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Feasibility Study On Using Small Plastic Phantoms for Auditing Radiation Output of MR-Linac Systems

Z Wen

Z Wen*, P Alvarez , G Ibbott , UT MD Anderson Cancer Center, Houston, TX


SU-E-J-208 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To study feasibility of using small plastic phantoms designed for conventional linac output auditing to measure the output of MR-Linac systems.

Methods: For simulations, the CT scan of an IROC(formerly RPC) acrylic block phantom designed for 8 MV beams was imported in a research version of the treatment planning system (Monaco). Dose delivered to three TLDs in the block was calculated with a Monte Carlo algorithm and a beam model based on an MR-linac prototype with and without a magnetic field (B=1.5T). In a large mathematical water phantom, the same beam was used to calculate dose in full scatter conditions. The block factor (F) was calculated as the ratio of the average dose to the block TLDs to the dose at the reference point in the mathematical phantom. For experimental measurement, four IROC blocks were irradiated with the MR-linac prototype, and data were analyzed by IROC.

Results: The F factor without a B field was 1.053. When a B field was applied, it changed the dose distribution in the block, especially on the edges. With a B field parallel to the long axes of the TLD, F was 1.038. However, with a perpendicular B field, F factor increased slightly to 1.075. In the IROC report, the output determined with two blocks parallel to the B field was 2.3% higher than the output by the two blocks perpendicular to the B field. The average of all four blocks was within 2% of machine output measured with an ion chamber.

Conclusion: It may be feasible to expand the utility of the acrylic block phantoms for radiation output auditing from conventional linacs to MR-linacs. However, the scatter correction factor can change due to the B field and its orientation to the block. More symmetric phantom designs may be less prone to mistakes.

Funding Support, Disclosures, and Conflict of Interest: We acknowledge research support from Elekta

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