[A Case of Advanced Recurrent Colorectal Cancer with Complete Response to Pembrolizumab Chemotherapy].

The efficacy of pembrolizumab has been demonstrated for all solid tumors showing high frequency microsatellite instability- high(MSI-High). It is a possible treatment option even in cases which do not respond to other forms of chemotherapy. We report a case of a 69-year-old man with MSI-High recurrent colorectal cancer with complete response(CR)after pembrolizumab therapy. Sigmoidectomy, extensive lymph node dissection, and partial bladder resection were performed for sigmoid colon cancer at another hospital. Histopathological examination revealed a T4a, N0, M0, Stage Ⅱb tumor. Six months after the operation, Hartmann's operation and partial resection of the small intestine were performed for local recurrence. However, the tumor invading the retroperitoneum was unresectable. Postoperative SOX therapy was performed, but it was discontinued due to Grade 3 diarrhea during the first course. The laboratory test showed MSI-High during the first course. Pembrolizumab chemotherapy was introduced as second-line therapy. Computed tomography examination after 2 courses (6 weeks)revealed reduction in the major axis of the tumor by 30% or more. After 4 courses(12 weeks), the tumor was further reduced, and a partial response(PR)was diagnosed. The tumor completely disappeared after 6 courses, and a complete response was achieved after 8 courses. The CR has been maintained for about 7 months.