# Factors Affecting Implementation of Radiological Protection Aspects of Imaging in Radiotherapy
## 4 Dosimetry Methods for Cone Beam CT Systems  
Optimization of radiological protection requires a knowledge of patient doses from imaging, but the survey has shown that only 50% of radiotherapy centres in European countries recorded patient doses and less than 10% in other parts of the world record patient doses (Figure 4). There is a need for raising awareness of imaging doses, and adaptation and development of the dose quantities displayed on imaging equipment used in radiotherapy to allow calibration and assessment to be performed more readily. The imaging technique used most widely during radiotherapy is kV CBCT [14, 16], so this study is focused on that technique. The challenge is then to find measures of dose that can be made by centres in a variety of countries, many with limited resources. Few centres have a methodology available that could be used to measure CBCT dose currently. Bearing this in mind, another mentee project has been initiated to investigate ways for measuring CBCT doses that could be applied more widely. The aim of the project is to trial ways in which dosimetry measurements could be carried out on CBCT systems on radiotherapy linacs. The dosimetry equipment and phantoms available in different countries vary widely and methods are required that could be used in centres with varying facilities, and ways developed in which comparisons of results could be made between centres. The dose metric displayed on most CBCT systems is a version of the CT dose index (CTDI), which gives a measurement of dose with a 100 mm long ionization chamber. Measurements made at the centre and periphery of head or body phantoms made of polymethyl methacrylate (PMMA) and having diameters of 160 or 320 mm, respectively, and lengths of 150 mm, are combined to give a weighted result (CTDIw) that is suitable \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**Country**} & **Use Vendor** & **Adapt Protocols** & **Adjust Image** & **Adjust Image** & **Involvement of** \\  & **Prococks (\%)** & **Exposures for** & **Field Size for** & **Field Size for** & **Diagnostic** \\  & **Individuals (\%)** & **Adults (\%)** & **Paediatric (\%)** & **Physicists (\%)** \\ \hline Germany & 50 & 10 & 60 & 33 & 22 \\ Australasia & 55 & 36 & 55 & 55 & 45 \\ USA & 97 & 43 & 43 & 60 & 30 \\ Cyprus & 100 & 0 & 0 & 0 & 100 \\ Saudi Arabia & 75 & 50 & 50 & 67 & 25 \\ Malaysia & 100 & 14 & 14 & 66 & 0 \\ Colombia & 92 & 31 & 38 & 53 & 14 \\ Algeria & 100 & 12 & 50 & 100 & 11 \\ Egypt & 100 & 0 & 0 & 0 & 50 \\ \hline \end{tabular}
\end{table} 
Table 3: Percentages of radiotherapy centres in each country adapting protocols for individual patients and involvement of diagnostic medical physicists in optimization [14]. for dose surveys and optimization of practices. CTDI\({}_{\text{w}}\) values are not accurate reflections of doses to individual patients of varying size, but nevertheless provide a starting point through which comparisons can be made with doses from CT scanning more generally. The CTDI values displayed on CBCT scanners will depend on the model, and may be the standard CTDI for narrow beam CT or a measure for a wide beam, as this is an area that is evolving, so care is required in choice of any method for calibration. All vendors are encouraged to include displays of assessed imaging dose quantities that can be used for patient dose surveys. The calibration of the displayed quantity should be verified to allow future comparison against national or international standards. Various methods have been developed for dose measurement and calibration of CBCT systems. The CTDI\({}_{\text{w}}\) concept has shortcomings in that it does not capture all the scattered radiation that would contribute to patient dose [23; 24; 25] and it is designed for CT scanners in which the fan beam is narrower, so that X-rays are incident at close to a perpendicular angle to the scanner axis. Various modifications have therefore been made to adapt the concept for CBCT.