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Submitted by Hugo de las Heras Gala, EFOMP Working group on unification of CT and CBCT
   Commenting on behalf of the organisation
Document Radiological Protection in Cone Beam Computed Tomography (CBCT)
 

Group comment:

Our EFOMP working group is working on a unifying procedure for quality control in CT and CBCT. A few colleagues inside the working group are working on the dosimetry part. We welcome your timely document and we are very thankful that you opened the draft for comments.


We agree that large radiation cones make it necessary to change the paradigm used for dosimetry and we find your proposed methods appropriate, in particular the CTDI measurements (with a single standard phantom or 200 mm long ICRU phantoms) at acceptance tests and in-air CTDI for constancy tests.


However, you have noted in page 37 that “dosimetry for CBCT is not yet standardized”. In the field of dental CBCT, the standard DIN 6868-161 contains a dosimetry method for dental CBCT devices and it was published in January 2013. We think that it would be a mistake of your document to ignore this standard.


In you find it appropriate, we think that you could include the following paragraph in the chapter about dosimetry:


-          The German standard DIN 6868-161 describes a dosimetry method for dental CBCT systems. Although these guidelines are published for dental applications, the method is valid for all devices having an accessible detector surface (including dental devices, angiography systems and C-arms). The procedure is very simple: a dosimeter is fixed on the detector surface and then an empty scan is performed. The reading of accumulated dose (at the entrance of the detector) is corrected by geometrical factors to obtain a final result of dose at the isocentre. This value is valid for any geometry (full cone or a section of a cone) and it corresponds to CTDIair for standard cone-beam geometries.


We are aware that the DIN group has already uploaded a comment describing this method, so please refer to their comments if you have any question.


One advantage of this method is that it can be performed using tools (dosimeters) that are already present in most hospitals, so that any facility in the world can perform their tests on a routine basis.


We would be happy to help if you have any questions.


 


Best regards,


Hugo de las Heras, Osvaldo Rampado, Christie Theodorakou, Jonas Andersson, Vesna Gershan, Federica Zanca

(and other members of the EFOMP working group for unification of quality controls for CT and CBCT)


Please find below separate comments from single members of our group.



Additional comments by Alberto Torresin:


Page 9  line 262          


Radiotherapeutic applications should be introduced from the beginning of the text


Page 9  line 299          


RDSR may be compulsory after the first year of publication of the document


Page 15           line 528          


and in radiotherapy


Page 37           line 1391        


Radiation Dose Structured report can be used to report the modality output following the existing RDSR in CT and angiography


KAP values for the different orientation of the beam can be reported inside the Radiation Dose Structured Report when step and shoot acquisition techniques are used


Page 42           line 1636        


This consideration is too much related to dental acquisition. This consideration seems appropriate for CBCT  "general comments". Please change it.


Page 62           line 2519        


Where is the absorbed dose? Should effective dose be used?


Page 86           line 3345        


The daily control should be moved into weekly


 


Additional comments by Jonas S. Andersson:


In regard to the present discussion on the ICRP draft and the recommendations contained therein on employing AAPM Reports No. 111 and 204 (Equilibrium dose and SSDE, respectively) I think that it is very important to differentiate between QA and dosimetry.


 The AAPM formalism for dosimetry is built upon the CTDI metric for QA, where corrections are added for wide beams and scatter (Report 111) and patient size (Report 204). As pointed out by a member of our WG, it is not reasonable that everyone should be able to use the dosimetry approach in the dose equilibrium concept. On the other hand, these correction factors can be supplied for a given technology and examination type, e.g. as done in AAPM Report 204 for body scans and MDCT. This is, in my opinion, the most robust approach for refining dosimetry in diagnostic radiology, where the strengths of a basic QA metric can be used (in the case of the CTDI: isocenter metric, in phantom, including table and holder attenuation) while weaknesses are corrected for.


 If we use another metric for QA, e.g. KAP, we will lose the possibility of employing this established formalism for connecting QA and dosimetry.


 


Additional comments by Federica Zanca:


What I maybe would have liked to see also discussed in the document is the following:


- How to monitor CBCT doses (dental/h&n..) in terms of patient doses and how to compare these to CT doses?


- Should there be an annual obligation for dose monitoring, similar to conventional CT? How to track CBCT doses with (emerging) dose monitoring software tools? A simple implementation of the requirements for conventional CT is not possible, as some CBCT systems do not report nor CTDI nor DLP. the problem is the absence of a common dose parameter between CBCT and CT. 


