Invasive lobular breast carcinoma with diffuse gastrointestinal metastases: a visual diagnostic pitfall.

Invasive lobular carcinoma (ILC) of the breast demonstrates a distinct infiltrative growth pattern and a predilection for metastasis to the gastrointestinal tract, often without forming discrete masses. We report on a 64-year-old woman with a history of estrogen receptor-positive, human epidermal growth factor receptor 2-negative ILC who presented with progressive nausea, vomiting, and diarrhea. Recent surveillance imaging had shown no evidence of metastatic disease. Computed tomography revealed nonspecific findings despite extensive mucosal involvement that was later identified on endoscopy. Upper endoscopy and colonoscopy demonstrated diffuse nodular and congested mucosa throughout the duodenum and colon without focal lesions. Histologic evaluation revealed poorly cohesive tumor cells infiltrating the lamina propria, and immunohistochemistry was positive for cytokeratin 7, GATA3, and estrogen receptor, confirming metastatic breast carcinoma. Diffuse gastrointestinal involvement by ILC is uncommon and may mimic inflammatory, infectious, ischemic, or medication-related conditions, contributing to delayed diagnosis. This case highlights an important diagnostic pitfall and underscores the need to consider metastatic disease in patients with a history of lobular breast carcinoma who present with unexplained gastrointestinal symptoms, even in the absence of radiographically apparent disease.