Ablative Five-Fraction Stereotactic Body Radiation Therapy for Inoperable Pancreatic Cancer Using Online MR-Guided Adaptation

PURPOSE: Patients with inoperable pancreatic adenocarcinoma have limited options, with traditional chemoradiation providing modest clinical benefit and an otherwise poor prognosis. Stereotactic body radiation therapy for pancreatic cancer is limited by proximity to organs-at-risk (OAR). However, stereotactic magnetic resonance-guided adaptive radiation therapy (SMART) has shown promise in delivering ablative doses safely. We sought to demonstrate the benefits of SMART using a 5-fraction approach with daily on-table adaptation. METHODS AND MATERIALS: Patients with locally advanced, nonmetastatic pancreatic adenocarcinoma were treated with 50 Gy in 5 fractions (biologically effective dose(10) 100 Gy) with a prescribed goal of 95% planning target volume coverage by 95% of prescription, prioritizing hard OAR constraints. Daily online adaptation was performed using magnetic resonance-guidance and on-table reoptimization. Patient outcomes, treatment factors, and daily adaptation were evaluated. RESULTS: Forty-four patients were treated with SMART at our institution from 2014 to 2019. Median follow-up from date of diagnosis was 16 months (range, 6.7-51.6). Late toxicity was limited to 2 (4.6%) grade 3 (gastrointestinal ulcers) and 3 (6.8%) grade 2 toxicities (duodenal perforation, antral ulcer, and gastric bleed). Tumor abutted OARs in 35 patients (79.5%) and tumor invaded OARs in 5 patients (11.1%). Reoptimization was performed for 93% of all fractions. Median overall survival was 15.7 months (95% confidence interval, 10.2-21.2), while 1-year and 2-year overall survival rates were 68.2% and 37.9%, respectively. One-year local control was 84.3%. CONCLUSIONS: This is the first reported experience using 50 Gy in 5 fractions for inoperable pancreatic cancer. SMART allows this ablative dose with promising outcomes while minimizing toxicity. Additional prospective trials evaluating efficacy and safety are warranted.