| Summary: | There are few reports of conversion surgery (CS) after nivolumab monotherapy because it is considered as a third-line standard chemotherapy for unresectable or recurrent gastric cancer. Here, we report a rare case of stage IV gastric cancer effectively treated with CS after nivolumab monotherapy as a third-line chemotherapy. A 73-year-old man was referred to our hospital with loss of appetite and abdominal discomfort. Stage IV gastric cancer with liver metastasis was diagnosed via upper gastrointestinal endoscopy and CT. Twelve courses of capecitabine, cisplatin, and trastuzumab were administered as the first-line treatment, 25 courses of paclitaxel plus ramucirumab as the second-line treatment, and 31 courses of nivolumab monotherapy as the third-line treatment. After 31 courses of nivolumab monotherapy, CT showed that the primary tumor shrank with no liver metastasis or ascites. Diagnostic laparoscopy was performed with no peritoneal dissemination (P0), and the peritoneal lavage cytology was negative (CY0). CS was performed with total gastrectomy and D2 lymph node dissection (R0 resection). The pathological diagnosis was U, Ant-Less, Type 2, 70 × 63 mm, poorly differentiated adenocarcinoma (ypT3N0M0 ypStage IIA). R0 resection was performed, and the histological response was grade 1a. The patient did not show recurrence for 9 months after CS.
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