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

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.