Effect of thermoplastic mask hole diameter on dose distribution in the build-up region of medical LINAC

Abstract

This study aims to investigate the effect of variations in the hole diameter of unstretched thermoplastic masks offered by a single company on the dose distribution in the build-up region. 2, 2.5, 3 and 4 mm hole diameters masks with the same properties were irradiated with 6 MV X-rays and dose distribution was analyzed according to the open field. Dose measurements were obtained with a parallel plane ionization chamber. All irradiations were repeated for 5 × 5, 8 × 8, 10 × 10 and 15 × 15 cm2 beam fields. It was observed that the mask had a bolus effect in all measurements. It was observed that this bolus effect varied especially depending on the hole diameter in the mask. In a 10 × 10 cm2 field, the dmax depth without mask was 15 mm, while the dmax for masks with 2, 2.5, 3 and 4 mm hole diameters was 11, 12, 13 and 13 mm, respectively. It was observed that the mask hole diameter affected the dmax depth. Thermoplastic masks with large hole diameters increase the surface dose less than those with small hole diameters.