ESTRO 2025 - Abstract Book

S3286

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

Purpose/Objective: To describe the commissioning and clinical implementation of Comprehensive Motion Management (CMM) using the Unity MR-Linac (Elekta AB, Stockholm, Sweden) at our centre. This work focuses on integrating CMM into routine clinical practice, enhancing precision and workflow efficiency in MR-guided adaptive radiotherapy (MRgART). Material/Methods: The commissioning of CMM aimed to validate system performance across various gating scenarios. Measurements and analyses were conducted using the Modus QUASAR™ MRI4D Motion Phantom (Modus Medical Devices, Ontario, Canada) and in-house scripts, following vendor recommendations and national/international guidelines [1,2]. Anatomical Position Monitoring (APM) checks with moving targets in all directions confirmed registration accuracy, while latency assessments measured system responsiveness. Dosimetric output under gated conditions was evaluated for consistency. End-to-end (E2E) testing with EBT4 Gafchromic™ films (Ashland Specialty Ingredients, NJ, USA) verified integration and functionality of system components. CMM protocols were implemented and optimised across various treatment sites, employing strategies like free breathing and exception gating to manage both respiratory and non-respiratory motion. Techniques included the Volumetric Criterion Envelope (VOICE) threshold and baseline shift (BLS) plans to address intrafractional drifts. Results: The commissioning confirmed consistent system performance, with stable dosimetric output under gated conditions and minimal variability compared to non-gated baselines (Table 1). The absolute dose for gated delivery showed a mean difference of +0.2 ± 0.2%. Latency measurements revealed values for the 5th percentile (P5%) and 95th percentile (P95%) for beam-on switching at -96.6 ± 7.6 ms and 91.0 ± 17.5 ms, respectively. For beam-off measurements, the P5% and P95% values were -61.3 ± 7.5 ms and 88.4 ± 11.5 ms, respectively. APM tracked targets with 1.3 mm accuracy for 95% of sampled points. Clinically, from May to October 2024, 74% of MRgART treatments employed CMM strategies, using a VOICE gating threshold of 100% to manage respiratory and non-respiratory motion (Figure 1). CMM was applied mainly to prostate, pancreas, and oligometastatic nodes, but excluded bladder cases due to target filling challenges. BLS plans were applied in 15% of fractions, with generation taking approximately one minute, ensuring target accuracy. Without CMM, similar cases would have required completion plans or acceptance of potential target misses due to intrafraction motion.