The Use of Effective Dose as a Radiological Protection Quantity

Introduction

The following review was prepared by scientists and clinical practitioners at Duke University, USA. The following individuals contributed to the review: 

Aiping Ding, PhD 

David Erickson, MS 

Wanyi Fu, BS 

Jocelyn Hoye, BS 

Anuj Kapadia, PhD 

Francesco Ria, PhD 

Ehsan Samei, PhD 

Shobhit Sharma, BS 

Justin Solomon, PhD 

Joshua Wilson, PhD 

Overall summary

Overall this is an important document that describes the utility of effective dose in radiation protection. The document comes from a recognized authority in the field or radiation protection (ICRP) and has been put together by a team of qualified and eminent scientists. Although the document presents important information and guidelines for radiation protection, there are some shortcomings that make it difficult to grasp the stance of the ICRP on recommendations being made. For example, the recommendation that Effective Dose be used for setting diagnostic reference levels (DRLs) is in direct conflict with a previous publication from ICRP (#135) which states that “Effective dose is not appropriate as a DRL quantity”. In that sense, the document is vague in clarifying the main recommendation of the ICRP regarding radiation protection. The definition of organ dose and its role/value in protection should be better clarified. Although the document acknowledges that age and gender should be taken into account, there is no mention of how these factors should be considered or what role they may play in lifetime risk estimation. An age- and gender-dependent effective dose is already in use; however, is that sufficient? Some discussion along these lines would be useful.

For the best and most efficient impact of this document, we recommend that the authors consider adding a single consolidated section that summarizes the key recommendations and what is needed going forward. This section could appear either at the start or the end of the document and would provide the key recommendations in a single, easy-to-find section. It would be tremendously helpful if this section could utilize clear math-based evidence (rather than vague ambiguous language) to highlight which recommendations need to be followed. A brief list of bullet points appears at the start of the document; it would be useful to mention where in the report each point is addressed. 

In the following pages, a short overall review of the document is provided first, followed by additional comments for each different section. 

Overview and main comments 

The document clarifies the use of Effective Dose (E) in radiation protection. In particular, E is recognized as a metric that can provide a risk-adjusted measure of total body dose from external and internal sources in relation to risks of cancer and hereditary effects. It has proved to be a robust quantity for Justification, Optimization and for setting limits, constraints, and reference levels. E takes into account the assumption of a linear-non-threshold dose-response relationship between dose and risk, and it is calculated for sex-averaged reference persons of specified ages.

It is interesting to note that the document states how E is used in medical applications to provide a generic indication for classifying and comparing different medical procedures for the purpose of communicating risks to clinicians and patients. In this scenario, E provides a measure of detriment that can be prospectively used as an indicator of radiation detriment in justification decisions. 

Furthermore, the document clarifies the use of Effective Dose in risk communication: “When only the possibility of stochastic effects is involved (the majority of cases), effective dose is an appropriate quantity for straightforward communication and to facilitate comparisons of the possible health risks of an exposure with risks from other exposure scenarios.”. Standard dose ranges and terminology for describing risks from different medical procedures are also proposed. 

Lastly, the Commission proposes to discontinue the use of the Equivalent Dose (H); instead Absorbed Dose (D) in Gy should be used to set limits on organ/tissue doses to prevent deterministic effects, whereas the control of stochastic effects relies on the use of Effective Dose. Furthermore, for medical procedures in which a single radiosensitive organ receives the majority of the dose (e.g., mammography, iodine therapeutic administration), mean absorbed doses to the tissues of interest should be used rather than effective dose.

However, compared with many positive clarifications and definitions, the following topics require further discussions.

• CONFLICTING INFORMATION:The Commission proposes the use of E for setting Diagnostic Reference Levels. Instead, another ICRP publication (135) states: “Effective dose is not appropriate as a DRL quantity. Effective dose is not a measurable quantity and does not assess the amount of ionising radiation used to perform a medical imaging task. Its use could introduce extraneous factors that are not needed and not pertinent for the purpose of DRLs.” Also, the same draft in exam reports that “for comparison of doses from the same procedure in different facilities and for setting diagnostic reference levels, measurable dose quantities are preferable” (pg. 56, 2073-2075).  

