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Estimation of the dose at the maze entrance for x‐rays from radiotherapy linear accelerators
Author(s) -
AlAffana I. A. M.
Publication year - 2000
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.598887
Subject(s) - collimated light , monte carlo method , linear particle accelerator , photon , physics , percentage depth dose curve , beam (structure) , dosimetry , radiation , nuclear medicine , collimator , imaging phantom , optics , photon energy , ionization chamber , computational physics , ionization , medicine , mathematics , ion , laser , statistics , quantum mechanics
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, and13.1 × 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 be9.1 × 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 .