Occult Metastatic Invasive Lobular Breast Carcinoma Presenting as Gastric Outlet Obstruction and Peritoneal...

Invasive lobular carcinoma (ILC) of the breast is a common type of breast cancer and is known to have a distinct metastatic pattern, with a predilection for the gastrointestinal tract and peritoneum rather than the lungs and liver. Gastric outlet obstruction (GOO), as the initial manifestation of occult metastatic breast carcinoma, is rare and may closely mimic primary gastrointestinal malignancy. A 67-year-old woman presented with progressive nausea, vomiting, poor oral intake, weight loss, and symptoms consistent with GOO. Computed tomography demonstrated gastric wall thickening, ascites, and peritoneal abnormalities concerning for primary gastric malignancy. Initial endoscopic biopsies of the pyloric channel revealed erosive chemical gastropathy without evidence of malignancy. Given the high clinical suspicion, endoscopic ultrasound-guided fine-needle aspiration (EUS-FNA) was performed, which demonstrated adenocarcinoma. Immunohistochemistry showed positivity for CK7, GATA3, and TRPS1, with negativity for CK20 and CDX2, favoring a breast primary. Subsequent diagnostic laparoscopy with omental, peritoneal, and mesenteric biopsies confirmed metastatic carcinoma with attenuated E-cadherin expression, consistent with invasive lobular breast carcinoma. Despite extensive biopsy-proven peritoneal disease, peritoneal fluid cytology was negative for malignant cells. Biomarker analysis demonstrated estrogen receptor positivity (91%-100%), progesterone receptor positivity, HER2 negativity (1+), and a Ki-67 of 10%-15%. This case highlights several diagnostic challenges in metastatic ILC, including false-negative mucosal biopsies, negative peritoneal cytology despite extensive carcinomatosis, and the need for deeper tissue sampling. Recognition of contemporary immunophenotypic markers, including GATA3 and TRPS1, with attenuated E-cadherin expression, is essential for distinguishing metastatic breast carcinoma from primary gastrointestinal malignancies and for facilitating appropriate systemic therapy.