A case of invasive lobular breast cancer mimicking occult gastric metastasis and atypical paraneoplastic syndrome eight years postoperatively.

[BACKGROUD] Breast cancer is the most common malignant tumor in women, and postoperative recurrence and metastasis significantly impact prognosis. Occult gastric metastasis with a long latency after breast cancer surgery is extremely rare. We report a case of occult gastric metastasis occurring 8 years after breast cancer surgery, emphasizing the importance of monitoring for such conditions postoperatively.

[CASE SUMMARY] A 57-year-old woman underwent a modified radical mastectomy for left breast cancer 8 years ago. Postoperative pathology confirmed invasive lobular carcinoma, followed by six cycles of chemotherapy and 8 years of endocrine therapy. She previously maintained good health but developed nausea, belching, and reduced appetite 5 months prior without obvious cause. PET/MRI revealed no significant gastric wall thickening, enhancement, or abnormal metabolism. Gastroscopy showed multiple nodular protrusions in the stomach and chronic superficial gastritis. Biopsies from multiple gastric sites indicated (gastric body) signet-ring cell carcinoma. Detailed gastroscopy revealed scattered irregular nodules and discolored lesions in the gastric fundus and body, making surgical resection margins difficult to define. Considering her history and pathology, laparoscopic radical total gastrectomy with Roux-en-Y esophagojejunostomy was performed. Gross examination of the specimen revealed multiple shallow mucosal ulcers. Immunohistochemistry demonstrated strong positivity for GATA-3, confirming metastatic carcinoma of breast origin.One year following the surgical treatment for gastric metastatic tumor, the patient developed unexplained weakness and numbness in both lower extremities, rendering them unable to ambulate.Based on the comprehensive clinical assessment, atypical paraneoplastic neurological syndrome secondary to malignant tumors was highly suspected.Due to financial constraints, the patient and their family declined intravenous immunoglobulin therapy. As of April 2025, the patient has not received any specific treatment.

[CONCULSION] This case highlights that occult gastric metastasis with a prolonged latency after breast cancer surgery may evade detection during routine follow-up. It aims to raise clinical awareness and provide reference for future practice.