SU-E-T-139: Evaluation of Surface, Peripheral,and Buildup Dose in MR-IGRT

Abstract

Purpose: To determine the effect of the magnetic field on surface and buildup dose in radiotherapy using MR-IGRT, specifically measured with a Co-60 system. Methods: A commercially-available MR-IGRT system was used to measure surface, peripheral, and build-up dose due to thermoplastic head immobilization masks. Both radiographic (Kodak EDR2) and radiochromic (EBT2) film types were used for surface measurements. A small-volume ionization chamber was used for peripheral dose measurements. Several types of thermoplastic masks from three different manufacturers were chosen; radiochromic film was used to measure possible dose to skin under the masks. Comparisons were made using these masks with 6-MV photon beams delivered by a Varian Truebeam machine. Results: For a 10×10 cm field size at 100 SSD, the surface dose as normalized to maximum dose (at 0.5 cm) was determined to be about 30% as measured by both EDR and EBT2 films. Peripheral dose with an open 10x10 field at 10 cm depth matched previously published data for Co-60. Skin dose due to thermoplastic mask buildup was at least 30% higher (depending on mask construction) with Co-60 as compared to 6-MV due to its lower energy. Conclusion: While the magnetic field of an integrated MR-IGRT system is expected to reduce skin dose by sweeping away contamination electrons, the use of Co-60 introduces the added challenge of carefully choosing immobilization devices in order to preserve the skin-sparing effect of the MR. Drs. Green, Hu, and Mutic have served as consultants to ViewRay, Inc.