Effects of immobilization mask material on surface dose

Abstract

This work investigates the increase in surface dose caused by thermoplastic masks used for patient positioning and immobilization. A thermoplastic mask is custom fit by stretching a heated mask over the patient at the time of treatment simulation. This mask is then used at treatment to increase the reproducibility of the patient position. The skin sparing effect of mega-voltage X-ray beams can be reduced when the patient?s skin surface is under the mask material. The sheet of thermoplastic mask has holes to reduce this effect and is available from one manufacture with two different size holes, one larger than the other. This work investigates the increase in surface dose caused by the mask material and quantifies the difference between the two samples of masks available. The change in the dose build-up was measured using an Attix parallel plate chamber by measuring tissue maximum ratios using solid water. Measurements were made with and without the mask material on the surface of the solid water for 6 and 15MV X-ray beams. The effective thickness of equivalent water was estimated from the TMR curves and the increase in surface dose was estimated. The build-up effect was measured to be equivalent to 2.2-0.6mm for masks that have been stretched by different amounts. The surface dose was estimated change from 16% and 12% for 6MV and 15MV respectively to be 27-61% for 6MV and 18-40% for 15MV with the mask samples.