Developing Quality Assurance Procedures for Multi-Plane Gated Beam Delivery In 0.35T Mrgrt: Ensuring Dosimetric Accuracy
Abstract
Purpose
Multi-plane target tracking in 0.35 T MR-guided radiotherapy (MRgRT) is beginning to emerge in clinical practice. To support safe implementation, robust and repeatable quality assurance (QA) procedures are needed. This study proposes a practical QA methodology to evaluate the dosimetric performance of single- and dual-plane gated beam delivery, with exploratory assessment of triple-plane gating, using a motion phantom and ion chamber measurements.
Methods
The QUASAR MRI4D motion phantom was used with two PTW ionization chambers connected to ADCL-calibrated electrometers and positioned in the phantom’s acrylic inserts. The chamber in the offset insert served as a stationary reference, while the central chamber was subjected to programmed sinusoidal and rotational motion to simulate tumor motion. Using the ViewRay gating system, a 6 MV FFF, 1 Gy beam was delivered and measured (nC) under the following conditions: (1) single-plane gating/no motion, (2) single-plane gating/translational motion, (3) dual-plane gating/translational motion, (4) single-plane gating/rotational motion, (5) dual-plane gating/rotational motion, and (6) triple-plane gating/rotational motion. Single-, dual-, and triple-plane gating corresponded to sagittal; sagittal + coronal; and sagittal + coronal + axial tracking, respectively. Measurements were compared across conditions using a predefined acceptance criterion of <2% difference.
Results
Ion chamber measurements demonstrated consistent dose delivery for single- and dual-plane gating, with all corresponding cases within 2% of reference measurements for both translational and rotational motion. In contrast, triple-plane gating did not meet the acceptance criterion, indicating deviations in dosimetric consistency under the tested conditions. The gating parameters for all cases used a ROI value of 0 and a 75% confidence interval.
Conclusion
This study demonstrates a feasible and reproducible QA methodology for evaluating multi-plane gated beam delivery on the ViewRay system. The results support routine QA implementation for single- and dual-plane gating, while highlighting the need for continued investigation into triple-plane gating performance.