Conversion surgery after pembrolizumab for initially unresectable MSI-H small bowel adenocarcinoma: a case report and brief analysis

Small-bowel adenocarcinoma (SBA) is rare and often diagnosed at an advanced stage. We report the case of a 61-year-old man with locally advanced unresectable upper jejunal SBA secondary to metastatic lymph nodes involving the superior mesenteric artery. Initial chemotherapy with FOLFOX (oxaliplatin, fluorouracil, and folinic acid) was initiated; however, a microsatellite instability-high (MSI-H) status was identified, and the treatment was promptly switched to pembrolizumab. After four cycles, marked regression of the metastatic lymph nodes was observed, and conversion surgery was performed. Partial jejunectomy with lymphadenectomy was performed to achieve an R0 resection. Pathological examination revealed a moderately differentiated adenocarcinoma with extensive fibrosis in the metastatic lymph nodes, indicating a substantial response to immunotherapy. The patient remained disease-free for 9 months postoperatively. Additionally, a brief meta-analysis of 10 studies comprising 72 patients with MSI-H/mismatch repair-deficient SBA revealed an objective response rate to immune checkpoint inhibitors of 65.3%. This case highlights the potential of pembrolizumab for the curative resection of an initially unresectable MSI-H SBA. Cancers of the small intestine are rare and are usually detected when surgery is often not possible. Some of these cancers have a feature called “microsatellite instability-high” (MSI-H), which may positively influence the response to immune-based drugs. A 61-year-old man had small intestinal cancer with lymph nodes stuck to a major blood vessel, making surgery unsafe. Standard chemotherapy was initiated; however, testing revealed that the cancer was MSI-H. The treatment was switched to the immunotherapy drug, pembrolizumab. Following four treatment cycles, the reduction in lymph node size enabled safe surgical resection. Part of the intestine and lymph nodes were removed, and the tests showed major shrinkage of the tumor. Advanced cancers that are initially considered unresectable may sometimes become operable and curable with immunotherapy, although only a small number of patients may benefit from this approach, and further investigation is required.