Gating evaluation of an MRGRT system using a novel MR-compatible motion phantom.
Kim J, Barberi E, Chetty I, Movsas B, and Glide-Hurst C. Gating evaluation of an MRGRT system using a novel MR-compatible motion phantom. Med Phys 2018; 45(6):e519.
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 MRbased 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 6 MV 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 MRsafe 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/min, 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 (2 cm). Chamber and film measurements were acquired over sessions lasting from 3 to 6 h without degradation in motor operation. Average percent differences of ion chamber measurements between stationary and moving targets without gating for oneand nine-field plans were 30.8% and 32.6%, respectively. For gated treatments, gating windows of 0.5 mm and 0.3 mm 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.