Room: Exhibit Hall | Forum 2
Purpose: Due to the growing number of Magnetic Resonance-guided Radiotherapy (MRgRT) systems, a pressing need exists for robust MR-compatible motion phantoms for use in commissioning and routine QA of MR-based gating. This study uses a novel MR-compatible motion phantom to assess the gating performance of a low-field MR-linac.
Methods: Tests were performed using a 0.35T MR-Linac delivering a 6MV flattening-filter-free beam. The motion phantom comprises a hollow oval body with two insert cavities, a set of hollow cylindrical inserts with changeable stems, an MR-safe piezoelectric motor drive unit, and non-MR-safe electronics control box and power supply. Cine images were acquired of a 4D motion insert containing a prototype imaging gel to reduce potential flow artifacts. Gating evaluation was performed using an MR-compatible A26 ionization chamber and radiochromic film for stationary and moving targets (15 breaths/minute, 2 cm peak-to-peak amplitude along the S-I direction) incorporating a variety of margin settings. 3D conformal plans (single and multiple fields) were generated to a target at mid-exhalation position to maximize gating effects.
Results: S-I motion of the MRI 4D motion insert measured in cine images was consistent with programmed amplitude (2cm). Chamber and film measurements were acquired over sessions lasting from 3-6 hours without degradation in motor operation. Average percent differences of ion chamber measurements between stationary and moving targets without gating for one- and nine-field plans were 30.8% and 32.6%, respectively. For gated treatments, gating windows of 0.5mm and 0.3mm around the target reduced deviation from stationary readings to ~16.0% and 12.9%, respectively. Film results demonstrated consistent results, where decreasing the gating window improved the film profile agreement with stationary deliveries.
Conclusion: Gating performance, including ion chamber and film measurements, was assessed for an MR-linac using a novel MR-compatible motion phantom. Future work will incorporate latency measurements and deriving disease-specific optimal gating margins.
Funding Support, Disclosures, and Conflict of Interest: The submitting institution holds research agreements with Philips Healthcare, ViewRay, Inc., and Modus Medical. Research partially supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA204189.