Synchronous right colon adenocarcinoma and sigmoid endometriosis mimicking a second primary carcinoma: a case report.

Intestinal endometriosis is a benign gynecological condition that frequently involves the rectosigmoid colon. Its synchronous occurrence with colorectal cancer is exceptionally rare and diagnostically challenging, particularly when mimicking a second primary malignancy. We report a case of a 48-year-old woman presenting with altered bowel habits. Colonoscopy revealed synchronous lesions: an adenocarcinoma at the hepatic flexure and a suspicious mass in the sigmoid colon. Repeated biopsies of the sigmoid lesion were inconclusive, ranging from dysplasia to chronic inflammation, raising concern for a second primary tumor. The patient underwent right hemicolectomy with partial sigmoidectomy. Intraoperative frozen section unexpectedly identified endometriosis, subsequently confirmed by definitive pathology and immunohistochemistry. At 12-month follow-up, the patient remained well with no evidence of recurrence.