Evaluation of a novel MR-compatible 3D scanner

Purpose: In MR-guided radiotherapy, there exists an unmet need for robust, 3D scanning equipment that can be used for data acquisition during acceptance/ commissioning and periodic QA testing. Avendor-provided MR-conditional prototype 3D scanning system was recently developed and tested on a clinical MR-guided Linac. Methods: Tests were performed using a 0.35 T MR-Linac delivering a 6 MV flattening filter-free beam with a bore diameter of 70 cm. The certified MR-conditional scanning system comprises webbased software, a water reservoir, an electronics control box, a hand pendant, and a 3D rectangular water tank. Scanning was performed over three sessions using 85 and 80 cm SSDs, 5 and 10 cm depths, and field sizes (FS) ranging from 0.4 × 0.2 cm2 to 27.2 × 24.1 cm2. MR-compatible A26 (field) and A1SL (reference) ionization chambers were used for FS \u3e 2.5 × 2.5 cm2 and an Edge diode detector for smaller FS. For each session, inline and crossline profiles as well as percent depth dose (PDD) measurements were acquired. Profiles and PDDs were compared across sessions and to Monte Carlo simulations and commissioning measurements acquired using either a 1D tank with manual changes in detector position applied between measurements or 2D ion chamber array. Results: For PDDs at 10 × 10 cm2 FS, dmax was constant and PDD10x was within 1% (mean: 1.4 cm and 61.6%, respectively) across all sessions. At 80 cm SSD, dmax was within 0.5 mm and PDD10x within 1% of baseline commissioning measurements. Both crossline and inline flatness and symmetry at 10 × 10 cm2 and 27.2 × 24.1 cm2 FS were within 1% of baseline measurements acquired during commissioning and field widths for all FS were within 1 mm of nominal values. Conclusion: Performance of a novel, MR-compatible 3D scanning water tank was assessed in a clinical MR-Linac system. Profile and PDD measurements were acquired using ion chambers and diodes. Data was consistent between scanning sessions and with previously acquired commissioning data

Investigating the clinical utility of gafchromic ebt3 film dosimetry in an MR-guided linac.

Purpose: To investigate the magnitude of magnetic field effects on Gafchromic™ EBT3 film in order to assess its clinical utility in a 0.35 T MR-guided linac. Methods: Perturbations induced by the magnetic field on EBT3 film were introduced by exposing the film to the magnetic field overnight at treatment isocenter prior to irradiation. These films were compared to films that had been irradiated without magnetic field exposure on a conventional linear accelerator. EBT3 films were placed at machine isocenter (90 cm SAD for films exposed to magnetic field, 100 cm SAD for unexposed films). Calibration plans consisting of comparable 3x3 grids of dose levels at machine isocenter were generated for both films. Polynomial fitting was utilized to generate calibration curves for both films. Film dosimetry was assessed in a set of square fields (4.94×4.94 cm2, 9.96×9.96 cm2, 14.94×14.94 cm2, and 19.92×19.92 cm2) by comparing film measurements to calculated dose and ion chamber array (ICprofiler) measurements. Small, open-field (0.83×0.83 cm2, 1.66×1.66 cm2) film measurements were compared to calculated dose and diode water tank measurements. Film dosimetry was utilized in patient specific QA for ten conventional and ten stereotactic body radiotherapy (SBRT) cases. Results: Effect of magnetic field on EBT3 films was negligible; R2=0.99 for unexposed film data points compared to curve generated using exposed film. For large fields, the average profile difference between film and ICprofiler measurements was 1.91%, and between film and calculated dose was 1.21%. For small fields, the average difference between film and diode measurements was 1.23% and between film and calculated dose was 1.56%. The 3%/3 mm (dose difference/DTA) gamma pass rates in conventional QA cases were 96.67 ± 2.23%, and 3%/1 mm in SBRT QA cases were 96.21 ± 1.12%. Conclusion: Minimal effects were observed in Gafchromic EBT3 films exposed to the 0.35T magnetic field. Film dosimetry results were consistent with calculated dose, ion chamber, and diode measurements