Study of 6 MV Photon Beam Dose Profile Peaks, Investigation and Evaluation of the Backscattering Effects on the Beam Dose Profiles

Abstract

The dose profile peaks are mainly produced by the flattening filter of the head of the medical linear accelerator due to the backscattering effects on the beam dose profiles; in this study we have investigated the backscattering effects on the beam profile peaks; the purpose was to evaluate the backscattering effects at the beam edge due to the flattening filter shape by calculating the increasing relative dose on the beam profile peaks as functions of depth along the central axis and filed size. The accent is on the photon beam dose profiles at different depths in a water phantom along central beam axis for a set of irradiation field sizes. The measures of dose profiles of the 6 MV photon beam in a water phantom, source to surface distance is 100 cm, for the set of the irradiation field sizes of 3×3 cm², 6×6 cm², 10×10 cm², 15×15 cm², and 20×20 cm² at different depths on the beam central axis, the linear accelerator is VARIAN Clinac 2100. First, we have plotted the dose profiles as function of the normalized depth to show the beam profile peaks; then we have calculated the increasing relative dose on the beam profile peaks as a function of the field size and depth along central beam axis from the measurements data. Then we have normalized the increasing relative dose on the beam profile peaks to the increasing relative dose on the beam profile peak of the standard filed size of 10×10 cm². We have plotted the results for interpretation and evaluation the backscattering effects on the beam dose profiles; the pervious study have shown that the flattening filter produced a flat profile at a depth of 10 cm and beam profile peaks; and in our study we have quantified the increasing relative dose on the beam profile peaks; then, we have shown that the variation of the normalized increasing relative dose presented the peaks at depth of 10 cm along the central beam axis, e.g. We have found that the increasing relative dose at depth of 10 cm along central beam axis for the field size of 25×25 cm² is almost 20 times of the standard field size at the same depth, but the increasing relative dose is very close for both filed sizes at a depth of 20 cm along central beam axis. Throughout this work we have use a method to evaluate the backscattering effects on the dose profiles; we have found the backscattering effects are very high at a depth 10 cm for the large irradiation field size.