# Factors Affecting Implementation of Radiological Protection Aspects of Imaging in Radiotherapy
## 4 Dosimetry Methods for Cone Beam CT Systems
The International Electrotechnical Commission (IEC) has proposed making measurements with a narrower reference beam and adjusting these based on a ratio of CTDI\({}_{100}\) measurements in air [26; 27; 28] to provide a measurement that is relatively independent of beam width [29]. An alternative approach taken by the American Association of Physicists in Medicine (AAPM) task group is based on measuring the cumulative dose with a small ion chamber in the centre of a phantom that is sufficiently long to include most of the contribution from scattered radiation [30; 31]. However, neither of these techniques is straightforward to perform. The IEC method requires a series of CTDI measurements at different positions to replicate a longer ionization chamber, while the AAPM method requires the availability of a larger phantom. Since imaging patient dose surveys are new to radiotherapy treatment imaging exposures, any survey that plans to obtain results from a large number of centres will require an approach that is easy to follow. A dose survey needs comparisons of performance across as many systems as possible in order to be successful. Use of a measurement of cumulative dose at the centre of a standard 150 mm long CT phantom with either a 0.6 cc Farmer ionization chamber or a 100 mm CTDI chamber with a wide beam are considered to be the easiest practical solutions for deriving a typical cumulative dose for a CBCT scan in mGy mAs\({}^{-1}\) (Figure 5). The reason for using different chambers at this stage is that access to 100 mm chambers is limited in many radiotherapy centres, and use of a 0.6 cc chamber would be a viable alternative. The final choice of which chamber to use will depend on the availability in different countries, but at this stage the use of either is proposed. Conversion factors between the two would then be established from Monte Carlo calculations and experimental measurements. Monte Carlo calculations employing 10\({}^{9}\)-10\({}^{10}\) photon histories can reduce uncertainty to the order of 2% or lower [32]. It is proposed that measurements are made for the maximum field size used in clinical practice for the kV/filter combinations used according to a set protocol provided in a spreadsheet. Measurements would be combined with the accepted 2:1 weighting for CTDI measurements [28], to derive a weighted value of the cumulative dose per mAs for any given kV/filter combination. Because the beam will be larger than the phantom, the efficiency in terms of making a measure of dose to tissues in the body will only be 40-50% [32]. But the fact that this is not an absolute measure of dose to a patient does not matter at this stage, as these measurements are designed primarily for comparing the performance of CBCT protocols. They capture a simple measure related to the level of dose to the patient, and the effects of kV and filtration. Placing more constraints on measurement of the dose quantity to make a measure more meaningful in terms of dose to the patient would increase the difficulty of the measurement and inevitably reduce the number of centres able to take part. Mentees have been asked first to take measurements at their own centres to establish the technique and then to ask colleagues to carry out similar measurements at their centres. The next stage will be to collect exposure factor data on standard treatment protocols for several specified treatments at each centre. The measurements can then be combined with exposure data in clinical protocols to obtain some information on patient dose levels. The measurements proposed should give dose values suitable for benchmarking and comparing the performance of equivalent CBCT protocols for the same clinical indication. The project is still at an early stage, but measurements have been made at a few centres and reasons for differences in results are being investigated.