and give an indication that further optimisation may be required for countries whose current national DRL values are above the regional DRLs.
(r) National and regional DRLs should be revised at regular intervals of 3–5 years, or more frequently when substantial changes in technology, new imaging protocols, or improved postprocessing of images become available.
(s) As national DRLs for x-ray procedures need large surveys or registries, and these can require substantial effort to perform and analyse, they are not always as responsive to changes in technology. Where it is apparent that further optimisation is being achieved locally, or where no national DRL values exist, ‘local DRLs or typical values’ based on surveys might be introduced to further assist the optimisation process (Table 2.2). Examples of their use are to account for the substantial dose reduction that could be achieved through the application of iterative reconstruction techniques in CT, the replacement of computed radiography with flat-panel digital radiography, and the introduction of digital radiography detectors into dental radiography. Another example is the introduction of new methods for postprocessing of images. In small healthcare facilities, ‘typical values’ can be used (Table 2.2).
5. Using DRLs for optimisation of protection
(t) Median values of the appropriate DRL quantity for medical imaging procedures for a specific x-ray room or for a radiology department or other facility should be compared with local, national, or regional DRL values to identify whether the data for the location are substantially higher or lower than might be anticipated.
(u) A DRL value is considered to be ‘consistently exceeded’ at a facility when the median value of the DRL quantity at the facility for a representative sample of patients within an agreed weight range is greater than the local, national, or regional DRL value.
(v) If a DRL value for any procedure is exceeded, an investigation should be undertaken without undue delay to determine possible reasons. If corrective action is required, a plan should be implemented (and documented) without undue delay.
(w) DRLs are not intended to be used for individual patients or as a trigger (alert or alarm) level for individual patients or individual examinations. Also, DRL values are not limits.
(x) The highest priority for any diagnostic examination is achieving image quality sufficient for the clinical purpose, so that the images from the whole examination provide all the diagnostic information required and the clinical purpose is not jeopardised.
6. Considerations for paediatric examinations
(y) The radiation exposure for examinations of children, whether from radiological imaging or nuclear medicine, can vary tremendously due to the great variation in patient size and weight. This variation in patient radiation dose is appropriate. However, variation in patient radiation dose due to inappropriate technique or failure to adapt imaging protocols from adults to children to account for both paediatric diseases and different patient sizes is not appropriate, and requires optimisation of protection.
(z) Weight bands (see Section 6) are recommended for establishing paediatric DRLs for x-ray examinations of the trunk, and should be promoted for paediatrics.
Concise Summary
The International Commission on Radiological Protection (ICRP) introduced Diagnostic Reference Level (DRL) in 1996, with an updated publication in 2001. It was apparent that additional advice was needed, which paved the way for publication 135.
Publication 135:
• suggests quantities for use as DRLs for various imaging modalities,
• provides information on the use of DRLs for paediatric imaging and interventional procedures,
• recommends changes in DRL surveys and
• emphasizes the importance of DRLs in training programs for healthcare workers.
The target audience for Publication 135 includes national, regional, and local authorities, professional societies, and facilities staff using ionizing radiation for medical purposes.
General Summary
The ICRP first coined the term Diagnostic Reference Level (DRL) in 1996. Then, in 2001, ICRP gave additional guidance on the use of DRLs. A DRL can be viewed as a tool for keeping patients safe during imaging tests using ionising radiations and interventional treatments. DRLs provide benchmarks in terms of measurable dose quantities against which dose levels for groups of patients can be compared. Their use enables procedures or facilities where doses are higher than expected to be identified. However, a DRL will only be of value if it is used in regular audit of patient doses and action is taken whenever doses exceed or approach the DRL value. Setting of DRLs can become an end in itself, but their use is only the first step along the road to optimisation of radiological protection.
As time passed, it became clear that there was a need for further advice on the use of DRLs. There were issues with definitions of terms used, such as:
• the determination of values for DRLs for specific procedures and their use in different facilities,
• the proper intervals for evaluating and updating those DRLs,
• the correct use of DRLs in clinical practice and their application to groups of patients not individuals,
• the methods for the sensible application of DRLs, and
• the use of the DRL idea in newer imaging technologies.
Publication 135 is an added source of information and guidance for those topics. It recommends:
• quantities for use as DRLs for various imaging modalities,
• changes in the manner in which DRLs are used in patient dose surveys given the increased availability of electronic storage of patient dose data
• adopting the median dose for groups of patients as the value for use in both setting DRLs and making dose comparisons
• adapting the methodology for assessing interventional procedures where there can be large variations in doses for procedures for individual patients
• deriving DRLs for groups of paediatric patients based on weight and size rather than age
The publication emphasizes the importance of DRLs in training programs for healthcare workers, but stresses that establishing a DRL is only the first stage of the patient dose audit process. Once a procedure or group of procedures at a facility have been identified as giving doses that exceed or approach the DRL, it is imperative that action is taken to optimise the radiological protection aspects of the procedure, if the programme of optimisation is to be successful.
The target audience for this publication is:
• National, regional, and local authorities,
• professional societies,
• facilities that use ionising radiation for medical purposes,
• and responsible staff within these facilities.
Technical Summary
The International Commission on Radiological Protection (ICRP) first introduced the term diagnostic reference level (DRL) in 1996 in Publication 73. The concept was developed further, and practical guidance was provided in 2001. DRLs provide benchmarks in terms of measurable dose quantities against which dose levels for groups of patients can be compared. The DRL has proven to be an effective tool that aids in the optimisation of protection in the medical exposure of patients for diagnostic and interventional procedures through identifying procedures for which the doses are greater than the level (DRL) that is generally considered acceptable. However, a DRL will only be of value if it is used in regular audit of patient doses and action taken whenever doses exceed or approach the DRL value. Setting of DRLs can become an end in itself, but their use is only the first step along the road to optimisation of radiological protection.
