Ascitic Fluid Cytology Provides Diagnostic Clues in a Case of Gastric Metastasis from Invasive Lobular Carcinoma of the Breast

INTRODUCTION: Breast cancer metastasis to the gastrointestinal tract is rare, and invasive lobular carcinoma (ILC) shows a higher propensity for dissemination than invasive ductal carcinoma. Accurate diagnosis is often challenging because of nonspecific clinical and pathological findings. CASE PRESENTATION: A 53-year-old woman presented to our hospital with a diagnosis of gastric cancer discovered during gastrointestinal cancer screening. Fourteen years earlier, she had undergone a right mastectomy for ILC and subsequently received treatment for local recurrence and bone metastasis. The gastric lesion was initially diagnosed as primary gastric cancer, and adjuvant chemotherapy was administered after gastrectomy. However, postoperative ascites gradually worsened. Five years after gastrectomy, cytological examination of the ascites revealed tumor cells positive for GATA3 and estrogen receptor, and negative for E-cadherin, leading to a diagnosis of peritoneal metastasis from breast cancer.ILC is characterized by a loss of E-cadherin and diffuse metastatic patterns, including peritoneal and gastric involvement. This case highlights the diagnostic difficulty of differentiating primary gastric cancer from metastatic breast cancer. Although tumor markers and imaging suggested gastric cancer recurrence, cytological examination revealed otherwise. Ascitic fluid cytology is essential for definitive diagnosis, emphasizing its diagnostic value for patients with ambiguous gastric lesions and a history of breast cancer. CONCLUSIONS: Metastasis should be considered in the differential diagnosis of patients with prior breast cancer presenting with gastric lesions. Ascitic fluid cytology can be a useful adjunct for an accurate diagnosis, guiding appropriate treatment, and avoiding unnecessary interventions.