ESTRO 2025 - Abstract Book

S1799

Clinical – Upper GI

ESTRO 2025

Conclusion: Robotic SBRT of liver lesions is an effective local treatment with high rate of LC and minimal acute toxicities. Patient and tumor characteristics are critical in identifying candidates who will benefit from this approach. Notably, BED exceeding 120 Gy does not significantly impact LC rate. In HCC, tumor size and consequently PTV volume are key factors influencing the LC rate.

Keywords: SBRT,Liver,Cyberknife

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Digital Poster Dose accumulation and its correlation with toxicity and clinical outcome in MR-guided pancreatic SABR Rekaya Shabbir 1,2 , Mairead Daly 1,2 , Ruksana Sivakaran 3 , Eliana Vasquez Osorio 1 , Ganesh Radhakrishna 3 , Ananya Choudhury 3,1 , Cynthia L Eccles 2,1 1 Division of Cancer Sciences, The University of Manchester, Manchester, United Kingdom. 2 Radiotherapy, The Christie NHS Foundation Trust, Manchester, United Kingdom. 3 Clinical Oncology, The Christie NHS Foundation Trust, Manchester, United Kingdom Purpose/Objective: Online adaptive radiotherapy on the magnetic resonance linear accelerator (MR-Linac) is a promising approach for delivery of stereotactic ablative body radiotherapy (SABR) in locally advanced pancreatic cancer (LAPC). We investigated how dose accumulation correlates to clinical outcomes and toxicity profiles in MR-guided adaptive SABR for LAPC. This work compared 1) the difference between accumulated dose to targets and OARs and the reference plan (Ref Plan) in pancreatic SABR using organ-wise (OW) deformable image registration (DIR) [1], and 2) associated radiotherapy dose with patient toxicity and treatment outcomes to evaluate the effectiveness of adaptive MR-Linac SABR. Material/Methods: 10 patients with LAPC, enrolled in the MOMENTUM trial (NCT04075305), were treated with 4000cGy in 5 fractions using online adaptive SABR. Pre-treatment MRI (FxMRI), Ref plan, and planning MRI (pMRI) were imported into RayStation (V12.0.100.0). Voxel-wise accumulated doses were calculated based on OW registrations, previously reported to perform well in this setting [1]. The differences in GTV D 99 and D 0.5cc for the OARs were calculated. Patient toxicity was assessed at baseline and follow-up. The relationship between toxicity and dose for GTV and selected GI OARs was assessed. Local and distant disease progression was also reported. Results: The accumulated dose differences compared to the Ref Plan dose were determined for GTV and OARs in all patients. Table 1 summarizes the mean percentage dose difference for GTV and OARs. The estimated accumulated dose to D 0.5cc in OARs was lower than Ref Plan for all OARs (Figure 2). Although the planned small bowel dose to D 0.5cc exceeded tolerance for patient 3, the delivered dose was notably lower (Figure2.B). There was an increase to D 0.5cc in the stomach in 5 patients. However, the delivered dose remained within tolerance (Figure2.C). 4 patients were alive at time of analysis. At 1 year post SABR, local and distant progression occurred in 3/10 and 4/10 patients, respectively. Preliminary analysis demonstrated a moderate positive association between accumulated D min for the GTV and time to local progression (r=0.64, p =0.08).