All aspects of medicine have developed rapidly in the last few decades, but this applies particularly to imaging and ionising radiation treatments. Not surprisingly, it is challenging for healthcare professionals to keep up with such changes and meet the expectations of an increasingly demanding public. A significant problem for optimisation of radiological protection is that since the move to use of digital x-ray images, which are optimised for viewing, there is no way of knowing whether the patient dose may be much higher than is necessary. As an advisory body, ICRP aims to provide relevant and up-to-date guidance for different sectors, including medicine; hence, this latest publication on DRLs. The language of radiological protection is complex and, at times, confusing to those who are not experts in the field. As such, it is essential for ICRP to provide practical information on the application of DRLs for different types of imaging and to clarify terminology and to describe ways in the concept should be applied. A few facts regarding DRLs are also easy to overlook. They are not intended to be applied to individual patients and should not be used as dose limits. Instead, a DRL is an essential tool in the optimisation process for exposures of patients, as dose limits are not relevant to medical exposures. The DRL is a supplement to professional judgement. It does not provide a dividing line between good and bad medical practice as groups of patients are different, but it gives an indication about whether the dose level is appropriate. In surveys performed to acquire dose information for different procedures, it is essential to identify radiation doses that are too high and too low, as both may have consequences for the patient. In the former case, exposing patients to unnecessarily high radiation levels and in the latter, potentially producing an image that may not be of sufficient quality for diagnosis.
DRLs are set in terms of measurable dose quantities that apply to individual imaging examinations or procedures. For x-ray techniques these are often displayed on the x-ray equipment, while for nuclear medicine they relate to the amount of radioactivity administered. They are derived from surveys of these quantities for groups of patients and the data will generally be collected locally and can be passed on to contribute to national or regional collations. When dose data is collected for a group of patients, there will be a distribution of the doses. The median value of the distribution is selected as being the representative dose of the group. The median value (rather than mean value) is used for the DRL quantity as this is considered to be more robust and representative of the patient population. It is recommended that DRL surveys take advantage of automated reporting of radiation-dose-related quantities now available as this can enable far larger numbers of patients to be included in surveys. When numbers of patients are smaller, it may be necessary to impose weight restriction in the collection of dose data, as patient weight can have a significant influence on doses from x-ray procedures. A special group for which dose reduction is important but for whom dose surveys are more difficult is paediatric patients. Children are growing continuously, and their size has a substantial impact on the amount of radiation required to produce a diagnostic image. There are challenges in obtaining sufficient data for each patient size and moreover the differences between individuals of the same age are large. Patient age categories have been used in the past to define groups of children for dose comparisons, but it has become apparent that age alone is not a good indicator. Comparisons of paediatric patients based on weight and size rather than age is recommended for examinations of the body.
The distribution of median values for a particular type of examination from many hospitals across a country or region can be combined and used for setting a national DRL. Traditionally a national DRL is set at the 75th percentile, such that three quarters of the hospitals across a country have doses below the DRL. A local DRL for a large grouping of local hospitals can also be set in this way. Thus, the DRL highlights the quarter of the hospitals for which doses are higher and where optimisation is required. Once a DRL has been set then the median doses from future surveys are compared with that DRL to check whether further optimisation may be required. The effectiveness of DRLs can be documented by comparing data over time and reviewing trends. However, optimisation of radiological protection and resulting reductions in a DRL through regular audit of patient doses will only occur if action is taken whenever doses exceed or approach the DRL value. Originally DRLs were used for diagnostic procedures, but their use has broadened with the introduction of newer medical technology and procedures, and DRL values have also been established for interventional procedures. This is a challenge due to the wide distribution of patient doses for the same procedure, even in the same facility. However, data have been collected for many different interventional procedures, and databases have been established successfully. This publication describes approaches that can overcome some of these limitations.
This publication introduces or clarifies the terminology:
• ‘DRL quantity’ is the measurable dose quantity used for the DRL.
• ‘DRL value’ is the value of the DRL quantity to be used for comparison with the median dose for that procedure in the facility.
• ‘National (or regional) DRL’ is a DRL value set nationally that can be used by facilities within the country for dose comparisons.
• ‘Local DRL’ may be set for procedures for which there is no national DRL available, or where local equipment or techniques have enabled a greater degree of optimisation to be achieved.
• ‘Typical value’ is the median of the distribution of the data for a DRL quantity for a clinical imaging procedure for a particular healthcare facility that has several x-ray rooms obtained from a local survey. Typical values can be used as a local comparator that can be tracked to encourage further optimisation. Publication 135 is intended as an additional source of information and guidance on issues like determination of the values for DRLs, the appropriate interval for evaluating and updating these values, proper use of DRLs in clinical practice, methods for practical application of DRLs, and application of the DRL concept to newer imaging technologies. In addition, the publication recommends quantities for use as DRLs for various imaging modalities and provides information on the use of DRLs for interventional procedures and in paediatric imaging. All individuals who have a role in subjecting a patient to a medical exposure should be familiar with DRLs as a tool for optimisation of protection and it is important that information on DRLs is included in training programmes for healthcare workers. The target audience for this publication is national, regional, and local authorities; professional societies; facilities that use ionising radiation for medical purposes; and responsible staff within these facilities. A complete set of the Commissions recommendations is also provided.
ICRP, 2017. Diagnostic reference levels in medical imaging. ICRP Publication 135. Ann. ICRP 46(1).
