Rectal metastasis originating from breast cancer: A rare case report.

Introduction: Gastrointestinal tract involvement in breast cancer is rarely encountered clinically. Data about this condition is limited and mostly from case reports. Case presentation: We report a case of rectal metastasis originating from breast cancer, which presents after a long-term latency of initial diagnosis. The patient had a history of diagnosis and treatment of stage II triple-negative breast cancer with mastectomy and adjuvant chemotherapy and radiotherapy. She showed no signs of recurrence up to eleven years, then presented with hematochezia and mild constipation. A rectal lesion was found on colonoscopy, which raise the initial suspicion of primary rectal cancer, but surprisingly, immunohistochemistry staining of the rectal specimen confirmed the origin of breast cancer. Clinical discussion: Breast cancer with rectal metastasis is very rare. Immunohistochemistry combined with medical history is essential for definitive diagnosis in this situation. Mammaglobin and GCDFP-15, CDX2, CK20, and CK7 help differentiate the origin from the breast or the rectum. Conclusion: Though breast cancer metastasizing to the rectum is a rare event, physicians should be aware of this differential diagnosis, even in patients with a remote history of breast cancer.