Long-term complete response of early gastric cancer to nivolumab: report of a case with disease-free survival for six years.

Although immune checkpoint inhibitors have demonstrated efficacy in advanced gastric cancer, reports of long-term complete remission in early-stage disease are lacking. We report a case of a 75-year-old man who presented with concurrent malignant melanoma and early gastric cancer (cT1bN0M0). Following surgical excision of melanoma with positive sentinel lymph node biopsy (T3bN1M0 Stage III), we prioritized treatment of advanced melanoma over early gastric cancer and initiated nivolumab adjuvant therapy every 2 weeks. After six cycles, treatment was discontinued due to drug-induced pneumonitis with elevated KL-6 levels. Repeat endoscopy performed 2 months after initial examination revealed partial tumor regression, with complete disappearance of the gastric lesion confirmed 4 months later. Histopathological examination showed no malignant findings. The patient achieved complete response of gastric cancer with only six cycles of nivolumab, despite negative PD-L1 expression (combined positive score 0%). During 6 years of follow-up, endoscopic and CT examinations consistently showed no gastric cancer recurrence, maintaining complete response. This case demonstrates sustained long-term efficacy of nivolumab against early gastric cancer, achieving 6-year disease-free survival with minimal treatment exposure. These findings suggest that immune checkpoint inhibitors may represent potential treatment options for patients with early gastric cancer contraindicated for surgery, even in PD-L1-negative cases.