Authors on behalf of ICRP
E. Vaño, D.L. Miller, C.J. Martin, M.M. Rehani, K. Kang, M. Rosenstein, P. Ortiz-Lopez, S. Mattsson, R. Padovani, A. Rogers
Abstract - The International Commission on Radiological Protection (ICRP) first introduced the term ‘diagnostic reference level’ (DRL) in 1996 in Publication 73. The concept was subsequently developed further, and practical guidance was provided in 2001. The DRL has been proven to be an effective tool that aids in optimisation of protection in the medical exposure of patients for diagnostic and interventional procedures. However, with time, it has become evident that additional advice is needed. There are issues related to definitions of the terms used in previous guidance, determination of the values for DRLs, the appropriate interval for reevaluating and updating these values, appropriate use of DRLs in clinical practice, methods for practical application of DRLs, and application of the DRL concept to newer imaging technologies. This publication is intended as a further source of information and guidance on these issues. Some terminology has been clarified. In addition, this publication recommends quantities for use as DRLs for various imaging modalities, and provides information on the use of DRLs for interventional procedures and in paediatric imaging. It suggests modifications in the conduct of DRL surveys that take advantage of automated reporting of radiation-dose-related uantities, and highlights the importance of including information on DRLs in training programmes for healthcare workers. The target audience for this publication is national, regional, and local authorities; professional societies; and facilities that use ionising radiation for medical purposes, and responsible staff within these facilities. A full set of the Commission’s recommendations is provided.
© 2017 ICRP. Published by SAGE.
Keywords: Diagnostic reference levels; Patient doses; Optimisation.
AUTHORS ON BEHALF OF ICRP E. VAÑÓ, D.L. MILLER, C.J. MARTIN, M.M. REHANI, K. KANG, M. ROSENSTEIN, P. ORTIZ LÓPEZ, S. MATTSSON, R. PADOVANI, A. ROGERS
Key Points
8.1. General
(358) The DRL process should be used to evaluate whether, in routine circumstances, the amount of ionising radiation applied for a medical imaging procedure at a local healthcare facility, when assessed for a representative sample of patients (not individual patients) for a defined clinical task, is too high or too low. The DRL process allows identification of equipment and procedures for which radiation dose levels are high, so that optimisation of protection can be undertaken.
(359) A DRL value is considered to be consistently exceeded when the local median value of the appropriate DRL quantity for a representative sample of patients within an agreed weight range is greater than the local, national, or regional DRL value. Here, ‘consistently’ means ‘in a majority of cases’ and not ‘over a period of time’.
(360) DRLs may be established by authorised bodies. The numerical values of DRLs are advisory. However, an authorised body may require implementation of the DRL concept.
(361) Organisations responsible for different components of the tasks of collating data on DRL quantities and setting national DRLs should be identified in each country or region.
(362) DRL values shall not be used for individual patients or as trigger (alert or alarm) levels for individual patients or individual examinations.
(363) Comparison of local practices with DRL values is not sufficient, by itself, for optimisation of protection. Action is required to identify and address any deficiencies. The highest priority for any diagnostic imaging examination is achieving image quality sufficient for the clinical purpose. Image quality or, more generally, the diagnostic information provided by the examination (including the effects of postprocessing), must be evaluated as part of the DRL process, and methods to achieve optimisation should be implemented.
(364) All individuals who have a role in subjecting a patient to a medical imaging procedure should be familiar with the DRL process as a tool for optimisation of protection.
(365) The concept and proper use of DRLs should be included in the education and training programmes of the health professionals involved in medical imaging with ionising radiation.
8.2. DRL quantities
(366) Quantities used for DRLs should assess the amount of ionising radiation applied to perform a medical imaging task, and should be easily measured or determined. DRL quantities assess the amount of ionising radiation used for a medical imaging procedure, not absorbed dose to a patient or organ. The one exception is mammography, for which DG may be used.
(367) DRL quantities should be appropriate to the imaging modality being evaluated, to the specific study being performed, and to the specific size of the patient.
(368) The Commission stresses that the radiation protection quantity ‘effective dose’ (used for other purposes in the ICRP radiological protection system) should not be used as a DRL quantity. It introduces extraneous factors that are neither necessary nor pertinent for the purpose of a DRL.
(369) For projection radiography, two DRL quantities are recommended – Ka,e (or Ka,i) and PKA – in order to simplify assessment of proper use of collimation, especially in paediatrics.
(370) DRL values developed for advanced digital radiographic techniques (e.g. tomosynthesis, dual-energy subtraction, contrast-enhanced subtraction, cone-beam CT) need to take the ‘multiple image’ aspect of the technique into account, and should distinguish these procedures from more standard ones.
(371) For mammography, the recommended DRL quantity is one or more of Ka,i, Ka,e, and DG, with the choice of quantity depending on local practices and regulatory requirements.
(372) For mammography, a simple approach could be setting DRL values for breasts of 5.0 0.5 cm thickness. Establishing DRL values for different breast thicknesses is a more complex but better approach to refine the DRL process for mammography.
(373) For interventional radiology, all of the following DRL quantities are recommended (if available): PKA, cumulative air kerma at Ka,r, fluoroscopy time, and the number of radiographic images (e.g. cine images in cardiology and digital subtraction angiography images in vascular procedures).
(374) The recommended DRL quantities for CT are CTDIvol and DLP. The number of scan sequences in the examination may be helpful as well. SSDE provide more accurate estimates of paediatric patient doses than CTDIvol or DLP, which are both indices of the dose to standardised phantoms and may be used as an additional aid in optimisation.
(375) The recommended CTDIvol value to be used is the CTDIvol for each sequence. The recommended DLP value is the cumulative DLP for the entire examination. DLP values for individual scan sequences can also be useful, and may be used in addition to the cumulative DLP.
(376) For nuclear medicine, the ideal DRL quantity would be the administered activity per body weight of a specific radionuclide for a specific clinical task and, if relevant, the radiopharmaceutical used. The Commission recommends that weightbased administered activities should be used for children, adolescents, and low-weight patients, and considered for other groups. Setting a fixed maximum administered activity for very obese patients may also be considered. It is recognised that, in many countries, a standard activity is used in clinical practice for adult patients.
