title: 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 manufacturer with two different sizes of 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 buildup was measured using an Attix parallel plate chamber by measuring tissue maximum ratios (TMRs) using solid water. Measurements were made with and without the mask material on the surface of the solid water for 6-MV and 15-MV X-ray beams. The effective thickness of equivalent water was estimated from the TMR curves, and the increase in surface dose was estimated. The buildup effect was measured to be equivalent to 2.2 mm to 0.6 mm for masks that have been stretched by different amounts. The surface dose was estimated to change from 16% and 12% for 6 MV and 15 MV, respectively, to 27% to 61% for 6 MV and 18% to 40% for 15 MV with the mask samples.
authors.affiliation: Department of Radiation Oncology Physics, The University of Michigan, Box 0010, 1500 E. Medical Center Drive, Ann Arbor, Michigan 48109, USA. swhadley@.umich.edu
authors.forename: Scott W
authors.initials: SW
authors.lastname: Hadley
authors.forename: Robin
authors.initials: R
authors.lastname: Kelly
authors.forename: Kwok
authors.initials: K
authors.lastname: Lam
mesh_terms: D001672:Biocompatible Materials / Q000737:chemistry*; D001822:Body Burden; D019544:Equipment Failure Analysis; D005145:Face / Q000502:physiology*; D006801:Humans; D007103:Immobilization / Q000295:instrumentation*; D008397:Masks; D008422:Materials Testing; D011829:Radiation Dosage; D011832:Radiation Injuries / Q000517:prevention & control; D011874:Radiometry; D012062:Relative Biological Effectiveness; D018570:Risk Assessment / Q000379:methods; D012307:Risk Factors; D012879:Skin Physiological Phenomena; D013499:Surface Properties; D014965:X-Rays
chemical_list: D001672:Biocompatible Materials