Ablative stereotactic MR-guided radiotherapy for >10 cm solid pseudopapillary tumor of the pancreas with liver oligometastases: a case report and literature review.

Background: Solid pseudopapillary tumors (SPTs) of the pancreas are rare, low-grade malignancies that generally have favorable outcomes when diagnosed at an early stage and can be resected. However, treatment options for patients with unresectable or metastatic disease are limited, and the role of radiotherapy (RT) is not well defined, especially for lesions near dose-limiting organs at risk (OARs). Advances in magnetic resonance-guided radiotherapy (MRgRT) can facilitate safe dose escalation for anatomically unfavorable tumors that may improve clinical outcomes. We report the use of dose-escalated MRgRT for >10 cm pancreatic SPT with liver oligometastases. Case summary: A 63-year-old female was incidentally found to have a 7-cm mass in the pancreatic body and three hepatic lesions with biopsy confirmation of SPT. Surgery was attempted after 3 months of chemotherapy but aborted intraoperatively due to extensive vascular involvement. She received four cycles of FOLFOX followed by definitive RT to the primary tumor that had enlarged to 10 cm. Given the tumor's size and extensive abutment of the stomach and bowel, she was treated on a 0.35-Tesla MR-Linac using a dose-painted, simultaneous integrated boost with 50.4 Gy in 28 fractions prescribed to the primary tumor and 75.6 Gy in 28 fractions prescribed to a central tumor volume. Concurrent chemotherapy was not used. One month later, two of the three liver metastases were treated with MRgRT to 40 Gy in five fractions. One sub-centimeter liver metastasis was observed due to concerns about cumulative liver dose, although due to eventual slight progression, it was treated 19 months later with the MR-Linac to 35 Gy in one fraction. All RT courses were well tolerated with no grade ≥2 toxicity. She is alive, asymptomatic, and without evidence of progression nearly 36 months after initiating RT and achieved a favorable radiographic response in all treated lesions. Conclusion: This case report demonstrates that dose-escalated MRgRT can treat anatomically unfavorable pancreatic SPT and achieve favorable long-term efficacy without significant toxicity. These data add to the growing body of evidence that MRgRT may significantly improve the therapeutic ratio over computed tomography (CT)-guided RT for complex abdominal tumors by facilitating tumor dose escalation while meeting OAR constraints.