Navigating diagnostic dilemmas toward precision therapy: a case report and literature review on gastric metastasis from breast cancer.

Gastric metastasis from breast cancer (GMBC) is a rare but diagnostically challenging condition, whose clinical and imaging features often mimic primary gastric cancer or treatment-related adverse effects. This study integrates a detailed case of a 54-year-old woman with Luminal B invasive lobular carcinoma who developed gastric metastasis during systemic therapy, with a systematic review of 35 recent cases (2019-2024) to delineate the clinical profile and management of GMBC. In the reported case, immunohistochemical analysis revealed phenotypic evolution, with hormone receptor expression shifting from ER 80%/PR 5% in the primary tumor to ER 30%/PR negative in the gastric metastasis. Literature synthesis identified invasive lobular carcinoma as the predominant histology (57.14%), abdominal pain as the most common symptom (54.29%), and highlighted the diagnostic utility of immunohistochemical markers-particularly GATA3 (positive in 71.43% of tested cases). Treatment remains primarily systemic, with endocrine therapy demonstrating survival benefit in hormone receptor-positive disease. We emphasize the need for heightened clinical suspicion in breast cancer patients with upper gastrointestinal symptoms and propose a structured diagnostic pathway centered on endoscopic deep biopsy and comprehensive immunohistochemical profiling. Re-biopsy to assess phenotypic evolution is crucial for guiding personalized therapy, while surgical intervention should be reserved for palliation of complications or selected cases of oligometastatic disease.