A case of multiple advanced colon cancers with spontaneous regression of only one lesion after biopsy: a case report and literature review.

A 90-year-old man underwent a colonoscopy due to abdominal distension, revealing half-circumferential Type 2 advanced cancers in the ascending and transverse colon. No distant metastasis was detected, and 3 months later, laparoscopic extended right hemicolectomy was performed for both lesions. Pathological examination revealed ulcers and mucus retention in the ascending colon lesion without tumor components. A small number of signet ring cell-like tumor cells were found in the regional lymph node of the ascending colon, while well-to-moderately differentiated tubular adenocarcinoma was observed on the serosal surface in the transverse colon. Tumor regression was observed in the ascending colon cancer and lymph-node metastasis. Mismatch repair (MMR) protein immunostaining was conducted on biopsy tissues from both lesions. The ascending colon lesion showed weak positivity for MLH1, positivity for MSH2 and MSH6, and negativity for PMS2, indicating MMR deficiency, whereas the transverse colon lesion showed positivity for all of them, indicating MMR-proficient tumor. This is the first case report of multiple advanced colon cancers, where only one lesion exhibited spontaneous regression after biopsy, suggesting a potential link between MMR deficiency and the spontaneous regression of colorectal cancer.