(377) Weight-based administered activities may not be appropriate for examinations where the radiopharmaceutical is concentrated predominantly in a single organ (e.g. thyroid scans, lung perfusion scans).
(378) As DRL values for nuclear medicine procedures and CT procedures apply to radiation from very different modalities, and use different DRL quantities, for hybrid imaging procedures (SPECT-CT, PET-CT), it is appropriate to set and present DRL values for each modality independently.
8.3. Use of median values of the national survey distribution
(379) Compliance with DRL values does not indicate that the procedure is performed at an optimised level with regard to the amount of radiation used. The Commission recognises that additional improvement can be obtained by using the median value (the 50th percentile) of the distribution used to set the national DRL value.
(380) This median value of the national distribution can serve as an additional tool to aid in optimisation, may be a desirable goal at which to aim using standard techniques and technologies, and represents a situation closer to the optimum use of the applied radiation.
(381) When the facility’s median value of a DRL quantity is lower than the median value of the national distribution, image quality (or diagnostic information, when multiple images are used) might be affected adversely. Image quality should be considered as a greater priority in the review if additional optimisation efforts are undertaken.
8.4. DRL surveys and registries
(382) The Commission recommends setting local and national DRL values based on DRL quantities for imaging examinations and procedures performed on patients. The use of phantoms is not sufficient in most cases. When phantoms are used, the effects of operator performance, the selected imaging protocol, and patient variability are not taken into account.
(383) The use of phantoms is important in the investigation of x-ray equipment performance, and is important in evaluating the performance of fluoroscopy and CT equipment with respect to the amount of radiation used during the optimisation of protection.
(384) Data on DRL quantities may be collected using surveys, registries, or other automated data collection methods.
(385) Calibrations of all dosimeters, kerma-area product meters, etc., used for patient dosimetry should be performed regularly and should be traceable to a primary or secondary standard laboratory.
(386) The accuracy of DRL quantity data produced by and transferred from x-ray systems should be verified periodically by a medical physicist.
(387) The examinations/procedures included should, in general, represent the most frequent examinations performed in the region for which dose assessment is practicable, with priority given to those that result in the highest patient radiation dose.
(388) DRLs are not intended for use in radiation therapy, but they should be considered for imaging for treatment planning, treatment rehearsal, and patient set-up verification in radiotherapy.
(389) National surveys and registries for setting DRL values should normally include medium- and large-sized healthcare facilities that have a sufficient workload to ensure that data for a representative selection of patients can be obtained. The sample should also cover the range of healthcare providers.
(390) For large countries, a survey of a random selection of a small proportion of all the healthcare facilities in the country can provide a good starting point for setting national DRL values, if no national registry or method for automated data collection exists. Results from 20–30 facilities are likely to be sufficient in the first instance. In a smaller country with fewer than 50 healthcare facilities, an initial survey of 30–50% of them may suffice.
(391) A survey for a particular examination in a facility should normally involve collection of data on DRL quantities for at least 20 patients, and preferably 30 patients for diagnostic fluoroscopy and CT examinations, and 50 patients for mammography. For paediatrics, these figures may need to be decreased for facilities where relatively few children are examined. For registries, all available and appropriate data should be used.
(392) There should be some standardisation of weight for adult patients included in surveys of diagnostic procedures if data are collected from fewer than 50 patients (e.g. patients with weights between 60 and 80 kg for a mean weight of 70 ± 5 kg).
(393) HIS and RIS can provide data for large numbers of patients, but may not include patient weight. As with all DRL surveys, the results rely on the accuracy of data entry.
(394) RIS and associated software may permit data on DRL quantities to be obtained in an automated fashion, either locally or through a national registry. When automated processes are used, the data for all cases of a specific procedure should be obtained and used for optimisation.
8.5. Setting DRL values
(395) The DRL value should be tied to defined clinical and technical requirements for the selected medical imaging task.
(396) The appropriate image quality or diagnostic information needed for the clinical task should be a priority when setting DRL values. DRL values may differ for different clinical tasks, especially for CT where visualisation of differences in the internal structure of tissues or identification of nodules is often important. Different tasks may require use of different image filters with varying exposure levels.
(397) It is important when developing DRL values that all data collected come from similar procedures across all participating facilities. This ensures that comparisons among facilities remain valid and useful.
(398) It may be important to specify, in detail, the views normally included and the clinical task associated with the procedure. This may be required where differing exposure factors or different views (or numbers of views) are employed for different clinical indications.
(399) When two imaging modalities are used for the same procedure (e.g. PET-CT, SPECT-CT), it is appropriate to set and present DRL values for both modalities independently.
(400) DRL values are dependent on the state of practice and the available technology (including postprocessing software) at a particular point in time.
(401) Median values (not mean values) of the distributions of data collected from a representative sample of patients within an agreed weight range should be used for comparison with DRL values. The mean can be affected substantially by a few high or low values.
(402) National DRL values should be set as the 75th percentile of median values obtained in a sample of representative centres.
(403) If regional (multi-national) DRL values are created, they should be set as the median value of the national DRL values (each of which is set at the 75th percentile) for the countries in the region. If the sample of available data is small, other approaches may be used by agreement among the involved countries.
(404) The process to set and to update DRL values should be both flexible and dynamic. Flexibility is necessary for procedures where few data are available (e.g. interventional procedures in paediatric patients), or from only one or a few centres. A dynamic process is necessary to allow initial DRL values to be derived from these data while waiting for a wider survey to be conducted.
(405) When a procedure is not performed on a regular basis in most hospitals, local DRL values may be determined using the data from a single large hospital with a relevant workload of procedures (e.g. a specialised paediatric hospital).
