Effects of Thermoplastic Mask on Skin Dose and Build-up Characteristics

With the growing demand for more reproducible of head and neck patient positioning on the couch, thermoplastic immobilization masks have been one of the commonly used devices in radiotherapy. The aim of the present work was to obtain in-phantom measurements on the surface dose for patients undergoing 6 MV IMRT for hypopharynx and to compare this surface dose to that for patients treated with 3D-CRT, with and without the mask, respectively. The Varian 21EX and Siemens Primus linacs were used for all irradiations, the former is capable of delivering 6 and 10 MV photons while the latter is capable of delivering 6 and 15 MV photons. Measurements of TMR were performed using a PTW 23343 parallel-plate Markus chamber. The surface doses were measured by placing the TLD-100 clips on the surface of Rando phantom and underneath the thermoplastic mask. The surface dose was estimated to vary from 26.5% and 18.4% for 6 MV and 10 MV, respectively, to 34.5% to 65.6% for 6 MV and 24.3% to 47.1% for 10 MV with different stretched masks. The surface dose was also estimated to vary from 25.2% and 14.4% for 6 MV and 15 MV, respectively, to 40.7% to 63.0% for 6 MV and 23.2% to 37.9% for 15 MV with different stretched masks. The results of the presented measurements indicate that for treatment planning it may be necessary to account for the thickness of mask material dependence on surface dose, especially for lower energy.