Nivolumab exerts therapeutic effects against metastatic lesions from early gastric adenocarcinoma with a small proportion of neuroendocrine carcinoma after gastrectomy: a case report.

Gastric neuroendocrine carcinoma (NEC) is an aggressive disease with high metastatic ability. Gastric cancer has intra-tumoral and intra-patient heterogeneity and may contain NEC. We discuss the case of a 75-year-old man who underwent distal gastrectomy for early gastric cancer. Tumor pathology revealed that nearly all of the tumor (> 95%) was well-differentiated adenocarcinoma, with NEC detected in a small area (< 5%) at the invasion front. No metastasis was identified in the dissected lymph nodes (LN). Multiple liver metastases were detected 3 months after surgery. The metastatic lesion was resistant to both chemotherapy regimens; namely, SOX (S-1 combined with oxaliplatin) and ramucirumab combined with paclitaxel. However, tumor regression was detected after nivolumab treatment. The tumor regression continued for 26 cycles (13 months). Irinotecan treatment was then administered. After 18 irinotecan treatment cycles for 11 months, the para-aortic LN rapidly enlarged. Following biopsy, the swollen para-aortic LN was diagnosed as the recurrence of NEC components of the gastric cancer. Treatment with carboplatin combined with etoposide has been effective and continued. We report a case of NEC para-aorta LN metastases from early gastric cancer with a low proportion of NEC (< 5%). A partial response of the distant metastatic lesions was observed with nivolumab therapy.