O. Yariv1,2, A. Elamir1, S. Al Mutar3, S. Kazmi3, S. M. Beg3, M. Porembka4, N. N. Sanford1, P. Polanco4, S. N. Badiyan1, N. Rich5, R. Kainthla3, A. Jones3, A. Mallik3, J. Lohrey3, S. Cole3, R. D. Timmerman1, and T. A. Aguilera1; 1Department of Radiation Oncology, UT Southwestern Medical Center, Dallas, TX, 2Radiation Oncology Branch, National Cancer Institute, National Institutes of Health, Bethesda, MD, 3Department of Internal Medicine, Division of Hematology/Oncology, UT Southwestern Medical Center, Dallas, TX, 4Department of Surgery, University of Texas Southwestern Medical Center, Dallas, TX, 5Division of Digestive and Liver Diseases, Department of Internal Medicine, UT Southwestern Medical Center, Dallas, TX
Purpose/Objective(s): Stereotactic ablative radiotherapy (SAbR) is commonly used for the treatment of gastrointestinal (GI) cancers. However, proximity to organs at risk (OARs) or retreatment can limit the safe delivery of SAbR. Personalized Ultrafractionated Stereotactic Adaptive Radiotherapy (PULSAR) is a strategy to deliver SAbR in pulses separated by 1-4 weeks to allow for adaptation, lower risk to OARs, and leverage potential benefits of immune responses. Here we present our initial experience with PULSAR for the treatment of patients with GI cancer. Materials/
Methods: A single center retrospective study was conducted comprising all patients with GI cancer treated with PULSAR between 2020 and 2023. Eligibility for PULSAR treatment included retreatment after prior SAbR, palliation, treatment of high toxicity risk area, or continuation of systemic therapy during radiotherapy. Following an MR or CT simulation, patients were treated with 3 to 5 fractions administered at intervals of at least one week. Adaptive plans were created to preserve OAR constraints, account for change in tumor size, and/or escalate PTV dose, based on re-imaging for selective pulses. Treatment schedules, dosimetric features, and patient outcomes were reviewed. Results: 22 patients were treated with PULSAR, 73% for pancreaticobiliary cancer, and 50% for metastatic cancer across several sites (pancreas, colorectal, anus). Treatment intent was definitive in 64% (14). The most common indications for PULSAR were reirradiation after SAbR (n=8, 36%) and high toxicity risk due to adjacent OARs (n=8, 36%). Median total dose was 4000 cGy (range, 2400-5000), The median number of administered pulses was 5, at different schedules; monthly- 36% (8), biweekly- 32% (7), weekly- 27% (6), and every 3 weeks- 5% (1). Three patients were treated with online adaptive radiotherapy on MR-Linac and 5 patients with adaptive CT-Linac. The median PTV volume and margins were 37.67 cc (range, 15.7-1284.3) and 3 mm (range, 0-5), respectively. Concurrent systemic treatment was given to 36% (8), including chemotherapy (CHT) for 7 patients and immunotherapy for one patient. The median follow-up was 6.4 months. Two months after completion of treatment 18%, 35%, and 29% of evaluated patients had complete, partial response, and stable disease, respectively. Median overall survival (OS) was 20 months (95% CI), and progression-free survival (PFS) was 11.5 months (95% CI). Three patients (14%) progressed in the RT field within a median time of 11 months (range, 4-12). Acute grade 1-2 GI toxicity occurred in 18% of patients (4). One patient with pancreatic cancer and preexisting ulcer on anticoagulation had a grade 4 GI hemorrhage 2 months after treatment. Conclusion: The results of this patient series suggest that PULSAR used to treat GI cancers can safely deliver sufficient radiotherapy dose to achieve local control, especially in high-risk patients. Further prospective studies to validate these findings and better elucidate the benefits of PULSAR are warranted.
