S4091 The Breast Is Not History: Exploring a Rare Case of Metastatic Breast Cancer (MBC) to the Rectum

Introduction: Breast cancer, a leading cause of cancer deaths for women, commonly metastasizes to bone, liver, and lung. Despite treatment advances, there is significant mortality associated with metastatic breast cancer (MBC). Metastases to the luminal gastrointestinal (GI) tract is rare and varies by subtype, commonly affecting the stomach or small intestine. Rectal metastasis is particularly uncommon, with few reported cases. Targeted systemic therapy initiation for luminal GI metastases has shown a positive effect on survival. We describe a rare case of MBC to the rectum. Case Description/Methods: A 63-year-old woman with a history of stage IIIB breast cancer (invasive ductal carcinoma, estrogen receptor (ER) and progesterone receptor (PR) positive, diagnosed 4 years prior with lymph node involvement status post double mastectomy, chemotherapy, and radiation, on anastrozole) presented to GI clinic with generalized abdominal pain, non-bloody diarrhea, and fecal urgency for 3 months. Colonoscopy 2 years prior only showed external hemorrhoids. Labs were unremarkable. Computerized tomography with contrast showed diffuse circumferential rectal/anal canal thickening (1 cm thickness) with mucosal enhancement. Flexible sigmoidoscopy showed erythematous, edematous, and mildly nodular mucosa in the distal rectum, 15 cm from the anal verge. Rectal biopsies revealed adenocarcinoma, with findings (including ER positivity) suggesting metastases from a breast primary. Anastrozole was switched to fulvestrant (ER antagonist) and ribociclib (cyclin dependent kinase 4/6 inhibitor, held 3 months later due to rash and drug related pneumonitis). Surgical consultation was obtained, with no recommended intervention. On follow up, her symptoms, particularly fecal urgency, had improved. Discussion: This case emphasizes the need for thorough evaluation of new GI symptoms in breast cancer survivors given treatment advancements have further increased life expectancy. Although rare, GI metastases should not be overlooked, as early detection and treatment can improve outcomes. Clinicians should maintain a low threshold for biopsy, as mucosal irregularities may represent metastases. In our case, presentation was thought to be from inflammation or infection. Biopsy results showing breast cancer metastases was unexpected given prior treatment and maintenance on anastrozole. The histologic findings guided transition to targeted systemic therapy, which highlights a new role of endoscopy in guiding MBC treatment.