S2182
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
Material/Methods:
− 10% for overlap volume, leading to discordance between planned and delivered constraint evaluations in 20% of patients. In both cases, constraint violations absent at planning emerged by the end of treatment. Conclusion: Simple anatomical parameters from simulation CT can accurately predict bladder dose constraint violations during salvage SBRT planning. Early identification of high-risk patients may enable tailored preparation protocols and adaptive strategies. Preliminary sCT- based dose recalculations suggest that delivered dose generally aligns with planned dose, reinforcing the clinical relevance of these anatomical predictors. Ongoing work will evaluate accumulated dose across the entire cohort to confirm these findings and to elucidate the relationship between constraint violations and urinary morbidity. Keywords: Predictors, salvage SBRT, bladder constraints Improving Radiotherapy Treatments for Locally Advanced Pancreatic Cancer Patients (PREFER study) Michael W Wake 1 , Natalie Harrold 1 , Peter Sitch 1 , Fabio D'Andrea 1,2 , Linnéa Freear 1 , Alberto Salas Mellado 3 , Marianne Aznar 2 , Ganesh Radhakrishna 4 , Robert Chuter 1,2 1 Christie Medical Physics & Engineering, The Christie NHS Foundation Trust, Manchester, United Kingdom. 2 Department of Physics and Astronomy, The University of Manchester, Manchester, United Kingdom. 3 Hospital Clínico Universitario de Santiago, CHUS, Santiago de Compostela, Spain. 4 Clinical Oncology, The Christie NHS Foundation Trust, Manchester, United Kingdom Digital Poster 4325 Purpose/Objective: MR-guided Stereotactic Ablative Body Radiotherapy (SABR) delivery with daily adaptive planning accounts for the significant inter/intra-fractional motion of Gastrointestinal Organs at Risk (GI-OARs), which is critical for patients with locally advanced pancreatic cancer [1,2]. This study compares the dosimetric impact of daily anatomical changes in pancreatic SABR plans delivered with Volumetric-Modulated Arc Therapy (VMAT), Proton Beam Therapy (PBT), and clinically delivered MR-Linac treatments. The aim was to quantify differences in target coverage and GI-OAR sparing across modalities to inform optimal selection for pancreatic SABR.
Each patient’s planning CT image was deformed onto daily planning MR scans from fraction 1-5 to generate a “daily anatomy” CT using Velocity™. The deformed CTs (dCT) were registered with the planning MR image, and clinician approved contours registered onto the dCT. A reference plan of 40 Gy in 5-fractions was made for each treatment modality (Figure 1) and recalculated on each fraction’s anatomy. Clinical acceptability was assessed using ITV coverage metrics (D95, D mean, D98) and GI-OAR dose metric (D0.5cc) for duodenum, stomach, small bowel and large bowel. For each metric, Δ Dose was calculated as the difference between the fractional and reference plans. Currently, data for three patients has been analysed; the full cohort (n=20) remains ongoing. Results:
Per-fraction Δ Dose showed intra/inter-fractional differences across modalities. Figure 2 displays ITV coverage remained stable, with deviations within ±1.5 Gy of the reference plan for all techniques. Greater variation was observed in the GI-OARs metrics, with early signs that PBT showed the largest deviations from the reference plan, with individual fractions differing up to (±15 Gy) due to gas near the target and inherent range sensitivity. VMAT demonstrated moderate variability (±4–6 Gy) while MR-Linac plans
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