Assessing the Octavius 4D MR Phantom for VMAT Validation on the 1.5 T MR-Linac
Abstract
Purpose
MR-linacs enable online adaptive radiotherapy, which increases dose conformality and thus potentially improves therapeutic ratios. A recurring drawback with respect to C-arm linacs is the lack of VMAT leading to decreased efficiency. Recently, it was demonstrated that VMAT is technically feasible on the 1.5 T Unity MR-linac (Elekta, Sweden) resulting in clear delivery efficiency gains with respect to IMRT. Here we report on the suitability of the OCTAVIUS 4D MR (PTW Freiburg, Germany) phantom for VMAT-specific MR-linac and patient QA.
Methods
Measurements were performed on a 1.5T MR-linac in research mode, enabling VMAT deliveries, with the OCTAVIUS 4D MR. The array OCTAVIUS 1500 Detector MR (1405 ion chambers, 27x27 cm2 grid) was placed on the central rotational axis of the cylindrical phantom. The phantom software was modified to enable continuous rotation synchronized with the linac-reported gantry angle. Its performance was verified by logfile analysis. Dynamic angular dependency was tested with the phantom in iso-center and measuring square fields (200 MU) dynamically and statically (30° intervals, averaged) while avoiding the cryostat pipe and highly-attenuating couch structures. Beam profiles were compared within 80% of the field size to exclude penumbra effects. Patient QA was tested with the phantom on the treatment couch with a two-arc (180°-5°, 21°-180°) 5x7.25 Gy prostate VMAT plan, created in Monaco research v6.29.0.0 (Elekta, Sweden).
Results
The phantom’s rotation slightly trailed the linac-reported gantry angle (stream-rate: 1.3 Hz) by 1.1° (RMSE). Dynamic square field measurements agreed with the static ones: mean-absolute (95th-percentile) errors of 0.3% (0.6%) and 0.5% (1.1%) for 10x10 cm2 and 20x20 cm2, respectively. The prostate plan passed with a 96% global gamma pass-rate (3%/3mm).
Conclusion
The OCTAVIUS 4D MR with modified software is a suitable phantom for VMAT-specific machine and patient QA on the MR-linac. Better gantry synchronization could yield further performance gains.