Gastric and Bone Metastases Arising from Invasive Lobular Carcinoma With Ductal Carcinoma Occurring in one Breast Synchronously

Background/Aim: We report the case of a patient with gastric and bone metastases arising from an invasive lobular carcinoma of the breast coexisting with ductal carcinoma at the same time. Case Report: A 68-year-old woman with gastric and right costal tumors was referred to our hospital. Esophagogastroduodenoscopy (EGD) revealed irregular, slightly elevated lesions extending from the gastric body to the antrum, and biopsy specimens revealed a poorly differentiated adenocarcinoma. Furthermore, abdominal contrast-enhanced computed tomography (CT) revealed extensive wall thickening with homogeneous enhancement in the stomach. 18F-2-deoxy-2-fluoro-glucose positron emission tomography (FDG-PET) showed intense FDG uptake in the right mammary gland and right third rib. Moreover, fine-needle aspiration of the third right rib lump and the right breast mass lesion was performed, and subsequent pathological investigations revealed metastatic adenocarcinoma and invasive ductal carcinoma, respectively. Immunohistochemical examination revealed that estrogen receptor was strongly positive (>95%) in breast cancer and focally positive (<5%) in gastric cancer with bone metastasis. In addition, another right breast tumor was detected by breast magnetic resonance imaging (MRI), and biopsy revealed invasive lobular carcinoma that matched the histological findings of bone and gastric lesions, including immunohistochemical examination. The patient was treated with an aromatase inhibitor, a CDK4/6 inhibitor, and a receptor activator of nuclear factor-kappa B ligand (RANKL) monoclonal antibody. She showed no symptoms or disease progression at 9-month follow-up after the initiation of systemic drug treatment. Conclusion: Invasive lobular carcinoma can metastasize to the gastrointestinal tract, and new treatment developments are expected as more cases will accumulate in the future.