(406) Local DRL values set by a group of radiology departments can play a role, where effort has already been invested in optimisation. The group could set a local DRL value based on more regular surveys of local practice that will normally be lower than any national DRL value. Where the number of facilities or x-ray rooms is small, the median of the distribution of values of the DRL quantity is recommended as a ‘typical value’. Typical values can also be set for newer technologies that enable decreased amounts of radiation to be used in achieving a similar level of image quality.
(407) Published DRL values should be accompanied by a statement of the local group, nation, or region from which the patient data were collected, the size of the ‘standard’ patient on whom the data are based, the details of the specific examination, as appropriate, and the date of the survey.
8.6. DRLs for interventional procedures
(408) The Commission recommends retaining the term ‘diagnostic reference level’ for the DRL process as applied to interventional procedures.
(409) For interventional procedures, complexity of the procedure may be considered in setting DRL values, and a multiplying factor for the DRL value may be appropriate for more complex cases of a procedure.
(410) If possible, the data from all interventional procedures performed (not just from a limited sample) should be collated to derive local and national DRL values.
8.7. Paediatric DRLs
(411) A single ‘representative patient’ should not be used to define DRLs for paediatric imaging, as weight in children can vary by a factor of more than 100 from a premature infant to an obese adolescent.
(412) The amount of radiation used for examinations of children can vary tremendously due to the great variation in patient size and weight, from neonates to adult-sized adolescents. This variation in patient radiation dose is appropriate. Variation in patient radiation dose due to incorrect technique or failure to adapt the imaging protocol from adults to children to account for both paediatric diseases and paediatric patient size is not appropriate.
(413) Weight bands are recommended for establishing paediatric DRL values for examinations of the trunk and should be promoted for paediatrics. Age bands can be used if age is the only available measure.
(414) Age groupings are recommended for establishing DRL values for examinations involving the head.
(415) For CT, the DRL quantities are CTDIvol and DLP, based preferably on calibration with a 32-cm-diameter phantom for body examinations and a 16-cm-diameter phantom for head examinations. Values for these quantities should be obtained from patient examinations. SSDE may be used as an additional source of information for optimisation.
(416) Modern CT scanners permit determination of effective diameter or patient equivalent thickness. This should be considered as an additional refinement for setting paediatric DRLs.
(417) For nuclear medicine imaging, consideration should be given to adjusting administered activities based on agreed factors linked to weight. Adjustments should be made for paediatric examinations.
8.8. Application of DRLs in clinical practice
(418) National and regional DRL values should be revised at regular intervals (3–5 years) or more frequently when substantial changes in technology, new imaging protocols, or postprocessing of images become available.
(419) Median values of the DRL quantity for medical imaging procedures in a representative sample of patients within an agreed weight range for a specific x-ray room, radiology department, or other facility should be compared with local, national, or regional DRL values to identify whether the data for that location are substantially higher or lower than might be anticipated.
(420) If a local or national DRL value for any procedure is consistently exceeded, an investigation should be performed without undue delay, and, if appropriate, corrective action should be taken.
(421) When corrective action is required, it is necessary to keep in mind that DRL values are not dose limits.
(422) Corrective action (optimisation of protection) should include a review of equipment performance, the settings used, and the examination protocols. The factors most likely to be involved are survey methodology, equipment performance, procedure protocol, operator skill, and, for interventional techniques, procedure complexity.
(423) In the optimisation process, account must always be taken of the level of image quality required for the medical imaging task. Image quality must always be adequate to provide the information required for the clinical purpose of the examination and the actual size of the patient irradiated.
(424) When a facility’s median value of a DRL quantity is substantially less than the DRL value, image quality (or diagnostic information, when multiple images are used) might be affected adversely. Image quality should be examined as a priority when the examination protocol is reviewed.
(425) The DRL audit process does not stop after a single assessment. Repeat evaluations are required following any optimisation, and the whole process should be repeated after an appropriate time interval.
(426) Local surveys of DRL quantities should normally be performed as part of the QA programme unless these data are continuously submitted to a registry, in which case, review of the registry data should be performed. A representative selection of examinations for each x-ray unit should be surveyed at intervals of about 3 years, and whenever substantial changes in technology or software have been introduced.
(427) Local surveys of DRL quantities, as part of the QA programme, should be performed annually for CT and interventional procedures, unless these data are continuously submitted to a registry, in which case review of the registry data should be performed at least annually. Annual surveys or review of registry data are also appropriate as part of the QA programme for SPECT-CT and PET-CT.
(428) If continuous collection of data on DRL quantities is possible through registries or automated collation of data from electronic databases, the dose management process may take the form of a regular review of all the data to identify any adverse trends.
(429) The method for managing and achieving optimisation for dental radiography differs from the method for other x-ray applications. Dental DRL values are set in terms of incident air kerma measured during routine tests. Based on test results, recommendations can be made on changes to protocols (equipment settings) and adjustments. The investigator should work with the dentist to optimise protection. Improvement in protection linked to new technology can be realised which otherwise might not be achieved.
Executive Summary
1. Introduction
(a) The Commission first introduced the term ‘diagnostic reference level’ (DRL) in Publication 73 (ICRP, 1996). The concept was subsequently developed further, and practical advice was provided in Supporting Guidance (ICRP, 2001a). This development and the 2001 advice are summarised in Annex A.
(b) As the Commission stated in Publication 103 (ICRP, 2007a), one of the principles of optimisation of protection in medical exposures is implemented through the use of DRLs. The DRL has proven to be an effective tool that aids in optimisation of protection in the medical exposure of patients for diagnostic and interventional procedures. DRLs are not intended for use in radiation therapy, but they should be considered for imaging for treatment planning, treatment rehearsal, and patient set-up verification in radiotherapy.
