[A case of metastatic gastric cancer showing long-term control with nivolumab after pseudoprogression].

The Japanese guidelines for the treatment of gastric cancer recommend nivolumab as third-line chemotherapy for metastatic gastric adenocarcinoma. We report a case in which long-term control of metastatic gastric adenocarcinoma was achieved with nivolumab after pseudoprogression. A man in his late 70s with advanced HER2-negative gastric cancer and liver metastasis underwent total gastrectomy to control tumor bleeding. He then underwent chemotherapy with S-1 plus oxaliplatin, followed by S-1 alone. After metastases in the liver and para-aortic and hilar lymph nodes regrew, the patient received ramucirumab plus paclitaxel as second-line chemotherapy, followed by third-line therapy with nivolumab. After four cycles of nivolumab, these metastases showed progression;however, the treatment was continued because levels of CA19-9 were decreased, and performance status was good. After five more cycles of nivolumab, the liver metastasis shrank, and CA19-9 levels decreased;therefore, we confirmed pseudoprogression. The patient suffered no immune-related adverse events and survived for 50 months after gastrectomy with 34 cycles of nivolumab treatment. Thus, at the beginning of treatment with an immune checkpoint inhibitor, oncologists must consider the possibility of pseudoprogression in cases of tumor growth associated with decreasing tumor marker levels and good performance status.