PHOTON DOSE CALCULATION INCORPORATING EXPLICIT ELECTRON-TRANSPORT

Significant advances have been made in recent years to improve photon dose calculation. However, accurate prediction of dose perturbation ef fects near the interfaces of different media, where charged particle e quilibrium is not established, remain unsolved. Furthermore, changes i n atomic number, which affect the multiple Coulomb scattering of the s econdary electrons, are not accounted for by current photon dose calcu lation algorithms. As local interface effects are mainly due to the pe rturbation of secondary electrons, a photon-electron cascade model is proposed which incorporates explicit electron transport in the calcula tion of the primary photon dose component in heterogeneous media. The primary photon beam is treated as the source of many electron pencil b eams. The latter are transported using the Fermi-Eyges theory. The sca ttered photon dose contribution is calculated with the dose spread arr ay [T. R. Mackle, J. W. Scrimger, and J. J. Battista, Med. Phys. 12, 1 88-196 (1985)] approach. Comparisons of the calculation with Monte Car lo simulation and TLD measurements show good agreement for positions n ear the polystyrene-aluminum interfaces.