Abstract
An analytical model has been developed for the calculation of rad R along the central axis of circular fields. The input data required by the model are photon fluence, phantom density, mass attenuation and mass energy absorption coefficients and differential Compton scattering cross-sections. The accuracy of the model was assessed by comparing (a) calculated and experimental central axis depth dose for 137Cs and 60Co gamma -ray beams and (b) f-factors calculated by the model with those published in Handbook 85 for 0.66, 1.25, 5.0 and 10 MeV. The principal source of error is attributed to an approximation in the calculation of dose from tertiary scatter. The model has been used to compare plastic and water dosimetry phantoms for the calibration of high energy X- and gamma -ray beams according to the recommendations of the AAPM. The results indicate that the use of lucite or polystyrene dosimetry phantoms will result in an overestimation in the determination of dose to water by less than 1% for most photon energies.
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