-      The use of  ‘effective dose’ requires conversions factors from dose indications reported in CBCT to effective dose. There is no such common factor published / internationally accepted. This leads to confusing situations in which different parties use different approaches in publications.


 


Additional comments by Anne Thilander-Klang:


I am myself working part time as medical physicist at an oral and maxillofacial radiology department. At this department there are several dental CBCT and one MDCT. The staff is very interested in patient dosimetry and it is challenging to try to estimate the radiation dose to the patient from the CBCT units especially as the CTDI concept not is valid as the irradiated volume can be as small as 3 cm x 4 cm in the rotation axis. To me it is obvious that PKA (KAP) is a better measure of the radiation dose to the patient (this is supported in the text in (68)). If we are going for effective dose we need specific conversion factors for specific X-ray units as they have different irradiation field sizes (3x4 to 17x17, to my knowledge), different rotation angel (180 to 360 degrees), and different motion patterns resulting in different areas in the patient being irradiated differently (i.e. a small 4x4 volume on the right side of the mouth will result in the highest dose at the rotation axis and a much lower dose levels at the left side. See reference: Lofthag-Hansen S, Thilander-Klang A, Ekestubbe A, Helmrot E, Gröndahl K. Calculating effective dose on a cone beam computed tomography device: 3D Accuitomo and 3D Accuitomo FPD. Dentomaxillofac Radiol 2008; 37: 72-79). How to handle the organ dose to the salivary glands – mean value? The salivary glands are a spread organ as it includes the parotic gland, the sublingual gland and the submandibular gland on both sides.


The reporting of radiation dose (RDSR) for each and every radiation exposure performed is essential and has to be in PKA (KAP) for dental CBCT units uses different field sizes and therefore irradiates different amount (both by volume and by absorbed dose) of the organs in the examined volume. A common conversion factor for the calculation of effective dose is not suitable. It has to be very specific. If using the effective dose one then must state the conversion factor used. Using the PKA (KAP) values will give a better possibility to compare ones results with other users.


I support the idea of common QA and QC test phantoms. I try to use the SENDENTEX CBCT phantom but it has many levels of freedom to place the inserts – a standardised way?


 


Additional comments by Christie Theodorakou
(you probably got these comments already in the review submitted by the IPEM)


Comment 1.


The draft ICRP reports summarises in a nice and simple way the current status of radiation protection in CBCT. Aspects such as dosimety, training, optimisation, patient and occupational protection are well described and summarised.


 


Comment 2.


However, the dental CBCT section could be further improved. 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:


·         Deman P, Atwal P, Duzenli C, Thakur Y, Ford NL. Dose measurements for dental cone-beam CT: a comparison with MSCT and panoramic imaging. Phys Med Biol. 2014 Jun 21;59(12):3201-22


·         Shin HS, Nam KC, Park H, Choi HU, Kim HY, Park CS. Effective doses from panoramic radiography and CBCT (cone beam CT) using dose area product (DAP) in dentistry.Dentomaxillofac Radiol. 2014;43(5):20130439


·         Dierckx D, Saldarriaga Vargas C, Rogge F, Lichtherte S, Struelens L. DOSIMETRIC ANALYSIS OF THE USE OF CBCT IN DIAGNOSTIC RADIOLOGY: SINUS AND MIDDLE EAR. Radiat Prot Dosimetry. 2014


·         Kim DS, Rashsuren O, Kim EK. Conversion coefficients for the estimation of effective dose in cone-beam CT. Imaging Sci Dent. 2014 Mar;44(1):21-9.


 


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.


 


Comment 3


I agree that the DIN report should be referenced.


 


[I do agree that CTDI measurements are important and easy to do, in general. However, the dental CBCT scanners have different patient set ups to angiography and radiotherapy scanners. They are mostly seated or standing. This means that placing the ion chamber at the isocentre requires a special jig to suspend the chamber in space. Also, there are scanners that offset the isocentre to cover a larger field of view than the detector size. Therefore, placing the dosemeter on the detector surface overcomes the above two issues. I do agree that phantom measurements are essential.]


 


Comment 4


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.


 


Comment 5


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.


 


Comment 6


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.


 


Comment 7


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.


 


Comment 8


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.


 


Comment 9


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’.


 


Comment 10


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.


 


Comment 11


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.


 


Comment 12


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’


 


Comment 13


Finally, I think that 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.