Regression-like Disappearance of Microsatellite-stable Rectal Adenocarcinoma before Surgery: A Case Report

Spontaneous regression of colorectal cancer is extremely rare and has most often been reported in mismatch repair-deficient or microsatellite instability-high tumors.Reports involving microsatellite-stable (MSS) colorectal cancer remain scarce.We describe a 76-year-old female with biopsy-proven rectal adenocarcinoma in whom no residual carcinoma was found after radical resection.She presented with hematochezia, and colonoscopy revealed a mass 15 cm from the anal verge.Biopsy showed well-to moderately differentiated adenocarcinoma.Abdominopelvic computed tomography demonstrated focal rectal wall thickening, and the lesion was clinically staged as cT2 or lower.Approximately 1 month later, robot-assisted low anterior resection was performed.No gross residual lesion was identified in the specimen, and intraoperative colonoscopy showed no remaining tumor.Final histopathologic examination revealed no residual carcinoma and no metastasis in 10 regional lymph nodes.Polymerase chain reaction-based microsatellite instability testing of the initial biopsy specimen confirmed MSS status.This case illustrates a rare regression-like disappearance of biopsy-proven MSS rectal adenocarcinoma before surgery, suggesting that this phenomenon may occur even in MSS tumors.