Poster — Thur Eve — 16: Dependences of Mucosal Dose on Small Photon Beams: A Monte Carlo Study

This study investigates the dependences of the mucosal dose in the oral or nasal cavity on the small photon fields with different beam energies, beam angles and mucosa thicknesses. Phase space files of 6 and 18 MV photon beams (field size = 1× 1 cm₂), produced by a Varian 21EX linac, were generated using the EGSnrc‐based BEAMnrc code. Mucosa phantoms (mucosa thickness = 1, 2 and 3 mm) with and without a bone under the mucosa were irradiated by photon beams with gantry angles varying from 0 to 30 deg. Doses along the central beam axis in the mucosa were calculated. For the 6 MV photon beams, it was found that the dose at the mucosa‐bone interface increased by 44.9% − 41.7%, when the mucosa thickness increased from 1 to 3 mm for the beam angle ranging from 0 to 30 deg. These values were lower than those (58.8% − 53.6%) for the 18 MV photon beams. For both the 6 and 18 MV photon beams, depth doses in the mucosa were found to increase with an increase of the beam angle. For the dose ratio (mucosal dose with bone to dose at the same point without bone), the dose enhancement due to the bone backscatter increased with a decrease of mucosa thickness, and was more significant at both the air‐mucosa and mucosa‐bone interface. The dosimetric information in this study is useful when searching for an optimized treatment strategy to minimize the mucosal complications in the head‐and‐neck IMRT.