ESTRO 2024 - Abstract Book

S4277

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

ESTRO 2024

EG has so far been performed in 48 lung SBRT patients (58 targets), between September 2019 and February 2023, on a TrueBeam platform (Varian Medical Systems) using 10MV FFF RapidArc VMAT to a prescribed dose of 30-60 Gy in 1 8 fractions (usually normalized to cover 95% of the PTV). Typically, the 30-70% exhalation phases were used for treatment but this was patient dependent. Although breathing phases were used for target delineation, limitations in phase-gated delivery require that treatment is performed with amplitude gating (based on the motion of an external marker block, Varian RPM system) after translating the respiratory phases to RPM amplitude thresholds. Some patients with for example a variable/fast breathing pattern and/or unstable expiration position, were given breathing instructions to try to regulate their breathing by counting 2-2-2 (in-out-exhale pause) for every breath. Positional set up was performed using EG CBCT prior to the 1st arc, with additional scans (usually 1 or 2) between subsequent arcs. For 31 targets online tumor position monitoring (template matching of each planar kV image followed by triangulation) during CBCT acquisition, using non-clinical RapidTrackRealtime (RTR) software was used to confirm that the motion of the tumor was within the gating window [3]. The present analysis is based on 51 targets in 42 patients who gave consent for use of their data. For all 51 targets, longitudinal tumor motion on the 10-phase (0-90%) free-breathing 4DCT was 5-35mm (mean 16, SD 7.6) compared with 1-9mm (mean 3.5, SD 2.2) for the clinical EG phases (Figure 2) - a reduction in the mean of 12.5mm (78%). In a subset of 10 patients, the phase 0-90% ITV was 1.2-30.3cm3 (mean 9.0, SD 7.8) compared with 0.8-20.5cm3 (mean 5.6, SD 5.5) for the clinical EG phases - a reduction in the mean of 38%. Intrafraction EG-CBCT-based shifts were small, with 94% ≤3mm and 84% ≤2mm (mean vertical 1.4±1.3mm/longitudinal 1.4±1.5mm/lateral 1.2±1.1mm). The overall time from first set-up imaging to end of the last arc for 233 fractions, was 8-56 minutes (mean 21, SD 10, mean 40 minutes for 1x30/34Gy) and 77% of fractions were ≤25 minutes. Imaging for setup before starting the treatment required on average 11 minutes (SD 8). Tracking with RTR confirmed that during EG, the tumor typically returned to within 2mm of the planned position and RTR positional verification agreed with the average CBCT shifts. 7% of fractions were interrupted during treatment for an extra verification EG-CBCT. All patients completed EG lung SBRT. Results:

Figure 2 Blue: longitudinal tumor motion in all 10 phases. red: longitudinal tumor motion in the clinical EG phases

Conclusion: