Effect of scattering and differential attenuation on beam profile in the presence of high-density intensity modifying compensator.

Abstract

The aim of this study is to investigate the effect of scattering and differential attenuation on dose profile of 6 MV photon beam in the presence of cadmium (Cd)-free compensator which has been used in compensator-based intensity-modulated radiotherapy. Totally, 10 slabs of Cd-free compensator having thicknesses ranging from 2.4 to 61.4 mm have been prepared. Dose profiles have been taken using computer-controlled radiation field analyzer for five field sizes from 30 mm × 30 mm to 200 mm × 200 mm and at three depths in water phantom. Off-axis dose variation (ODV) has been measured with off-axis percentage depth dose scan and with ion chamber by measuring point dose at two diagonal points with respect to dose at central axis point in a plane and at three depths. A decrease in beam flatness has been observed with increase in compensator thickness and depth in phantom. ODV has been found to increase with compensator thickness. Selective beam hardening has been observed due to differential attenuation from compensator. Point dose measurements show approximately 20% and 23% underdose region at 70 and 106 mm off-axis diagonal point, respectively, as compared to dose at central axis point for a field size of 200 mm × 200 mm at a depth of 15 mm, with 30.2-mm slab thickness. Significant increase in scattered penumbra has been observed with field size and thickness of compensator due to increase in scattered photon. The presence of compensator changes photon beam mean energy along the cross-section resulted in decreased beam flatness and increased scattering. This may lead to overestimation of dose along off-axis within radiation field if change in flatness is not taken into account and more exposure to healthy tissues in penumbral region due to large-angle scattering.