Dosimetric Effect of Beam Angle on the Unflattened and Flattened Photon Beams: A Monte Carlo study

Abstract

This study compared the variations of depth and surface dose on beam angle between the unflattened and flattened photon beams. Monte Carlo simulation based on the EGSnrc code was used in dose calculation. Phase-space files of the 6 MV photon beams (field size = 10 × 10 cm2) were generated with and without the flattening filter based on a Varian TrueBeam Linac. Depth and surface doses were calculated in a water phantom with angles of the photon beams turning from 0° to 15°, 30°, 45°, 60°, 75° and 90°. Our results showed that compared to the flattened photon beam, the unflattened beam had a higher dose in the build-up region but lower dose beyond the depth of maximum dose. With the Monte Carlo beams cross-calibrated to the machine monitor unit in simulations, dose ratios of the unflattened to flattened beams were in the range of 1.6 - 2.6 with beam angles varying from 0° to 90° in water. In addition, higher surface doses of about 2.5 times were found with beam angles equal to 0° and 15°. However, surface dose deviation between the unflattened and flattened beam became smaller with an increase of beam angle. Using the photon fluence incorporated to the monitor unit, variations of depth and surface dose on the beam angle were investigated and compared. For the unflattened and flattened photon beams, the surface dose and range of depth dose ratios (unflattened to flattened beam) decreased with an increase of beam angle.