Novel 3D Gel Dosimetry to Support High Fidelity MR-Guided Radiotherapy Delivery

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Conference Proceeding

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J Med Phys


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 = 1.6 ms/3.8 ms) 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 multitarget (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 intraprostatic 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 conebeam 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 MRvisible, 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.





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