Head scatter factor calculation considering characteristics of scattered photon

The head scatter factor S_h is one of the important parameters for MU calculation. It is defined as a ratio of absorbed dose of arbitrary field to that of reference field. The S_h can be determined by using two physical quantities. One is energy fluence of primary photons generated from target and scattered photons from several components of treatment head. Another is backscattered radiation from collimator jaws into monitor chamber. In this study, to determine S_h for intricate field shaped with MLC, primary and scattered photons from several head components were simulated using Monte Carlo method and output variation caused by backscattered radiation into monitor chamber was measured using a miniphantom. In the simulation, geometry and materials of head component were precisely coded on the BEAMnrc, and the particles from the head components and incident into isocenter region were sampled into the phase space files. The emit positions of scattered photons at the base of flattening filter were analyzed by using information of the phase space file and the energy fluence distribution of scattered photons as a function of radius at base of flattening filter was obtained. The S_h was determined by integration of the energy fluence distribution within range defined by the field shape and contribution of backscatter into monitor chamber. The calculated S_h agreed with the measured one within 0.3 %.