Ascending colon cancer with spontaneous regression of the primary tumor despite persistent isolated regional lymph node metastasis.

Spontaneous regression of cancer, particularly that of colorectal cancer, is rare. We encountered a rare case of spontaneous regression of ascending colon cancer with regional lymph node metastasis. A 70-year-old woman was referred for colonoscopy after a fecal immunochemical test yielded positive results. A colonoscopy revealed a 15-mm depressed lesion that was confirmed as adenocarcinoma. Regression of the primary tumor was observed over the course of multiple endoscopic examinations. Subsequently, robot-assisted laparoscopic right hemicolectomy and D3 lymph node dissection were performed. A histopathological examination revealed complete regression of the primary tumor. However, metastasis to a regional lymph node was observed. Immunohistochemistry indicated colon cancer with high microsatellite instability, infiltration of CD4-positive T cells, and predominant infiltration of CD8-positive T cells at the primary site and metastatic lymph node. Immunological studies of the regressed primary tumor and remaining lymph node metastasis instigated a discussion regarding spontaneous regression mechanisms. This case emphasizes the necessity for vigilant clinical management of similar cases because the potential for lymph node metastasis persists even when the spontaneous regression of the primary tumor is observed.