Abstract
BACKGROUND: Breast metastasis from extra mammary malignancies is rare. An incidence of 0.2%-1.3% has been reported in the literature, including that from different types of malignant neoplasms. CASE SUMMARY: We present a case of a 29-year-old nonsmoking woman with breast metastasis from lung adenocarcinoma. Computed tomography revealed atelectasis in the right middle lobe of the lung and ipsilateral pleural effusion. Additionally, on physical examination, a small mass was noted in her right breast. The patient underwent bronchoscopy, needle thoracentesis, and breast biopsy. Following cytology, histology and immunohistochemistry, primary lung adenocarcinoma with metastasis to the breast was diagnosed. Only 63 cases, including our patient, have been reported in the literature since 2000, and this is the second in a woman under 30 years of age. CONCLUSION: This atypical presentation may cause a significant diagnostic dilemma, but the contribution of immunohistochemistry is crucial to the accuracy of the final diagnosis.