P213] Influence of beam spoiler and air gap on dose distribution in build-up region for X6 MV static field

Abstract

Purpose Basing on CBCT images we noticed, that there are several different problems with reproducibility of bolus positioning, especially air gap occurrence. The aim of this study was to determine the influence of air gaps between bolus and phantom on dose distribution in build-up region for X6 MV. The novelty of the work is using low-density boluses fabricated with 3D printer. Methods Depth dose measurements were performed for X6 MV 10×10 cm2 open field, generated with Varian CL 2300 C/D. The measurements were carried out with the Markus plane parallel ionization chamber (PTW, 0.055 cm3 volume), connected to Unidos (PTW) electrometer. As 1 cm thick beam spoilers we used 3D-printed thermoplastic polyurethane (TPU) cuboids filled with 5%, 10%, 15% and 20% honeycomb structure (thickness of 0.5 g/cm2; 0.17 g/cm2; 0.22 g/cm2). To generate air gap we used homemade styrofoam frames. Measurements were performed at a physical depth of 1, 5, 10 and 15 mm for all beam spoilers without air gaps and with 10, 20, 30 and 40 mm air gaps, at constant 90 cm SSD. Results Our results showed that beam spoilers without air gap increased the dose in build-up region. The bigger spoiler filling is, the larger is surface dose. The same dependence can be seen on Percentage Depth-Dose curves, irrespectively of air gap size. Regardless of spoiler percentage filling with honeycomb structure, the larger air gap size is, the smaller dose on 1 mm depth. Conclusions Influence of beam spoiler filling and air gap on dose distribution in build-up region for X6 MV was investigated. Beam spoiler increased the dose in build-up region. The larger thickness of beam spoiler (larger filling) - the larger influence on build-up dose. Occurrence of air gaps lowers the surface dose. However, the influence of air gap in the range of 10–40 mm is not very visible. For bolus with density of 0.22 g/cm2; regardless of air gap size, the dose on 1 mm depth was always larger than 95% of maximum dose. The issue will be investigated for wedged and dynamic fields.