Clinical Outcomes of Ablative MR-Guided Adaptive SBRT for Pancreatic Lesions

Purpose: To assess the efficacy, safety and treatment‑course dynamics of ablative stereotactic magnetic resonance (MR)-guided adaptive radiation therapy (SMART) for unresectable pancreatic lesions. Materials and Methods: Sixteen consecutive patients (median age 76.5 years; 56% women) with unresectable pancreatic lesions (13 pancreatic ductal adenocarcinoma (PDAC), 3 renal cell carcinoma (RCC) metastases) received SMART on an MR‑Linac. Fifteen patients received 50 Gy in five fractions and one received 50 Gy in four fractions (biologically effective dose (BED10; α/β=10 Gy) ≈ 100 Gy). Daily on‑table adaptation was performed before every fraction. Gross tumor volume (GTV) was contoured at each fraction; inter‑observer variability was assessed when different clinicians contoured the same case. Serum CA 19‑9 was recorded before, during and after treatment. Primary end‑points were per‑fraction GTV change, contouring variability, one‑year overall survival (OS) and acute toxicity. Results: All patients completed treatment; 97.5% of fractions were adapted. Median GTV change from first to final fraction was +1.1% (range –17.2% to +26.6%); 73% of tumors stabilized or shrank between fractions 4 and 5. Mean inter‑observer coefficient of variation for GTV was 6.9%. One‑year OS for PDAC patients (n=13) was 84.6%, exceeding historical rates (~55%) for conventional chemoradiotherapy. No acute grade ≥3 gastrointestinal toxicity occurred. Conclusion: Ablative five‑fraction SMART is feasible, well tolerated and associated with encouraging early survival in pancreatic malignancies. The modest day‑to‑day tumor‑volume changes and low inter‑observer variation support the robustness of multi‑physician adaptive workflows.