Pathological Complete Response of Clinical T4b Ascending Colon Cancer after Preoperative Chemotherapy Using Pembrolizumab

Preoperative chemotherapy is efficacious in several cancers. However, it is not an established treatment for locally advanced colon cancer, particularly cases with microsatellite instability-high (MSI-H)/deficient mismatch repair. Herein, we report a case of pathological complete response of MSI-H clinical T4b ascending colon cancer to preoperative treatment with pembrolizumab. A 78-year-old man was diagnosed with ascending colon cancer that invaded into the iliacus muscle and enlarged regional lymph nodes. The tumor was classified as T4bN1bM0 stage IIIC according to the 8th Union for International Cancer Control guidelines, with MSI-H status. Based on our initial diagnosis, this tumor could not be resected completely. Thus, the patient underwent preoperative therapy with CAPOX (capecitabine and oxaliplatin combination) plus bevacizumab. After 4 cycles of preoperative CAPOX/bevacizumab, we observed tumor reduction corresponding to a partial response based on the Response Evaluation Criteria in Solid Tumors criteria. Nevertheless, tumor invasion of the iliacus muscle persisted. Since oxaliplatin-induced peripheral sensory neuropathy was observed, we discontinued treatment with oxaliplatin and changed the regimen to pembrolizumab in anticipation of the therapeutic effect of this immune checkpoint inhibitor against MSI-H tumors. After 2 cycles of therapy with pembrolizumab (200 mg/body on day 1 every 3 weeks), there was drastic tumor regression. In addition, computed tomography indicated that all lymph node metastases had disappeared. Therefore, the patient underwent laparoscopic right hemicolectomy with D3 lymph node dissection. Analysis of the resected specimen showed pathological complete response.