Spontaneous Regression of Poorly Differentiated Carcinoma in the Transverse Colon with Deficient Mismatch Repair: A...

Spontaneous regression (SR) of colorectal cancer (CRC) is exceptionally rare. The SR of malignant tumors occurs in approximately 1 in 80000-100000 cases, and CRC accounts for <2% of all spontaneous malignancy regressions. Recent studies have suggested that immunological mechanisms, particularly those related to deficient mismatch repair (dMMR) and high-frequency microsatellite instability, may play an important role in such tumor regressions.

A 68-year-old woman was referred to our hospital after a positive fecal occult blood test. Colonoscopy revealed a 12-mm nonpolypoid lesion (IIa + IIc) in the transverse colon. Biopsy specimens showed poorly differentiated carcinoma without glandular formation or mucin production, accompanied by marked tumor-infiltrating lymphocytes (TILs). Immunohistochemistry was negative for CK20, CDX2, and neuroendocrine markers. The majority of TILs were CD3-positive lymphocytes. CT revealed no lymph node involvement or distant metastasis (cT1bN0M0). Laparoscopic partial colectomy of the transverse colon with D3 lymphadenectomy was performed 50 days after biopsy. Macroscopically, the resected specimen showed only a small scar-like lesion, and histological examination revealed no residual carcinoma. All dissected lymph nodes were tumor-free. Additional immunohistochemical analysis of the biopsy specimen showed loss of MLH1 and PMS2 expression, consistent with a dMMR status. These findings highlighted the possibility of medullary carcinoma, but a definitive diagnosis was not possible due to the limited biopsy samples. The postoperative course was uneventful, and no recurrence was observed during the 18 months of follow-up period without adjuvant therapy.

We report an extremely rare case of SR of poorly differentiated CRC with dMMR and marked TILs. Enhanced tumor immunogenicity associated with dMMR and immune activation may contribute to CRC regression.