Monte Carlo calculation of effective attenuation coefficients for various compensator materials.

Effective attenuation coefficients for 6, 8, and 15 MV photon beams were derived and studied for various compensator materials for square beams with side lengths of 0.5, 1.0, 2.0, 3.0, and 5.0 cm. Calculations were based on depth dose data in water obtained from EGS4 based DOSXYZ Monte Carlo simulations. Depth dose data were calculated using different compensator materials as attenuators of variable thickness. The absorbed dose varied exponentially as a function of absorber thickness at any depth in water on the beam axis for all materials. The effective attenuation coefficient data were compared with measurements for wax, aluminum and brass with values from the literature. Theoretical narrow beam linear attenuation coefficients were calculated and compared with the Monte Carlo data. The effective attenuation coefficient data for all materials were parametrized as functions of field size and depth in water. The effective attenuation coefficient was also parametrized as a function of atomic number. It was found that the effective attenuation coefficients calculated from the DOSXYZ data using a simple source model correspond to measured data for wax, aluminum and brass and published data for lead.