S3668 Rectal Metastasis of Invasive Lobular Carcinoma Mimicking Neuroendocrine Tumor: A Diagnostic Challenge

Introduction: Breast cancer often metastasizes to lymph nodes, bone, brain, liver, and lungs, while gastrointestinal tract (GIT) metastasis is rare. The stomach is the most frequent GIT site; rectal metastasis is extremely rare. Distinguishing metastatic lesions from primary colorectal cancer is crucial as treatment and prognosis differ. Case Description/Methods: A 49-year-old woman with no significant past medical history presented with 5 months of worsening GI symptoms: constipation, urgency, mucus discharge, rectal bleeding, stool leakage, and a 20 lb unintentional weight loss. She reported increased stool frequency up to 20 times/day. Exam revealed a friable circumferential rectal mass causing significant stenosis. Colonoscopy was aborted due to inability to pass the mass. Biopsy initially suggested a neuroendocrine tumor based on patchy synaptophysin positivity. Forty-five days later, she returned with severe abdominal pain and distension. Imaging showed bowel obstruction, and she underwent transverse loop colostomy. One liter of ascitic fluid was aspirated. Cytology showed tumor cells in an “Indian file” pattern, suggestive of metastatic invasive lobular carcinoma (ILC) of the breast. Rectal biopsy staining was re-evaluated, showing strong positivity for CK7, GATA3, and ER, with weak mammaglobin and PR positivity, and negativity for CK20, CDX2, TTF-1, PAX8, and neuroendocrine markers, refuting the initial diagnosis. Further workup revealed bilateral suspicious breast lesions. Biopsies confirmed grade 2 ILC, ER 100%, PR 40%, HER2 negative. This case underscores a rare presentation of metastatic ILC to the rectum, mimicking a primary GI tumor. Careful histopathologic review was essential for accurate diagnosis. Discussion: Given ILC’s relapsing nature, bowel metastasis should be considered in patients with GI symptoms or positive fecal occult blood, especially with nodal involvement. The mechanism may involve E-cadherin loss and lobular cell morphology aiding GIT entrapment. computed tomography and MRI may show mural thickening or a linitis plastica-like pattern. Endoscopically, lesions can mimic colorectal cancer or Crohn’s disease. Immunohistochemistry is key: CK7+/CK20−, ER+, GATA3+, GCDFP-15+, and mammaglobin+ suggest breast origin. Treatment strategies differ, and ILC should be viewed as a chronic disease with potential for late GI relapse. This case highlights the value of long-term follow-up even during remission.