• Effective dose is calculated for sex and age averaged reference persons, and the Commission supports this approach by stating that: “Protections would not be improved by introducing separate considerations for males and females and for children of different ages” (pg. 55, 2005-2006) because optimization should ensure protection of all groups within populations. However, it is important to consider that the other two radiation protection principles (justification and limitations) are strictly related to patient/worker age and gender. Also medical procedures’ optimization can and should take into account age and gender (i.e.: designing different clinical protocols for pediatric populations). See, also:
  • pg. 56, 2058-2060;
  • pg. 56-57, 2080-2089. 

• The document does not clarify multiple effective dose calculations in CT. In particular, E from DLP, proposed in ICRP 102, is age specific while this draft specifies how E should be an age- and gender-averaged metric. Furthermore, the use of E predicted by DLP has been supported by other scientific committees: "The values of E predicted by DLP and the values of E estimated using more rigorous calculations methods are remarkably consistent, with a maximum deviation from the mean of approximately 10% to 15%" (AAPM report 96). This has not been proved in modern CT with Automatic Tube Current and kVp modulation systems. The document should simply say that the use of DLP should be discontinued. 

For these reasons, it might be better to propose that E can be used in medical applications only when predicted by accurate organ dose estimates.

Health Effects

Although the report does a good job at introducing the detrimental effects of radiation (both deterministic and stochastic) and the associated risks, it is vague at times in its description.

• From the perspective of radioprotection, ICRP recommends using a common criterion (same averaged tissue weighing factors for all ages and genders) for the whole population as the existing uncertainty in the risk estimates for each of these groups is large compared to their differences (line 713). Owing to this, using different criteria for different age and gender groups might not lead to benefits in terms of reduced risk to the exposed population. However, it would be useful to have specific guidance on when it is practically useful to isolate these groups.

• Although the report discusses radiation-related risks at multiple points, there is no guidance given on how to use and interpret those numbers, or how to extrapolate the nominal risk coefficients from a population to individuals (particularly important for risk communication). In addition, these risks need to be reevaluated in association with the benefits that radiation provides in populations with preexisting conditions.  

• In terms of the metric that should be used for radiological protection, ICRP recommends using absorbed dose for deterministic effects and effective dose for stochastic effects (line 454).  However, the role of organ dose in terms of quantifying the stochastic effects of radiation still needs to be clarified.

• Although there is sufficient discussion in the report about large uncertainties in risk estimation below 100 mSv (equivalent dose), no concrete figures have been provided that quantify these estimates.

• ICRP’s recommendation for radiation-induced circulatory disease (line 462) seems weak compared to the other thresholds recommended for various deterministic effects, which makes it difficult to interpret the final stance on this issue. Does the ICRP want the community to follow the thresholds mentioned, or is this simply an acknowledgment that they exist (and should perhaps be studied further through future research)? Also, the classification used for this particular effect is not clear because stochastic processes might be involved in disease development. 

• The description for the calculation of detriment (line 526) does not describe the exact difference from the method used in Publication 103, which might be useful here.
  
  

Dosimetry 

This document states that absorbed dose should be considered the most accurate value correlated with risk assessment. However, there is currently no reliable method to convert absorbed dose directly into risk, hence it may be beneficial to encourage research studies identifying a direct relationship between organ dose and risk. Furthermore, absorbed dose is an average value measured across a volume, which is not always constant (e.g., due to inhomogeneity in radiation fields). It may therefore also be beneficial to encourage research on dose quantities taking into account radiation field heterogeneity with in an organ.

Line854: The absorbed dose is “assumed” to be correlated with radiation detriment effect, and “sufficient for the purpose of radiological protection”. Given that the correction relationship is an assumption rather than a truth, more justification should be provided on why this assumption is sufficient for use in radiological protection?

Line889:  What is the “intended application” of effective dose?

