[A Case of MSI‒High Advanced or Recurrent Gastric Cancer Showed Complete Response to Pembrolizumab as Third‒Line Treatment].

A 52‒year‒old female with gastric cancer revealed by a medical examination. The patient was diagnosed with T4b(pancreas)N2M0, CY0P0, cStage ⅣA, and after 3 courses of docetaxel+oxaliplatin+S‒1(DOS)therapy as preoperative chemotherapy, laparoscopic distal gastrectomy with D2 lymph node dissection and Roux‒en‒Y reconstruction were performed. Pathological histological examination showed ypT3(SS)N2M1(CY+), Stage Ⅳ, HER2(-), and S‒1+oxaliplatin(SOX)therapy was initiated as first‒line treatment. Tumor markers elevated and CT scan showed ascites after 5 courses and the treatment was consequently changed to paclitaxel+ramucirumab(PTX+Ram)therapy as second‒line treatment. CT scan showed para‒aortic lymph node enlarged and increased ascites after 2 courses, MSI‒High was identified at the same time and the treatment was consequently changed to pembrolizumab(Pem)monotherapy(200 mg/body, q21 days)as third‒line treatment. CT scan showed partial response(PR)after 3 courses, and subsequently resulted in a complete response(CR). After 36 courses of Pem monotherapy, the chemotherapy was terminated at the patient's request. Four years after the initial onset, the patient is currently alive without recurrence. No serious adverse events have occurred since the start of Pem monotherapy.