(Tuesday, 7/16/2019) 9:30 AM - 10:00 AM
Room: Exhibit Hall | Forum 4
Purpose: To facilitate high precision MR-guided radiotherapy (MRgRT) with complex, modulated plans, a high resolution 3D dosimetry option is desirable. This work describes our initial experience using a novel 3D gel dosimeter for complex MR-linac treatment planning QA.
Methods: A novel, radiochromic gel dosimeter was prepared in a transparent vessel (15 cm diameter, 12 cm effective length) and immobilized for reproducible setup. 0.35T MRI (TrueFISP, TE/TR=1.6ms/3.8ms) and CT scans were obtained. Using a Monte Carlo algorithm, dose to the MRI dataset was calculated using registered CT values for electron density in two complex patient plans (single isocenter multi-target (SIMT) with 5 metastatic lesions (0.1-0.7 cc, 18 Gy/lesion) and hypofractionated prostate (36.25 Gy in 5 fx) with integrated boost (46 Gy) to a 3.6 cc dominant intra-prostatic lesion (DIL)). Gel dosimeters were localized with 0.35T MR-imaging, and treatment plans were delivered. Optical gel readout was performed using a Vista 16 cone-beam optical CT scanner. Resultant 3D scans were registered to planned doses using cross-correlation, and dose was calibrated using voxels in the high-dose region. 3D gamma analysis (2%/2 mm, 30% threshold) and relative dose differences were assessed.
Results: 3D gel dosimeters were MR-visible, enabling accurate MR-guidance for setup (shifts<1 mm). Overall, 3D gamma for the DIL plan was 95.2% with excellent dose profile agreement across the DIL volume. Using a single registration for all 5 lesions of the SIMT plan, the overall 3D gamma was 86.9% (range: 77.5%-97.9%). With local registrations, 3D gamma of the two individual lesions farthest from isocenter improved from 77.5% and 80.6% to 92.7% and 96.9%, respectively.
Conclusion: High-resolution 3D dosimetric verification using an MR-visible radiochromic gel is promising to support high precision MRgRT for complex plans using a single volumetric readout. Further benchmarking against other dosimetric methods and validation of additional off-axis plans are warranted.
Funding Support, Disclosures, and Conflict of Interest: The submitting institution holds research agreements with Philips Healthcare, ViewRay, Inc., and Modus Medical. Research partially supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA204189.
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