Comments to ICRP draft publication Radiological Protection in Cone Beam Tomography (CBCT)
Sent on behalf of DRSIG
The DRSIG on behalf of IPEM would like to thank the ICRP for the draft document and that it has been opened for comment. A number of our members would like to raise some queries and recommendations on this document. Please note that some of these comments may have also been submitted as part of comments from the EFOMP working party due to members being in both groups
The DRSIG is pleased that there are recommendations on training and optimisation as we are in agreement that it is being used by operators with little previous RP awareness. We feel that there should be a strong focus on ensuring operators and indeed any staff involved with purchasing, installing and use of CBCT equipment are fully aware of the issues such as installation location, protection of the surrounding environment and or personnel from scatter and how the patient doses compare to other similar techniques. The manufacturers should be discouraged from promoting equipment by demonstrating that image quality is ‘best’ when image quality should be adequate for the task required.
Specifically;
Recommendation 1 of section 5 requests manufacturers to adopt a standard for dose measurement units but does not recommend a specific one, although later in section 9 the UK recommendation of KAP for dental is acknowledged. We wonder if this recommendation would be more effective if ICRP was more specific here.
This recommendation also discusses issues with a 100mm chamber and makes the recommendation that dosimetry requires a 600mm phantom. We feel this document should be aware of the limitations of medical physics groups and the practicality of testing methodology along with downtime for acceptance and testing.
Recommendation 1 of section 6 states ‘Recording, reporting and tracking of radiation dose for a single patient should be made possible’. It is noted that this appears to be expanded upon in section 5.5. We feel this could be clarified further. How is the dose to be tracked and via what method? Would this be information within the DICOM header for example. If so, could the recommendation be more specific to encourage manufacturers to be consistent with their approaches
It is noted there is a section on Bismuth shielding. We feel this should be a more explicit recommendation along the lines of the AAPM recommendation on shielding for CT
Line 588 – is this a typo, should the line say “CBCT have superior contrast resolution...”?
Paragraph starting line 963 – It should be noted that linac gantry CBCT rotates 360 deg but then has to go back again. Definitions may require revising
For Dental CBCT, The authors are referring to the EC No 172 report but this report was prepared and submitted in 2011 so, it is already 3 years old. Just to name a few papers that have not been included in this draft ICRP report:
An up-to-date dosimetry table with references to the journal papers and not just the EC No 172 should be included. Up-to-date tables listing doses have been nicely done for the rest of the modalities e.g. vascular, non vascular etc.
Moreover, Table 7.9 should reference the journal paper and not the EC No 172 document. The reference is:
Theodorakou C, Walker A, Horner K, Pauwels R, Bogaerts R, Jacobs R; SEDENTEXCT Project Consortium. Estimation of paediatric organ and effective doses from dental cone beam CT using anthropomorphic phantoms. Br J Radiol. 2012 Feb;85(1010):153-60.
In addition, the following paper should be included in Table 7.9 as this study has looked into paediatric doses as well.
Ludlow JB, Walker C. Assessment of phantom dosimetry and image quality of i-CAT FLX cone-beam computed tomography. Am J Orthod Dentofacial Orthop. 2013 Dec;144(6):802-17.
It is important that the current literature in dental cone beam CT is reviewed and summarised.
Throughout the document the dental CBCT are referred as ‘dental CBCT’, ‘oral and maxillofacial’, ‘dental and maxillofacial’. For consistency purposes, the dental applications can be referred as ‘dental and maxillofacial’ throughout the document.
Paragraph 67; there are a number of clinic-based systems that offer a range of FOVs from a very small one (one or two teeth) to very large ones (maxillofacial). So, I do not agree with the statement that the main difference in dental and the rest clinic-based systems is the FOV. Probably the authors mean that the difference between dental CBCT and head and neck and bCT imaging is the FOV.
Line 363; Regarding patient-specific exposure settings in dental CBCT; the authors should explain what they mean by patient-specific exposure settings. Do the authors mean exposure settings tailored to individuals or to different size patients; small/medium/large child and small/medium/large adult? Deriving patient-specific exposure settings for individual patients is not practical. However, standard exposure settings based on size category would be more practical and feasible.
The content of section 5.3 is also applicable to dental CBCT scanners. Dental CBCT scanners exhibit asymmetrical axial dose distributions due to 180o or 220o plus the fan beam rotation angle.
Line 1781, page 46; the parotid gland is part of the salivary glands. The authors have not discussed the dose reduction to the thyroid gland having the X-ray tube rotating at the posterior side. What do the authors mean by patient dose; effective dose or organ dose? The authors state that the ‘patient dose’ may be lower when the tube is at the anterior side. However, in page 67 under the neurointerventions section, it is stated that the x-ray tube should be positioned on the opposite side of the body from radiosensitive organs such as the thyroid and the eyes.
Line 2804; ‘Most systems are seat-scanners’; there is a large number of dental CBCT systems that utilise a standing patient set-up. The authors should say: ‘Most systems are seat or standing scanners’.
Line 2819; the EC No 172 has also covered important aspects such the ‘referral criteria’ and ‘staff protection’. These two topics should be added to line 2819.
Paragraph 184, page 74; special attention should be paid to paediatric doses not just because of the higher organ radiosensitivity but also because of their smaller size. The size, shape and position of the radiosensitive organs are different in children than in adults.
Line 3317; it should read ‘X-ray’ instead of ‘x-ray’.
Line 3469; it should read ‘The users’ instead of ‘Theusers’
Line 2333; it should read ‘of occupational’ instead of ‘ofoccupational’
This document could have discussed more the issues related to image quality assessment; for example, lack of standardised phantoms and tests that can be applied to all scanners of a specific application e.g dental and maxillofacial imaging. Another important aspect of QA is the Level A tests performed by the radiographers/dentists/users in between the more complicated physicists’ tests. Currently, in dental CBCT only very few manufacturers offer test objects that the users can use routinely to check the performance of the system. This is not the case in conventional CT as the users are provided with phantoms for daily and weekly checks.