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

To assess the efficacy, safety, and treatment‑course dynamics of ablative stereotactic magnetic resonance (MR)-guided adaptive radiation therapy (SMART) for unresectable pancreatic lesions.

Sixteen consecutive patients (median age: 76.5 y; 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 5 fractions and one received 50 Gy in 4 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; interobserver 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, 1‑year overall survival (OS) and acute toxicity.

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 interobserver 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.

Ablative 5‑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 interobserver variation support the robustness of multiphysician adaptive workflows.