Photon beam softening coefficient determination with slab thickness in small filed size: Monte Carlo study

Abstract

Determination and understanding of photon beam softening using material soften photon beam for clinical usage is important for material study for attenuation and for beam modifier enhancements and linac improvements. Monte Carlo model was used to simulate 6 MeV photon beams produced by Varian Clinac 2100 accelerator with flattening filter thereafter the flattening filter was replaced by a slab of aluminum and copper with different of 0.5, 1, 1.5 and 2 mm. The Monte Carlo geometry was validated by a gamma index acceptance rate of 99% in PDD and 98% in dose profiles, the gamma criteria was 3% for dose difference and 3mm for distance to agreement. The purpose was to investigate the beam softening for small size and beam attenuation as a function of inserted slab thickness of copper and aluminum and also as a function of off-axis distance. For beam softening evaluation, variation amplitude of beam softening coefficient a1 was very high near the beam central axis and decreased with off-axis distance and also it was high for aluminum slab compared to copper slab. For aluminum slab, variation amplitude of beam softening coefficient a1 have a minimum at–0.5 cm–1 and a maximum at 0.5 cm–1 and for copper slab, variation amplitude of beam softening a1 have a minimum at–0.15 cm–1 and a maximum at 0.11 cm–1. Variation amplitude of beam softening coefficient a2 was very high near the beam central axis and decreased with off-axis distance and it was very high for aluminum slab compared to copper slab. For aluminum slab, variation amplitude of beam softening coefficient a2 have a minimum at–0.54 cm–2 and a maximum at 0.44 cm–2 and for copper slab, variation amplitude of beam softening coefficient a2 have a minimum at–0.111 cm–2 and a maximum at 0.0825 cm–2.