[A Case of Unresectable Colon Cancer Treated with Pembrolizumab That Underwent Conversion Surgery].

The patient is a man in his 60s referred with a diagnosis of ascending colon cancer with right ureteral invasion, hydronephrosis, and right psoas muscle and nerve invasion. Complete resection in a single stage was deemed difficult during surgery, and chemotherapy was selected as the treatment plan. Given the MSI-H status, pembrolizumab was selected as the primary therapy. Twelve courses were administered. Following the normalization of tumor markers and imaging-based tumor shrinkage, improvement in ureteral and psoas muscle invasion was observed, enabling conversion surgery (right hemicolectomy). Although residual viable cells were observed infiltrating the submucosal layer and muscularis propria, the surfaces exposed to the subserosal and serosal layers were covered with necrotic cellular tissue, allowing for complete resection. More than 6 months after surgery, the patient continued to attend follow-up appointments without recurrence.