Comparison of conversion factors for x-ray beams from a Philips SL15 and the NPL linear accelerator

The National Physical Laboratory provides a megavoltage photon calibration service for secondary standard dosemeter systems in terms of absorbed dose to water using a graphite calorimeter. It is therefore necessary to evaluate factors to convert absorbed dose calibrations from graphite to water for all energies provided by the calibration service. A portable graphite calorimeter is currently being developed at the NPL for measuring absorbed dose in the radiotherapy clinic (McEwen and Duane 2000 Phys. Med. Biol. 45 3675-91) and so factors are now required to be able to convert absorbed dose calibrations from graphite to water in the clinical beam. The factors used to convert absorbed dose calibrations from graphite to water which are currently in use at the NPL were determined in previous work by Burns and Dale (1990 NPL Report RSA (EXT) 7) for all photon energies provided by the high-energy x-ray calibration service. Nutbrown et al (2000 NPL Report CIRM 37) have since re-evaluated these conversion factors using two methods. In this paper the factors to convert absorbed dose calibrations from graphite to water for two clinical beams from a Philips SL15 LINAC (6 and 10 MV) that were determined using both methods are presented and compared with values for the NPL heavily filtered beams. The results from the measurements made on the clinical machines using both methods agree within 1sigma uncertainty. The weighted average of these results agrees to within 1sigma uncertainty with results given by Burns and Dale and Nutbrown et al. The uncertainty in the determination of the graphite to water conversion factors at the 1sigma level is estimated to be 0.4%.