(c) With time, it has become evident that additional advice is needed. There are issues related to definitions of some of the terms used in previous guidance, determination of the values for DRLs, the appropriate interval for re-evaluating and updating these values, appropriate use of DRLs in clinical practice, methods for practical application of the DRL process, and application of the concept to certain newer imaging technologies [e.g. dual-energy computed tomography (CT), positron emission tomography-CT (PET-CT), single photon emission CT (SPECT-CT), cone beam CT, digital radiography, tomosynthesis], and additional difficulties in paediatric practice, particularly due to the wide range in sizes.
(d) In this publication, the Commission recommends the use of four different DRL terms (defined in the Glossary):
DRL (a form of investigation level used as a tool to aid optimisation of protection in the medical exposure of patients for diagnostic and interventional procedures).
DRL quantity (a commonly and easily measured or determined radiation metric that assesses the amount of ionising radiation used to perform a medical imaging task).
DRL value (an arbitrary notional value of a DRL quantity, set at the 75th percentile of the distribution of the medians of distributions of the DRL quantity obtained from surveys or other means).
DRL process (the cyclical process of establishing DRL values, using them as a tool for optimisation, and then determining updated DRL values as tools for further optimisation).
(e) This publication is intended as a further source of information and guidance on these issues. Some terminology has been clarified. In addition, this publication recommends DRL quantities for various imaging modalities, provides information on the application of DRLs to interventional procedures and in paediatric imaging, highlights common errors in the determination and application of DRL values, suggests modifications in surveys for establishing DRL values that take advantage of automated reporting of radiation-dose-related quantities, and highlights the importance of including information on DRLs in training programmes for healthcare workers and in information for patients.
(f) The target audience for this publication is national, regional, and local authorities; professional societies; and facilities where ionising radiation is used for medical exposures, and responsible staff within these facilities.
(g) A full set of the Commission’s recommendations is provided in Section 8 of this publication. In addition, each section is preceded by a set of main points that summarise the principal concepts in that section. A limited summary of the most important points and recommendations is presented below for the convenience of the reader.
2. Diagnostic reference levels
(h) The principles of justification and optimisation of protection are key and complementary radiological safety tenets. DRL is the Commission’s term for a form of investigation level used to aid in optimisation of protection in the medical exposure of patients for diagnostic and interventional procedures. A DRL value is a selected level of a DRL quantity for broadly defined types of equipment for typical examinations for groups of patients within an agreed weight range or, in certain specific circumstances, a standard phantom. DRLs do not apply to individual patients. They are derived as an arbitrary threshold from radiation metric data obtained locally and collected nationally or regionally. A DRL is a supplement to professional judgement and does not provide a dividing line between good and bad medical practice. All individuals who have a role in subjecting a patient to a medical exposure should be familiar with DRLs as a tool for optimisation of protection.
(i) The application of the DRL process is not sufficient, by itself, for optimisation of protection. Optimisation is generally concerned with maintaining the quality of the diagnostic information provided by the examination commensurate with the medical purpose while, at the same time, seeking to reduce patient exposures to radiation to a level as low as reasonably achievable. Image quality or, more generally, the diagnostic information provided by the examination (including the effects of postprocessing) must also be evaluated. Methods to achieve optimisation that encompass both the DRL process and image quality evaluation should be implemented. In some cases, optimisation may result in an increase in dose.
(j) A dose below a DRL value does not, by itself, indicate that the procedure is performed at an optimised level with regard to the amount of radiation used. Therefore, the Commission recognises that additional improvement can often be obtained by using the median value (the 50th percentile) of the national distribution of values of dose-related quantities to provide additional guidance for further optimisation efforts. If local median values of the DRL quantity are below the national median value, image quality, rather than the amount of radiation used, should be considered as a greater priority in this additional optimisation process. The basis for this recommendation is that if practices at the local facility have already achieved levels of radiation use that are below the national median value, further reduction in the amount of radiation used is not the principal concern. When local practices result in levels of radiation that are below the national median value, ensuring that image quality is adequate should be a priority.
3. DRL quantities and values
(k) DRL quantities should be appropriate to the imaging modality being evaluated, should assess the amount of ionising radiation applied to perform a medical imaging task, and should be easily measured or determined. When two imaging modalities are used for the same procedure (e.g. PET-CT, SPECT-CT), it is appropriate to set and present DRL values for both modalities independently.
(l) An authorised body may require implementation of the DRL process as a tool to promote optimisation, but DRL values should be considered advisory. The numerical value of the DRL should be tied to defined clinical and technical requirements for the selected medical imaging task. The Commission recommends setting DRL values based on surveys of the appropriate DRL quantities for procedures performed on an appropriate sample of patients. The use of phantoms is not sufficient in most cases, as the effects of operator performance are not taken into account when phantoms are used.
(m) DRL values are not static. As optimisation of examinations continues or hardware and software improve, DRL values should be updated on a regular basis. When new imaging techniques are introduced, an effort should be made to measure suitable DRL quantities and set DRL values as soon as is practicable.
(n) For interventional procedures, the complexity of the procedure may be considered in setting DRL values, and a multiplying factor for the DRL value may be appropriate for more complex cases of a procedure.
4. Local, national, and regional DRLs
(o) Organisations responsible for different components of the tasks of collating data on DRL quantities used for patient examinations and setting DRL values should be identified in each country or region. The process to set and update DRLs should be both flexible and dynamic. Flexibility is necessary for procedures where few data are available (e.g. interventional procedures in paediatric patients), or where data are available from only one or a few centres. A dynamic process is necessary to allow initial DRLs to be derived from these data while waiting for a wider survey to be conducted.
