[A Case of Stage Ⅳ Lung Adenocarcinoma with Bilateral Breast Metastases Mimicking Inflammatory Breast Cancer].

Breast metastases from primary lung cancers with inflammatory breast cancer (IBC)-like features are rare. Here, we report a patient with bilateral breast metastases from an advanced lung adenocarcinoma that mimicked IBC. A 67-year-old woman was diagnosed with right middle lobe lung adenocarcinoma with pleural and peritoneal dissemination (cT2aN2M1c, cStage ⅣB) and received systemic therapy. Twenty-nine months later, skin erythema and edema appeared in both breasts, suggesting bilateral IBC. Core needle biopsies of both breasts revealed atypical carcinomas without intraductal or lobular in situ components. Immunohistochemistry showed thyroid transcription factor-1 and Napsin A positivity and GATA3 negativity, supporting the diagnosis of metastatic lung adenocarcinoma of the breasts. Comprehensive genomic profiling of the breast specimen revealed an EGFR exon 19 deletion, and osimertinib achieved disease control within 1 year. This report highlights a diagnostic pitfall in which lung adenocarcinoma metastasis to the breast mimics IBC and underscores the importance of biopsy, an appropriate immunohistochemical panel, and molecular testing to establish a correct diagnosis and guide optimal systemic therapy.