Purpose: The magnetic field (1.5-Tesla) of Unity MR-Linac (MRL) creates a high-intensity force on ferrous materials rendering many conventional QA detectors inoperable and poses significant challenges for clinical physicists. In this work, we evaluated using the Gafchromic EBT-XD film for the commissioning of SRS/SBRT in MRL.
Methods: Unity MRL is a combined 1.5T MR scanner and 7X-FFF Linac. The output factors (St), for 1x1, 2x2, 3x3cm², were measured with film in solid water at d=5cm, SAD=143.5cm. The results were compared with (1) the measurements by a PTW CV diamond detector (CVD) in 3D water phantom; (2) the calculation by Monaco TPS (MTPS) in water using 1.0mm dose-grid and 0.5% statistical uncertainty. The inline and crossline profiles, measured by films and the CVD, were also compared. An SRS plan with three targets (1-3cm) was created in MTPS and was measured with EBT-XD film in a StereoPhan® phantom. The 3x3cm² field size was used for film calibration with doses ranging from 0 to 28.21Gy. Water was added to the phantom-film-phantom interface to reduce the electron-return-effect (ERE). Films were scanned on an Epson scanner following the One-scan dosimetry protocol.
Results: The measured St between films and CVD were within 0.5% and 1.9% for 1x1 and 2x2cm² respectively. Compared with MTPS, films were within 1.5% and 2.4%. At the central B-field induced asymmetric region, films were within 0.6% on average from the CVD and the MTPS. 5-8% differences were observed between films and CVD in the 40%-5% gradient region in crossline profiles due to different magnitude of ERE. The differences in localization and dose were found to be 0.6mm and 3.3%. The γ(3%,2mm) and γ(5%,2mm) within 50% target dose region were 94.4% and 99.3%.
Conclusion: EBT-XD film agreed well with the diamond detector and is a valuable QA tool for SRS/SBRT commissioning in MRL.
Funding Support, Disclosures, and Conflict of Interest: One of the authors has a research grant from Ashland. This research was funded in part through the NIH/NCI Cancer Center Support Grant No. P30 CA008748.