ESTRO 2025 - Abstract Book

S3350

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

ESTRO 2025

Material/Methods: Eleven healthy volunteers, median age 38 (range: 31-69) years, were scanned using a 3T MRI with a 9-second T1 weighted 3D VIBE sequence. Volunteers inhaled to a comfortable level and held their breath for 30 seconds nine times, simulating a treatment session. Three scans were acquired within a single breath-hold three times: at the beginning, middle, and end of the session. Therefore, nine scans were acquired per volunteer, 99 scans in total. A fish oil capsule taped to the lower sternum served as an external marker, simulating a 0.3cm gating window (±0.15cm). Scans within and outside the gating window were analyzed separately. Points were placed at the external marker and three internal landmarks: the base of the heart, a peripheral lung feature in the lower right lobe, and the top of the liver. Positional uncertainties were assessed relative to the first scan per patient (intrafraction uncertainties) and the first scan of each breath-hold (intra-breath-hold uncertainties). Results: Forty-three scans were within the gating window for intrafraction analysis, and all scans were within for intra breath-hold analysis. Positional uncertainties were larger outside the gating window. Within the gating window, liver and lung uncertainties were on the order of 3-5mm (max 14.3mm) intrafraction and 1-3mm (max 8.9mm) intra breath-hold. Heart uncertainties were on the order of 1-2mm (max 7.5mm) for both analyses (Figure 1). Internal anatomy positions correlated poorly with the external marker (Figure 2).