Physical characteristics for high energy X-ray narrow beams

Recently, high-energy photon narrow beams, less than several cm in diameter, were going to use for the purpose of the stereotactic radiosurgery with a linear accelerator. In this paper, we analyzed the physical characteristics of high-energy X-ray narrow beams by means of a Monte Carlo simulation, and discussed their peculiar phenomena. When the beam size of high-energy X-ray became smaller, in addition to a decrement in the scattered doses, the decrease in the primary doses began from a certain beam size due to the failure of the electron equilibrium in the side direction. This caused sharp fall-off on the percentage depth doses, off-center ratios and output factors, with shallowing the maximum dose depth. In dose calculations for high-energy X-ray narrow beams, it is necessary to estimate the primary doses, which take into account the electron transport. (author)