Initial Outcomes and Treatment-Associated Toxicities Using Stereotactic MRI-Guided Adaptive Radiation Therapy for Pancreatic Cancer Patients Treated at A Single Institution

High dose stereotactic body radiotherapy (SBRT) may offer improved outcomes for pancreatic cancer patients, but treatment delivery remains a challenge due to tumor proximity to organs-at-risk (OAR) and daily anatomic variability. Stereotactic MRI-guided adaptive radiation therapy (SMART) with real-time tumor tracking allows for improved dose delivery and OAR sparing by accounting for anatomic changes with each treatment. This study reports initial outcomes, treatment-associated toxicities, and dosimetric outcomes in locally advanced, metastatic, and/or recurrent pancreatic cancer patients treated using SMART. We performed a retrospective analysis of locally advanced, metastatic, and/or recurrent pancreatic cancer patients who underwent adaptive planning with MRI-guided radiation treatments. Charts were reviewed for follow-up, imaging, and laboratory values, including CA19-9 and absolute lymphocyte count. A plan was generated for each patient after initial CT and MR-simulation. At each treatment, MR imaging was obtained for the daily anatomy and the initial (unadapted) plan was compared to obtain a predicted dose to target volumes and OARs using a prescribed dose ranging from 30-50 Gy in 3-5 fractions. If constraints were not met or coverage was inadequate, an adaptive plan was generated based on the anatomy imaged on the day of treatment. From June 2018 to February 2020, a total of 23 patients were analyzed. With a median follow up of 5 months, we observed 15/19 (79%) with local control, 12/19 (63%) distal metastatic control, and a median overall survival of 19 months from cancer diagnosis. We observed no grade 3 or higher treatment related toxicities. Two patients were found to have an absolute lymphocyte count less than 0.5 x 103 cells/μl within six months of radiation, and 11/17 (65%) had lower CA19-9 six months after radiation treatment compared to before treatment. Of the 23 patients treated, there were 71 adaptive plans available for analysis. Compared to unadapted plans, adaptive fractions had improved coverage of PTV (84.94% vs. 73.36% at 100% of prescribed dose) and CTV (94.62% vs 88.08% at 95% of prescribed dose). Unadapted plans were more likely to exceed OAR constraints with 63 of 71 (89%) unadapted fractions predicted to violate prescribed duodenal, stomach, or small bowel constraints. Moreover, 34/71 (48%) of unadapted fractions violated constraints for 2 of 3 abdominal OARs and 10/71 (14%) violated constraints for all 3 compared to none for adapted fractions. Stereotactic adaptive MRI-guided radiotherapy demonstrated minimal treatment-related toxicities, enhanced tumor volume coverage, and improved adherence to OAR constraints in pancreatic cancer patients. Additional follow up is needed to further assess outcomes of these patients.