EFFECTS OF BEAM MODIFIERS AND IMMOBILIZATION DEVICES ON THE DOSE IN THE BUILDUP REGION

Purpose: To analyze the effect that immobilization devices used in con junction with beam modifiers may have on the dose to the skin and buil d-up region. Methods and Materials: Central axis depth dose measuremen ts were made in a polystyrene phantom in the buildup regions using the 6 and 15 MV photon beams, at two different source-to-phantom distance s, and various field sizes. The effects of acrylic blocking trays, lea d wedges, and cerrobend blocks were assessed in conjunction with the e nhancement of dose in the build-up region due to immobilizing devices using plaster and thermoplastic casting materials of different thickne sses. Results: For the 6 MV photons, solid (3 mm) thermoplastic castin g material was found to have the greatest effect on surface dose: for a 12 X 12 cm field we measured 79% of maximum dose when treating throu gh the material versus 22% of maximum dose when no beam modifiers or i mmobilization devices are used. Measurements were also made to evaluat e the effect of the immobilization of patients receiving three-dimensi onal conformal treatments using a 15 MV photon beam. Conclusions: The relevance of these results to treatments in the pelvis, breast, and he ad and neck regions is discussed. For 6 MV beams, special consideratio n should be given if the need arises to treat through the immobilizati on device, as unacceptable skin reactions may result.