Rarity Within Rarity: Complete Response to Chemoimmunotherapy in a Patient with Polymerase epsilon ( POLE)-Mutated locally advanced Jejunal Carcinoma

Background Jejunal carcinoma is an exceptionally rare malignancy with limited therapeutic options beyond chemotherapy. Polymerase epsilon (POLE) mutations in this tumor type is even rarer, yet it may represent a predictive biomarker for response to immune checkpoint inhibitors in jejunal cancer and agnostically. Case Presentation We report a 58-year-old male patient diagnosed with a POLE-mutated jejunal carcinoma. He presented with left-sided abdominal pain and was found to have an extremely large mass in the left upper quadrant. Biopsy confirmed jejunal adenocarcinoma. He was initially treated with two cycles of FOLFOX and, upon detection of the POLE mutation, nivolumab was added. After four cycles of chemo-immunotherapy, imaging demonstrated significant tumor regression. However, radiologic and clinical signs of subocclusion prompted treatment discontinuation and surgical resection. Histopathologic analysis revealed a complete pathological response, with no viable tumor cells. The patient is disease-free since January 2025 Conclusion This case underscores the role of immunotherapy in POLE-mutated tumors regardless of the site of origin and highlights the potential usefulness of molecular profiling in rare malignancies. In line with the agnostic approach used for microsatellite instability, molecular-driven clinical trials should be prioritized over histology-based studies to optimize treatment strategies for these orphan diseases.