Line951: Defining WT as tissue (rather than “organ”) weighting factor is confusing. Although different tissues have different radiation sensitivity, WT is typically defined for each organ, because1) it is different for each organ, and 2) the same (single) value of WT is used to calculate overall organ dose. Therefore, it would be clearer to refer to W T as “organ weighting factor”.  

Line1027: Considerable work has been performed in the development of voxelized phantoms for dosimetry calculations. The document may cite the following work in creating voxelized phantoms to provide as a dosimetry reference to the community. 

Line1223: It may be beneficial to calculate collective organ dose.

Occupational Exposure

Line 1262: “Collective and individual effective dose estimates can then be used to optimise protection, ensuring that the reductions in exposures for some workers are balanced against the potential increase in the number of workers exposed to smaller doses.” – Some examples would be useful for this statement, particularly if this is of relevance to certain occupations.  

Line 1360: “Effective dose can provide an initial indication of whether exposures are such that tissue reactions could be observed and individual organ doses need to be considered in the control of any further exposures.” – Agreed 

Line 1467: “A number of possible cohorts containing people of various ages with different occupations, habits and food consumption rates would generally be considered to define the representative person.” – Unclear whether the representative person is defined based on an average of cohorts or different ages, occupations, habits and food consumption, or whether the individuals of these groups at highest risk would be considered the representative person. 

Line 1504: “The use of dose constraints and reference levels that apply to all members of the public (or all workers), together with optimisation, provides a pragmatic, equitable and workable system of protection that recognises age-, sex-, and population-related differences in risks per Sv but does not distinguish on an individual basis. The corollary is that, for practical radiation protection purposes, the use of a single set of tissue weighting factors remains entirely appropriate.” – This statement is true provided an appropriate weighting factor is selected to approximate the representative person (or persons corresponding to the cohort at risk). Care must be taken to not base protection needs on an approximated / unified tissue weighting factor that is not representative of the affected individuals. The following statement explains this better. 

Line 1565: “…it is recommended that when exposures occur over large populations, areas and time periods, such that individual doses range over several orders of magnitude, the collective effective dose should be split according to ranges of individual dose, also taking account of geographical locations and the time-course of dose delivery.”

Medical Exposure

The section includes some age/sex specific risk tables that will be very useful. 

Section 5.1, Lines 1639-1645: I disagree with this statement. They’re implying that estimating an effective dose for a single generic reference person is sufficient to “optimize protection through selecting the most appropriate technique”. I don’t think this is true due to the strong relationship between the necessary dose needed to achieve diagnostic quality, and the patient size. In other words, you cannot optimize a technique without considering how much more or less dose is needed across patient sizes. Further, because patient size is highly correlated with age and gender, the risk-to-benefit ratio could be highly variable across patient sizes if you optimize to a single reference patient. Probably if they soften the language to focus on what they actually want to say it would be acceptable. I think what they really want to say is that “its ok to use somewhat generic values of E when weighing what types of imaging exams might be appropriate for a patient. You don’t need to calculate a patient specific E to do this” This statement I agree with. They go too far in implying that you can optimize imaging techniques based on a single generic E and not consider the variability across different patients. 

Figure 5.1. Finally! After all the talk about uncertainty, a plot that actually shows that uncertainty! Bravo!

Line 1935 “Uncertainties in both dose and risk estimates should be considered” How? Is there a plan to provide a systematic mathematical framework to do this? 

Lines1723-1727: I disagree with this statement. It was clearly stated in the ICRP 135 (page 27) “Effective dose is not appropriate as a DRL quantity. Effective dose is not a measurable quantity and does not assess the amount of ionizing radiation used to perform a medical imaging task’.  Absorbed organ doses should provide an appropriate measure for comparison, instead of effective dose. The two conflicting statements in these documents need to be consolidated, with a clear guideline provided about which recommendation should be followed. 

Lines 1728-1742: I don’t think ‘the use of effective dose is appropriate’. Comparisons should still be based on dose to the tissues/organs, especially for individual patient dose estimation.

Line 1755: What does ‘a reference person’ look like?  Like the ICRP reference male or female person?

Additional comments

General feedback:

ICRP effective dose specific comments