(p) Data for determining national DRL values for x-ray procedures are obtained from surveys or registries. Values of appropriate DRL quantities from patient examinations are collected from several different health facilities. The 75th percentile value of the distribution of median values of a DRL quantity at healthcare facilities throughout a country is used as the ‘national DRL’.
(q) When national DRL values exist for many or most countries within a region (e.g. the European Union), regional DRL values may be determined by using the median value of the available national values. These may provide guidance on the need for optimisation or protection for neighbouring countries without their own DRL surveys or registries, and give an indication that further optimisation may be required for countries whose current national DRL values are above the regional DRLs.
(r) National and regional DRLs should be revised at regular intervals of 3–5 years, or more frequently when substantial changes in technology, new imaging protocols, or improved postprocessing of images become available.
(s) As national DRLs for x-ray procedures need large surveys or registries, and these can require substantial effort to perform and analyse, they are not always as responsive to changes in technology. Where it is apparent that further optimisation is being achieved locally, or where no national DRL values exist, ‘local DRLs or typical values’ based on surveys might be introduced to further assist the optimisation process (Table 2.2). Examples of their use are to account for the substantial dose reduction that could be achieved through the application of iterative reconstruction techniques in CT, the replacement of computed radiography with flat-panel digital radiography, and the introduction of digital radiography detectors into dental radiography. Another example is the introduction of new methods for postprocessing of images. In small healthcare facilities, ‘typical values’ can be used (Table 2.2).
5. Using DRLs for optimisation of protection
(t) Median values of the appropriate DRL quantity for medical imaging procedures for a specific x-ray room or for a radiology department or other facility should be compared with local, national, or regional DRL values to identify whether the data for the location are substantially higher or lower than might be anticipated.
(u) A DRL value is considered to be ‘consistently exceeded’ at a facility when the median value of the DRL quantity at the facility for a representative sample of patients within an agreed weight range is greater than the local, national, or regional DRL value.
(v) If a DRL value for any procedure is exceeded, an investigation should be undertaken without undue delay to determine possible reasons. If corrective action is required, a plan should be implemented (and documented) without undue delay.
(w) DRLs are not intended to be used for individual patients or as a trigger (alert or alarm) level for individual patients or individual examinations. Also, DRL values are not limits.
(x) The highest priority for any diagnostic examination is achieving image quality sufficient for the clinical purpose, so that the images from the whole examination provide all the diagnostic information required and the clinical purpose is not jeopardised.
6. Considerations for paediatric examinations
(y) The radiation exposure for examinations of children, whether from radiological imaging or nuclear medicine, can vary tremendously due to the great variation in patient size and weight. This variation in patient radiation dose is appropriate. However, variation in patient radiation dose due to inappropriate technique or failure to adapt imaging protocols from adults to children to account for both paediatric diseases and different patient sizes is not appropriate, and requires optimisation of protection.
(z) Weight bands (see Section 6) are recommended for establishing paediatric DRLs for x-ray examinations of the trunk, and should be promoted for paediatrics.
Concise Summary
The International Commission on Radiological Protection (ICRP) introduced Diagnostic Reference Level (DRL) in 1996, with an updated publication in 2001. It was apparent that additional advice was needed, which paved the way for publication 135.
Publication 135:
• suggests quantities for use as DRLs for various imaging modalities,
• provides information on the use of DRLs for paediatric imaging and interventional procedures,
• recommends changes in DRL surveys and
• emphasizes the importance of DRLs in training programs for healthcare workers.
The target audience for Publication 135 includes national, regional, and local authorities, professional societies, and facilities staff using ionizing radiation for medical purposes.
General Summary
The ICRP first coined the term Diagnostic Reference Level (DRL) in 1996. Then, in 2001, ICRP gave additional guidance on the use of DRLs. A DRL can be viewed as a tool for keeping patients safe during imaging tests using ionising radiations and interventional treatments. DRLs provide benchmarks in terms of measurable dose quantities against which dose levels for groups of patients can be compared. Their use enables procedures or facilities where doses are higher than expected to be identified. However, a DRL will only be of value if it is used in regular audit of patient doses and action is taken whenever doses exceed or approach the DRL value. Setting of DRLs can become an end in itself, but their use is only the first step along the road to optimisation of radiological protection.
As time passed, it became clear that there was a need for further advice on the use of DRLs. There were issues with definitions of terms used, such as:
• the determination of values for DRLs for specific procedures and their use in different facilities,
• the proper intervals for evaluating and updating those DRLs,
• the correct use of DRLs in clinical practice and their application to groups of patients not individuals,
• the methods for the sensible application of DRLs, and
• the use of the DRL idea in newer imaging technologies.
Publication 135 is an added source of information and guidance for those topics. It recommends:
• quantities for use as DRLs for various imaging modalities,
• changes in the manner in which DRLs are used in patient dose surveys given the increased availability of electronic storage of patient dose data
• adopting the median dose for groups of patients as the value for use in both setting DRLs and making dose comparisons
• adapting the methodology for assessing interventional procedures where there can be large variations in doses for procedures for individual patients
• deriving DRLs for groups of paediatric patients based on weight and size rather than age
The publication emphasizes the importance of DRLs in training programs for healthcare workers, but stresses that establishing a DRL is only the first stage of the patient dose audit process. Once a procedure or group of procedures at a facility have been identified as giving doses that exceed or approach the DRL, it is imperative that action is taken to optimise the radiological protection aspects of the procedure, if the programme of optimisation is to be successful.
The target audience for this publication is:
• National, regional, and local authorities,
• professional societies,
• facilities that use ionising radiation for medical purposes,
• and responsible staff within these facilities.
