Gastric Metastasis with Lobular Phenotype from a Primary Invasive Ductal Breast Carcinoma: A Case Report

Introduction: Breast cancer is a leading cause of cancer-related mortality in women, with common metastatic sites including bone, lung, liver, and brain. Metastasis to the gastrointestinal tract, particularly the stomach, is a rare occurrence, with a reported clinical incidence of 0.2-0.7%. Its nonspecific symptomatology and heterogeneous endoscopic presentation often lead to diagnostic confusion with primary gastric cancer or benign conditions. Case Presentation: We report the case of a 51-year-old woman with a history of Stage IIIC invasive ductal breast cancer, treated nine years prior with multimodal therapy. She presented with a six-month history of severe abdominal pain, significant weight loss (15 kg), vomiting, and diarrhea, initially managed as a Helicobacter pylori-associated gastric ulcer without improvement. Upper endoscopy revealed a large, raised-margined ulcer. Biopsy and immunohistochemical analysis were pivotal, showing a profile positive for CK7, GATA3, and Estrogen Receptors, and negative for E-cadherin, confirming metastatic breast carcinoma. Staging CT scans showed no other metastatic disease. The patient was initiated on systemic hormonal therapy with an aromatase inhibitor combined with a CDK4/6 inhibitor. Conclusion: This case underscores that gastric metastasis should be considered in breast cancer survivors presenting with persistent upper GI symptoms, even many years after initial remission. It highlights the critical role of immunohistochemistry in achieving a correct diagnosis, which is essential for guiding appropriate systemic therapy and avoiding unnecessary surgical intervention. A multidisciplinary approach is paramount for managing this rare manifestation of advanced breast cancer.