Abstract
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.