PHOTON-BEAM CONVOLUTION USING POLYENERGETIC ENERGY DEPOSITION KERNELS

In photon beam convolution calculations where polyenergetic energy dep osition kernels (EDKS) are used, the primary photon energy spectrum sh ould be correctly accounted for in Monte Carlo generation of EDKS. Thi s requires the probability of interaction, determined by the linear at tenuation coefficient, mu, to be taken into account when primary photo n interactions are forced to occur at the EDK origin. The use of prima ry and scattered EDKS generated with a fixed photon spectrum can give rise to an error in the dose calculation due to neglecting the effects of beam hardening with depth. The proportion of primary photon energy that is transferred to secondary electrons increases with depth of in teraction, due to the increase in the ratio mu(ab)/mu as the beam hard ens. Convolution depth-dose curves calculated using polyenergetic EDKS generated for the primary photon spectra which exist at depths of 0, 20 and 40 cm in water, show a fall-off which is too steep when compare d with EGS4 Monte Carlo results. A beam hardening correction factor ap plied to primary and scattered 0 cm EDKS, based on the ratio of kerma to terma at each depth, gives primary, scattered and total dose in goo d agreement with Monte Carlo results.