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