Abstract
PurposeCommercial independent monitor unit (IMU) check systems for high-magnetic-field MR-guided radiation therapy (RT) systems are lacking. We investigated the feasibility of adopting an existing treatment planning system (TPS) as an IMU check for online adaptive radiotherapy using 1.5-Tesla MR-Linac.MethodsThe 7-MV flattening filter free (FFF) beam and multi-leaf collimator (MLC) models of a 1.5-T Elekta Unity MR-Linac within Monte Carlo-based Monaco TPS were used to generate an optimized beam model in Eclipse TPS. The MLC dosimetric leaf gap of the beam in Eclipse was determined by matching the dose distribution of Eclipse-generated intensity-modulated radiation therapy (IMRT) plans using the Analytical Anisotropic Algorithm (AAA) algorithm to Monaco plans. The plans were automatically adjusted for different source-to-axis distances (SADs) between the two systems. For IMU check, the treatment plans developed in Monaco were transferred to Eclipse to recalculate the dose using AAA. A plug-in within Eclipse was created to perform a 2D gamma analysis of the AAA and Monte Carlo dose distribution on a beam’s eye view parallel plane. Monaco dose distribution was shifted laterally by 2 mm during gamma analysis to account for the impact of magnetic field on electron trajectories. Eclipse doses for posterior beams were corrected for both the Unity couch and the posterior MR coil attenuation. Thirteen patients, each with 4–5 fractions for a variety of tumor sites (pancreas, rectum, and prostate), were tested.ResultsAfter thorough commissioning, the method was implemented as part of the standard clinical workflow. A total of 62 online plans, each with approximately 15 beams, were evaluated. The average per-beam gamma (3%/3 mm) pass rate for plans was 97.9% (range, 95.9% to 98.8%). The average pass rate per beam for all 932 beams used in these plans was 97.9% ± 1.9%, with the lowest per-beam gamma pass rate at 88.4%. The time for the process was within 3.2 ± 0.9 min.ConclusionThe use of a second planning system provides an efficient way to perform IMU checks with clinically acceptable accuracy for online adaptive plans on Unity MR-Linac. This is essential for meeting the safety requirements for second checks as outlined in American Association of Physicists in Medicine Task Group (AAPM TG) reports 114 and 219.
Highlights
MRI provides a huge advantage for target and organ at risk (OAR) delineation because of the superior soft-tissue contrast [1]
The independent monitor unit (IMU) check used the same MUs from the Monaco plans as the input to the independent dose calculation and performed the quality assurance (QA) check by comparing the dose distribution calculated with the two algorithms
An Eclipse plugin was developed to perform the gamma analysis using the Eclipse Scripting Application Program Interface (ESAPI) software library provided by Varian and to generate a report for the QA record
Summary
MRI provides a huge advantage for target and organ at risk (OAR) delineation because of the superior soft-tissue contrast [1]. Recent developments in on-board MRI coupled to the teleradiotherapy unit are enabling online MR-guided adaptive radiotherapy planning [2–4]. The capability of evaluating daily geometric and anatomical changes along with real-time imaging of tumor position during beam delivery makes on-board MR superior to other imaging modalities for online treatment plan adaptation [1]. The clinical introduction of such MR-guided radiotherapy (MRgRT) systems using hybrid MR-Linac systems have prompted considerations of the potential impact of the static magnetic field on biological responses to radiation [5, 6]. Studies have reported initial results suggesting that the biological response to radiation in the presence of a static magnetic field (B-field) may be modestly different when compared to conventional radiotherapy in a zero B-field environment. While the mechanisms of interaction in the presence of a static magnetic field are still an active field of research, the calculation and validation of physical dose will be critical in isolating any biological effects due to treatment in a high-field MR-Linac system [4–6]
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