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 x 5, 8 x 8, 10 x 10 and 15 x 15 cm 2 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 x 10 cm 2 field, the d max depth without mask was 15 mm, while the d max 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 d max depth. Thermoplastic masks with large hole diameters increase the surface dose less than those with small hole diameters.