( a pdf-document including formulas and images will be sent to the secretary for better understanding )
to the ICRP draft - ref 4831-9482-1403 - 17 June 2014
As mentioned in the section “Scope of the document” line 774 it “was decided to restrict the current document to non-dental applications of CBCT, with a brief coverage of dental CBCT” seems not to be a clear restriction. Our comments are in parts to the “brief coverage of dental CBCT” and in other parts to general topics that are useful for dental and non-dental application.
From our point of view, the following additions should be made to the draft for the sake of completeness:
The German standard DIN 6868-161 defines a dosimetry method that is independent of the CBCT acquisition geometry. For this purpose, the method takes advantage of the fact that in all CBCT systems it is possible to measure the dose at the image receiver. The measured values are then consistently normalized with the aid of geometrical measures of the imaging system (see brief discussion in Annex A). The document states that there is no need to introduce a more sophisticated procedure for CBCT devices.
The DIN standard method has the potential to enable CBCT dosimetry in all facilities without the introduction of new materials and methods. When the scanned volume is irradiated homogeneously, the dose determined by this DIN method is equal to the CTDIfree-in-air. “Although CTDI has its limitations […]”
Add reference to DIN 6868-161.
“Image quality assurance in diagnostic X-ray departments: Part 161: RöV acceptance testing of dental radiographic equipment for digital cone-beam computed tomography“
The unofficial translation of the DIN standard to English is available and the official translation is under consideration by the DIN working group.
You have chosen three documents from the dental field and three documents from the radiotherapy fields, which we find appropriate. However, you may consider adding the acronym “DIN” to the following cells in the table:
4.1. All monthly tests related to image quality and image display
4.2. Annual test of isocentre coincidence and dose measurements
4.3. Add a new field called “combined figure of merit”. “DIN” should be then written in the cell for annual test.
4.4. Below the table, please add DIN: DIN standard 6868-161, 2012.
Except for the German DIN standard for dental CBCT devices (described in Annex A.3), the system for dose reporting is not yet standardized. […]
A.3. The German standard dosimetry method for dental CBCT devices.
The dose at the isocenter D is determined from the average of three independent measurements of the dose Dd measured at the detector and geometrical data (see Fig. X).
The dose at the isocenter is calculated following equation 2:
D=Dd * b/a * d/c where
a is the distance from the focal spot to the rotation centre;
b is the distance from the focal spot to the detector (place of dose measurement);
c is the horizontal diameter of the scanned volume;
d is the horizontal diameter of the radiation field at the detector (place of dose measurement).
The following figures may be included as an explanation:
[figure symmetric scan geometry]
Fig. 1 — Horizontal slice through a scan geometry (example 1)
[figure non-symmetric scan geometry]
Fig. 2 — Horizontal slice through a scan geometry (example 2)
Patient dose can be estimated using the DIN standard method using the following calculation:
Dp = D * Lz , where D is the dose at the isocentre and Lz is the length of the cone along the axis of rotation.