Author: Kin Yin Cheung, Chen-Yu Huang, Chi Wah Kong, Ka Ki Lau, Pei-Xiong Li, Pak Hang Nam, Mei Yan Tse, Jierong Wang, Bin Yang, Siu Ki Yu, Jing Yuan, Chi To Yung, Shang Peng Felix Yung 👨🔬
Affiliation: Medical Physics Department, Hong Kong Sanatorium and Hospital, Research Department, Hong Kong Sanatorium and Hospital 🌍
Purpose: Intrafraction motion management remains a major challenge in magnetic resonance-guided radiotherapy (MRgRT). The recent clinical release of an active motion management system aims to enhance treatment capabilities for moving targets. This study evaluates the dosimetric performance of the real-time gating system using MRI-compatible respiratory motion phantoms.
Methods: Experiments were conducted on an Elekta Unity 1.5T MR-Linac with the Comprehensive Motion Management (CMM) package. Four tracking strategies—respiratory, exhale, breath-hold, and exception gating—were assessed. A commercial MRI-compatible respiratory motion phantom facilitated point dose measurements, and custom plans in the Monaco treatment planning system mitigated artifacts during motion tracking caused by the ionization chamber. Delivery consistency was verified by comparing measurements at a static target position with and without gating. Film dosimetry evaluated coronal and sagittal dose distributions in a custom PMMA phantom. Measurements were performed for sinusoidal, cos6(x), and breath-hold motion patterns, with gating windows of ±2–3 mm. In exception gating a displacement larger than 5mm was introduced to enable the baseline shift plan workflow. Gamma analysis with 2%/2 mm and 3%/2 mm criteria was applied.
Results: The CMM system exhibited robust performance in managing intrafraction motion. Static point dose measurements showed linac delivery consistency within 0.8%. For moving targets, deviations from treatment planning system calculations were within +0.1% (standard deviation: 1.4%). Film dosimetry achieved >90% gamma passing rates for 2%/2 mm and >95% for 3%/2 mm criteria, demonstrating strong alignment between measured and planned dose distributions. With a ±3 mm gating window, the CMM system maintained a gamma passing rate of 93.8% ± 2.4% (2%/2 mm) for motions up to 20 mm.
Conclusion: The CMM system demonstrates high dosimetric accuracy and reliability in managing respiratory motion with a 1.5T MR-Linac. These results underscore its potential to enhance precision and consistency in MRgRT, particularly for moving targets.