Commissioning and clinical implementation of an MLC tracking system: An evaluation of AAPM TG-264 guidelines

Precise radiotherapy relies on accurately targeting tumours while minimising exposure to healthy tissue, yet patient and organ motion complicate treatment delivery. To address intra-fractional motion, multi-leaf collimator (MLC) tracking systems have recently been adopted, adapting beam shapes in real-time. The American Association of Physicists in Medicine (AAPM) Task Group 264 (TG-264) provides guidelines for safely commissioning such tracking systems, yet these guidelines were initially developed for conventional linear accelerators and require evaluation, especially for newer platforms such as Radixact Synchrony.

This study aimed to: (i) evaluate the clinical performance and dosimetric accuracy of the Radixact Synchrony MLC tracking system according to AAPM TG-264 guidelines, from commissioning to clinical implementation; and (ii) critically assess and suggest practical refinements to these guidelines based on experiences with this novel tracking technology.

Commissioning followed TG-264 recommendations, adapted for Radixact Synchrony, utilizing three tracking modes: fiducial-based, markerless adaptive, and marker-based adaptive tracking. Performance was assessed with multiple test systems, including the Delta4 Phantom+, HexaMotion, Quasar platform, and film dosimetry. Measurements included geometric accuracy of phantom trace tracking, dosimetric accuracy of delivered dose to movable phantom, and system latency. Clinical protocols established treatment planning, quality assurance (QA), safety procedures, and clinical decision pathways, focusing on prostate and lung cancer treatments.

The Synchrony system demonstrated substantial improvements in geometric accuracy compared to non-MLC-tracking approaches. Fiducial-based tracking achieved a root mean square error (RMSE) of 0.76 ± 0.27 mm compared to 3.99 ± 2.84 mm without tracking (p = 0.008), with a mean absolute error (MAE) reduction to 0.36 ± 0.12 mm. Markerless adaptive tracking resulted in similar accuracy (RMSE 0.80 ± 0.15 mm, MAE 0.68 ± 0.15 mm). Dosimetric evaluations revealed consistent improvements, with gamma pass rate ≥ 95% (criteria 2%/2 mm) for tracked plans, significantly outperforming static plans under dynamic conditions (V = 7.0, p = .037). System latency was measured one time at approximately 630 ms for fiducial tracking without external breathing monitoring, slightly exceeding TG-264's ideal threshold (500 ms), yet well within the manufacturer's tolerance (1.5 s). Clinical cases confirmed feasibility, showing median deviations of 2.0-3.9 mm for prostate tracking and around 3.3 mm for markerless lung tracking. Safety protocols and clinical pathways developed during implementation ensured treatment robustness.

The Radixact Synchrony MLC tracking system successfully met TG-264 guidelines, significantly improving geometric and dosimetric accuracy during real-time tumour tracking. However, practical implementation highlighted necessary adaptations to TG-264 recommendations for non-standard platforms such as Radixact, specifically regarding QA protocols, latency tolerance, and handling of the system's unique characteristics (pneumatic MLC, jaw tracking, and flattening-filter-free beams). Our findings underscore the importance of maintaining conservative margins initially, rigorous QA, specialized staff training, and careful patient selection strategies. Further clinical trials focusing on safe margin reduction strategies are essential for optimizing the clinical benefits of advanced tracking technologies.