S3966 Colon Metastases From Breast Cancer

Introduction: Breast cancer is the most common malignancy worldwide, accounting for approximately 2.26 million new cases annually. Mortality is typically due to widespread metastases, which most commonly involve the bones, lungs, liver, and brain. Metastases to the colon is very rare, occurring in only about 0.2%-3% of patients with metastatic breast cancer. STK11 is a tumor suppressor gene strongly associated with Peutz-Jeghers Syndrome (PJS), a hereditary condition characterized by mucocutaneous pigmentation and gastrointestinal (GI) hamartomatous polyps. VUS (variant of uncertain significance) STK11 is a mutation of unknown pathogenicity regarding the risk for developing PJS. Patients with PJS have a high lifetime risk of developing GI cancer and require frequent screening. Case Description/Methods: A 46-year-old woman with a history of stage III triple negative invasive ductal carcinoma (IDC) and an STK11 variant of uncertain significance presented to an outpatient gastroenterology clinic for colon cancer screening at the request of her oncologist. Surveillance computed tomography of the abdomen and pelvis revealed recurrent lymphadenopathy in the abdomen, pelvis, and inguinal area which prompted the referral. The patient denied a history of prior colonoscopies, mucocutaneous macules, GI symptoms, and a family history of GI malignancy and PJS. She later underwent colonoscopy which showed scattered ulcerated and occasionally necrotic lesions with heaped-up edges. Biopsies of the lesions revealed poorly differentiated metastatic carcinoma from breast primary which was positive for TRPS1, and negative for SATB2, ER, PR, and HER2. She was then started on paclitaxel and pembrolizumab therapy. Discussion: Colon metastases from primary breast cancer is very rare, and is most often associated with IDC. When metastasizing to the GI tract, breast cancer will more likely spread to the stomach or small bowel. This case highlights an unusual presentation of triple-negative IDC metastasizing to the colon. It also raises consideration for lowering the threshold for colonoscopic screening in patients with VUS STK11, particularly in the setting of a concurrent non-GI malignancy. While VUS mutations are not typically acted upon, this case highlights a population to potentially lower screening thresholds for.