The effects of thermoplastic masks used for immobilization of patients on surface and build-up for megavoltage radiation

Abstract

OBJECTIVESTo examine the effect of thermoplastic masks used for immobilization on surface dose at Co-60 and 6MV photon energies. METHODSThree pieces of Aquaplast masks were used. One of these masks was used intact as a standard. The hole diameters of the other masks were stretched 1.25 and 2-fold. The masks were numbered as 1, 2, 3. Doses were measured with Markus parallel-plate (pp) ionization chamber, EBT2 film and MOS-FET dosimetry. RESULTSThe 1, 2 and 3 numbered masks were increased the surface dose from 24.2% to %73.3, 55.6% and %40.6, respectively, for 10x10 cm 2 at Co-60. For 6MV, the surface dose of 14.4% increased to 50.1%, 34.7% and 23.7%, respectively. Markus pp results were compatible with the EBT2 film results. MOS-FET measured higher surface doses. CONCLUSIONThe surface dose increases in the presence of mask. Mask material thickness and hole diameter significantly affects the surface dose. The diameter of the holes should be enlarged or cut the relevant region.

Surface and Buildup Region Dose Measurements with Markus Parallel-Plate Ionization Chamber, GafChromic EBT3 Film, and MOSFET Detector for High-Energy Photon Beams

“…Our results are in good agreement with the results of MOSkin when the WET values Advances in High Energy Physics are considered. Elvan Erkan and Kemikler [23] examined the effect of thermoplastic masks used for immobilization on surface dose at Co-60 and 6 MV (Siemens Oncor Impression Plus linear accelerator) photon energies. They used MOSFET detectors which have a WET of 0.8 mm and their surface dose measurements were 32.2 ± 2.3%, 37.4 ± 2.3%, and 43.2 ± 2.4% for the same field sizes that we used, respectively.…”