Technical Summary
The International Commission on Radiological Protection (ICRP) first introduced the term diagnostic reference level (DRL) in 1996 in Publication 73. The concept was developed further, and practical guidance was provided in 2001. DRLs provide benchmarks in terms of measurable dose quantities against which dose levels for groups of patients can be compared. The DRL has proven to be an effective tool that aids in the optimisation of protection in the medical exposure of patients for diagnostic and interventional procedures through identifying procedures for which the doses are greater than the level (DRL) that is generally considered acceptable. However, a DRL will only be of value if it is used in regular audit of patient doses and action taken whenever doses exceed or approach the DRL value. Setting of DRLs can become an end in itself, but their use is only the first step along the road to optimisation of radiological protection.
All aspects of medicine have developed rapidly in the last few decades, but this applies particularly to imaging and ionising radiation treatments. Not surprisingly, it is challenging for healthcare professionals to keep up with such changes and meet the expectations of an increasingly demanding public. A significant problem for optimisation of radiological protection is that since the move to use of digital x-ray images, which are optimised for viewing, there is no way of knowing whether the patient dose may be much higher than is necessary. As an advisory body, ICRP aims to provide relevant and up-to-date guidance for different sectors, including medicine; hence, this latest publication on DRLs. The language of radiological protection is complex and, at times, confusing to those who are not experts in the field. As such, it is essential for ICRP to provide practical information on the application of DRLs for different types of imaging and to clarify terminology and to describe ways in the concept should be applied. A few facts regarding DRLs are also easy to overlook. They are not intended to be applied to individual patients and should not be used as dose limits. Instead, a DRL is an essential tool in the optimisation process for exposures of patients, as dose limits are not relevant to medical exposures. The DRL is a supplement to professional judgement. It does not provide a dividing line between good and bad medical practice as groups of patients are different, but it gives an indication about whether the dose level is appropriate. In surveys performed to acquire dose information for different procedures, it is essential to identify radiation doses that are too high and too low, as both may have consequences for the patient. In the former case, exposing patients to unnecessarily high radiation levels and in the latter, potentially producing an image that may not be of sufficient quality for diagnosis.
DRLs are set in terms of measurable dose quantities that apply to individual imaging examinations or procedures. For x-ray techniques these are often displayed on the x-ray equipment, while for nuclear medicine they relate to the amount of radioactivity administered. They are derived from surveys of these quantities for groups of patients and the data will generally be collected locally and can be passed on to contribute to national or regional collations. When dose data is collected for a group of patients, there will be a distribution of the doses. The median value of the distribution is selected as being the representative dose of the group. The median value (rather than mean value) is used for the DRL quantity as this is considered to be more robust and representative of the patient population. It is recommended that DRL surveys take advantage of automated reporting of radiation-dose-related quantities now available as this can enable far larger numbers of patients to be included in surveys. When numbers of patients are smaller, it may be necessary to impose weight restriction in the collection of dose data, as patient weight can have a significant influence on doses from x-ray procedures. A special group for which dose reduction is important but for whom dose surveys are more difficult is paediatric patients. Children are growing continuously, and their size has a substantial impact on the amount of radiation required to produce a diagnostic image. There are challenges in obtaining sufficient data for each patient size and moreover the differences between individuals of the same age are large. Patient age categories have been used in the past to define groups of children for dose comparisons, but it has become apparent that age alone is not a good indicator. Comparisons of paediatric patients based on weight and size rather than age is recommended for examinations of the body.
The distribution of median values for a particular type of examination from many hospitals across a country or region can be combined and used for setting a national DRL. Traditionally a national DRL is set at the 75th percentile, such that three quarters of the hospitals across a country have doses below the DRL. A local DRL for a large grouping of local hospitals can also be set in this way. Thus, the DRL highlights the quarter of the hospitals for which doses are higher and where optimisation is required. Once a DRL has been set then the median doses from future surveys are compared with that DRL to check whether further optimisation may be required. The effectiveness of DRLs can be documented by comparing data over time and reviewing trends. However, optimisation of radiological protection and resulting reductions in a DRL through regular audit of patient doses will only occur if action is taken whenever doses exceed or approach the DRL value. Originally DRLs were used for diagnostic procedures, but their use has broadened with the introduction of newer medical technology and procedures, and DRL values have also been established for interventional procedures. This is a challenge due to the wide distribution of patient doses for the same procedure, even in the same facility. However, data have been collected for many different interventional procedures, and databases have been established successfully. This publication describes approaches that can overcome some of these limitations.
This publication introduces or clarifies the terminology:
• ‘DRL quantity’ is the measurable dose quantity used for the DRL.
• ‘DRL value’ is the value of the DRL quantity to be used for comparison with the median dose for that procedure in the facility.
• ‘National (or regional) DRL’ is a DRL value set nationally that can be used by facilities within the country for dose comparisons.
• ‘Local DRL’ may be set for procedures for which there is no national DRL available, or where local equipment or techniques have enabled a greater degree of optimisation to be achieved.
• ‘Typical value’ is the median of the distribution of the data for a DRL quantity for a clinical imaging procedure for a particular healthcare facility that has several x-ray rooms obtained from a local survey. Typical values can be used as a local comparator that can be tracked to encourage further optimisation. Publication 135 is intended as an additional source of information and guidance on issues like determination of the values for DRLs, the appropriate interval for evaluating and updating these values, proper use of DRLs in clinical practice, methods for practical application of DRLs, and application of the DRL concept to newer imaging technologies. In addition, the publication recommends quantities for use as DRLs for various imaging modalities and provides information on the use of DRLs for interventional procedures and in paediatric imaging. All individuals who have a role in subjecting a patient to a medical exposure should be familiar with DRLs as a tool for optimisation of protection and it is important that information on DRLs is included in training programmes for healthcare workers. The target audience for this publication is national, regional, and local authorities; professional societies; facilities that use ionising radiation for medical purposes; and responsible staff within these facilities. A complete set of the Commissions recommendations is also provided.
