Collimator scatter in photon therapy beams.

Abstract

A model for collimator photon scatter calculations is presented. The scatter from a collimating block is separated into two categories, one being the scatter released from photons entering the block through the surface facing the source, and one from photons striking the block tangentially through the side tangential to the beam. Both sources of scatter are analyzed by means of scatter kernels, defined as the scatter fluence distribution from a narrow line beam striking a collimator block. Kernels for both cases are calculated analytically using first scatter models based on Klein-Nishina cross sections including corrections for binding effects and coherent scattering. For 1-MeV primary photons, Monte Carlo calculations (EGS4) are used to show that the collimator scatter is dominated by first scatter. The kernels for the two types of scattering geometries are shown to be related and thus the total collimator scatter kernel can be derived from the kernel for the photons which have entered the block through the source facing side. Using a set of photon beam spectra, the kernels are parameterized as functions of beam energy in a form suitable for implementation in treatment planning systems. The parameterization is used to derive collimator scatter distributions for broad beam geometries of clinical interest. It is shown that scatter from the primary collimator, due to its location close to the beam source, is a major source of scatter that amounts to several percent of the primary fluence. The scatter from beam shaping collimators, however, generally accounts for less than 1% of the primary fluence. Although the beam-shaping collimators generate little scatter, the model used to calculate the resulting dose component is simple to implement and separates the different sources of scattered photons from the accelerator head. The latter provides generality and accuracy in dose per monitor unit calculations in treatment planning. Due to the low magnitude of collimator scatter it is recommended to use the phrase "head scatter" instead of "collimator scatter" when addressing the overall subject of extra-focal radiation from medical accelerators.