Photon beam convolution using polyenergetic energy deposition kernels.

In photon beam convolution calculations where polyenergetic energy deposition kernels (EDKS) are used, the primary photon energy spectrum should be correctly accounted for in Monte Carlo generation of EDKS. This requires the probability of interaction, determined by the linear attenuation coefficient, mu, to be taken into account when primary photon interactions are forced to occur at the EDK origin. The use of primary 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 interaction, due to the increase in the ratio mu ab/mu as the beam hardens. 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 compared with EGS4 Monte Carlo results. A beam hardening correction factor applied 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 good agreement with Monte Carlo results.