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

Purpose/Objective. Based on CBCT images we noticed that for postmastectomy patients there are several different problems with the reproducibility of bolus positioning (E. Dąbrowska, et al., Investigation of reproducibility of bolus position based on kV CBCT imaging, Radiotherapy & Oncology, Journal of the European Society for Radiotherapy and Oncology, Volume 123, Supplement 1, 2017), especially air gap occurrence. This study aimed to determine the influence of air gaps between bolus and phantom on dose distribution in the build-up region for X6 MV. The novelty of the work is using a different density of boluses fabricated with the 3D printer. Materials and Methods. Depth dose measurements were performed for X6 MV 10x10 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 a 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 gap and with 10, 20, 30, and 40 mm air gaps, at constant 90 cm SSD. Results. The results of measurements will be presented on graphs. The beam spoilers increased the dose in the build-up region. The bigger is spoiler filling the larger is surface dose. The same dependence was present irrespective of air gap size. Regardless of spoiler percentage filling with honeycomb structure, the larger air gap size the smaller dose was on 1 mm depth. Conclusions. The influence of beam spoiler filling and air gap on dose distribution in the build-up region for X6 MV was investigated. Beam spoiler increased the dose in the build-up region. The larger thickness of the beam spoiler (larger filling) the larger influence on the build-up dose. The occurrence of air gaps lowers the surface dose. However, the influence of the air gap in the range of 10 - 40 mm is small. For bolus with the density of 0.22 g/cm² (filling 15% and 20%) regardless of air gap size the dose on 1 mm depth was always larger than 95% of the maximum dose. (author)