S3963 Metastatic Misroute: When Breast Cancer Finds the Colon

Introduction: The gastrointestinal (GI) tract is an uncommon site for breast cancer metastasis, with colon involvement being particularly rare. These metastases often present diagnostic challenges due to vague symptoms, delayed onset, and mimicry of other conditions. We present 3 cases of colonic metastases from primary breast cancer, alongside a literature review. Case Description/Methods: Case 1 is a 79-year-old woman with hormone receptor-positive (HR+) invasive lobular carcinoma, status post-mastectomy and aromatase inhibitor therapy, presented with nonspecific gastrointestinal symptoms. Despite previously negative screening colonoscopies, a rectal mass and widespread metastases were identified. Biopsy confirmed metastatic lobular breast carcinoma. Case 2 is a 74-year-old woman reported weight loss and dyspnea. Imaging revealed stage IV HR+ invasive ductal breast carcinoma and a concurrent stage IIA colon adenocarcinoma. A hemicolectomy revealed metastatic breast cancer in the colon. Case 3 is an 85-year-old woman with a history of HR+ lobular breast cancer presented with bowel obstruction. Hemicolectomy pathology revealed metastatic lobular carcinoma. Immunohistochemistry (IHC) showed markers GATA-3+, ER+, PR+, and HER2+, confirming breast origin. Discussion: Breast cancer is the most commonly diagnosed cancer in women, with mortality often due to metastasis. GI tract involvement is rare and primarily linked to invasive lobular carcinoma. Metastases may emerge years after the initial diagnosis, typically presenting with nonspecific GI symptoms that can be misattributed. Imaging alone may be insufficient; definitive diagnosis often requires endoscopy or biopsy. IHC is vital, distinguishing metastatic breast cancer from primary colorectal cancers by identifying markers like HR and GCDFP-15. Clinicians should consider GI metastases in patients with prior breast cancer who present with new GI symptoms, regardless of latency. Early recognition and diagnosis are critical for appropriate management.