Estimation of the dose at the maze entrance for x-rays from radiotherapy linear accelerators.

Recent measurements have shown that the NCRP formula to estimate the x radiation dose at the maze entrance of high energy radiotherapy rooms underestimates the dose by an order of magnitude. In the present work the Monte Carlo Code MCNP was used to model a radiotherapy room and investigate the NCRP formula. The dose of the scattered photons was calculated for 6-MV and 10-MV x-rays for the following situations: primary beam in vacuum, primary beam with air in the room, collimated primary beam (by the jaws) with air in the room and primary beam collimated with air in the room, and phantom at 100-cm SSD. It was found that for 6-MV x-rays the dose, when these materials were present in the beam path, was 1.2, 1.6, 5.3, and 13.1 x 10(-22) Gy photon(-1), respectively. Therefore the presence of all these materials together increased the dose by a factor of 11. The dose due to leakage was calculated separately to be 9.1 x 10(-22) Gy photon(-1). This adds another factor of 8. The 10-MV results were similar to those at 6 MV. There was good agreement between MCNP calculations and the published measurements. The spectrum and average energy of scattered photons at different locations in the radiotherapy room and the maze were also calculated by MCNP.