Dose rate and energy distributions of X rays from a linear accelerator at the maze entrance of a radiotherapy room by measurement and Monte Carlo simulation

The dose rates in a radiotherapy room and maze were measured and computed, using the Monte Carlo code for neutron and photon transport, MCNP, for 6 MV X rays at different field sizes and gantry angles. The contribution from leakage was also measured and computed. When a phantom (patient) was positioned at the isocentre the measured and computed doses at the maze entrance were increased by a factor of 2-3 times over that without a phantom present. The size of the phantom on which the primary beam was incident was not a sensitive factor in increasing the dose rate at the maze entrance. There was good agreement between the measurements and MCNP computations. The energy spectra of the photons (both the scattered and leakage components) near the maze entrance were measured and computed for a primary beam with a field size 20 mm x 20 mm for scatter and leakage.