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Develop a Patient-Specific QA Program for Radiation Therapy with On-Board MRI

H Li

H Li*, V Rodriguez , O Green , Y Hu , R Kashani , H Wooten , D Yang , S Mutic , Washington University in St Louis, St Louis, MO


TH-C-12A-1 Thursday 10:15AM - 12:15PM Room: 12A

Purpose: This work describes development of the first patient-specific quality assurance (QA) program for magnetic resonance imaging guided radiation therapy (MR-IGRT).
Methods: The program consisted of following components: 1) multipoint ionization chamber (IC) measurement using a 15 cm3 cubic phantom, 2) 2D stacked radiographic film dosimetry using a 30x30x20 cm3 phantom with multiple inserted ICs, 3) 3D ArcCHECK measurement with a centrally inserted IC, 4) machine delivery file verification, 5) 3D Monte-Carlo dose re-calculation with machine delivery file and phantom CT, 6) 2-head mode delivery validation in case of a malfunctioning head, and 7) independent beam-on time calculation for non-IMRT fields. Both ADCL calibrated ICs and ArcCHECK were MRI compatible. Experimental data were analyzed for the first 10 patients treated at our institution.
Results: The customized phantoms allowed measuring multiple points with ICs in one delivery. Absolute IC measurements were all within 3% in all phantom geometry/shape/material combinations. Despite known uncertainty associated with film dosimetry, passing rates greater than 90% were achieved in both absolute and composite modes using TG-129 criteria. Due to the simultaneous irradiation by three radiation sources, ArcCHECK was used as a 3D relative dosimeter with angular and energy dependences uncorrected. 95-100% passing rates were obtained and the centrally inserted IC measurement assured that the overall dose normalization was within 3%. Machine delivery file verification and MC re-calculated dose to the phantom results showed 98-100% passing rates, providing opportunity of moving from gamma passing rates to patient DVH-based QA metrics. Same results were obtained for the 2-head delivery mode. Manual beam-on time calculation for non-IMRT fields showed better than 5% agreement.
Conclusion: We have successfully developed the first MR-IGRT patient specific QA program by adopting experimental and computational dosimetry methods that were developed in the past decade for other radiation therapy modalities.

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