Photon dose calculation based on electron multiple-scattering theory: Primary dose deposition kernels

The transport of the secondary electrons resulting from high-energy photon interactions is essential to energy redistribution and deposition. In order to develop an accurate dose-calculation algorithm for high-energy photons, which can predict the dose distribution in inhomogeneous media and at the beam edges, we have investigated the feasibility of applying electron trans port theory [Jette, Med. Phys. 15, 123 (1988)] to photon dose calculation. In particular, the transport of and energy deposition by Compton electron a nd electrons and positrons resulting from pair production were studied. The primary photons are treated as the source of the secondary electrons and p ositrons, which are transported through the irradiated medium using Gaussia n multiple-scattering theory [Jette, Med. Phys. 15, 123 (1988)]. The initia l angular and kinetic energy distribution(s) of the secondary electrons (an d positrons) emanating from the photon interactions are incorporated into t he transport. Due to different mechanisms of creation and cross-section fun ctions, the transport of and the energy deposition by the electrons release d in these two processes are studied and modeled separately based on first principles. In this article, we focus on determining the close distribution for an individual interaction site. We define the Compton dose deposition kernel (CDK) or the pair-production dose deposition kernel (PDK) as the dos e distribution relative to the point of interaction, per unit interaction d ensity, for a monoenergetic photon beam in an infinite homogeneous medium o f unit density. The validity of this analytic modeling of dose deposition w as evaluated through EGS4 Monte Carlo simulation. Quantitative agreement be tween these two calculations of the dose distribution and the average energ y deposited per interaction was achieved. Our results demonstrate the appli cability of the electron dose-calculation method to photon dose calculation . (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)0 2608-5].