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3D Remote Dosimetry for MRI-Guided Radiation Therapy: A Hybrid Approach

L Rankine

L Rankine1,2*, S Mein3 , J Adamovics4 , B Cai1 , A Curcuru1 , T Juang3 , D Miles3 , S Mutic1 , Y Wang1 , M Oldham5 , H Li1 , (1) Washington University School of Medicine, Saint Louis, MO, (2) The University of North Carolina at Chapel Hill, Chapel Hill, NC, (3) Duke University Medical Physics Graduate Program, Durham, NC, (4) Rider University, Skillman, NJ, (5) Duke University Medical Center, Durham, NC


TH-CD-BRA-2 (Thursday, August 4, 2016) 10:00 AM - 12:00 PM Room: Ballroom A

Purpose: To validate the dosimetric accuracy of a commercially available MR-IGRT system using a combination of 3D dosimetry measurements (with PRESAGE(R) radiochromic plastic and optical-CT readout) and an in-house developed GPU-accelerated PENELOPE Monte-Carlo dose calculation system.

Methods: ⁶⁰Co IMRT subject to a 0.35T lateral magnetic field has recently been commissioned in our institution following AAPM's TG-119 recommendations. We performed PRESAGE(R) sensitivity studies in 4ml cuvettes to verify linearity, MR-compatibility, and energy-independence. Using 10cm diameter PRESAGE(R), we delivered an open calibration field to examine the percent depth dose and a symmetrical 3-field plan with three adjacent regions of varying dose to determine uniformity within the dosimeter under a magnetic field. After initial testing, TG-119 plans were created in the TPS and then delivered to 14.5cm 2kg PRESAGE(R) dosimeters. Dose readout was performed via optical-CT at a second institution specializing in remote 3D dosimetry. Absolute dose was measured using an IBA CC01 ion chamber and the institution standard patient-specific QA methods were used to validate plan delivery. Calculated TG-119 plans were then compared with an independent Monte Carlo dose calculation (gPENELOPE).

Results: PRESAGE(R) responds linearly (R²=0.9996) to ⁶⁰Co irradiation, in the presence of a 0.35T magnetic field, with a sensitivity of 0.0305(±0.003)cm⁻¹Gy⁻¹, within 1% of a 6MV non-MR linac irradiation (R²=0.9991) with a sensitivity of 0.0302(±0.003)cm⁻¹Gy⁻¹. Analysis of TG-119 clinical plans using 3D-gamma (3%/3mm, 10% threshold) give passing rates of: HN 99.1%, prostate 98.0%, C-shape 90.8%, and multi-target 98.5%. The TPS agreed with gPENELOPE with a mean gamma passing rate of 98.4±1.5% (2%/2mm) with the z-score distributions following a standard normal distribution.

Conclusion: We demonstrate for the first time that 3D remote dosimetry using both experimental and computational methods is a feasible and reliable approach to commissioning MR-IMRT, which is particularly useful for less specialized clinics in adopting this new treatment modality.

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