Calculation of three-dimensional photon primary absorbed dose using forward and backward spread dose-distribution functions

This paper describes a method of calculating three-dimensional (3-D) photon primary absorbed dose in a homogeneous or heterogeneous medium. The method is based on a technique of convolving a pair of forward and backward spread dose-distribution functions with the primary water collision kerma distribution. Both spread dose-distribution functions can be constructed by analyzing the zero-area tissue-maximum ratio, the primary absorbed dose near the beam exit surface, and the laterally spread dose distribution. Primary absorbed dose calculations are performed along the beam axis for 10-MV x rays. The characteristic patterns of primary dose in heterogeneous phantoms containing an aluminum or lunglike material slab can be obtained.