B80-5-03 Isolated Breast Metastasis From Small Cell Lung Cancer: A Rare Case of Atypical Extrathoracic Spread

Small cell lung cancer (SCLC) is a highly aggressive neuroendocrine tumor that typically arises centrally in individuals with a significant smoking history. It is characterized by rapid progression and early metastatic spread, most commonly to the mediastinal lymph nodes, liver, bones, adrenal glands, and brain.1 SCLC is also known to be associated with several paraneoplastic syndromes, including Cushing’s syndrome, Lambert-Eaton myasthenic syndrome, and syndrome of inappropriate antidiuretic hormone secretion (SIADH). Metastasis to the breast from SCLC is exceedingly rare, especially as an isolated site of metastases. Only sporadic cases have been reported in the literature, particularly as an isolated site of spread2. A 45-year-old female with a 30-pack-year smoking history presented with dyspnea. CT chest showed a right lower lobe mass measuring 5.7 cm and an adjacent right hilar component measuring 4.8 x 4.2 x 5.6 cm, with extensive hilar lymphadenopathy and bronchovascular encasement, concerning for malignancy. Bronchoscopy with endobronchial ultrasound-guided biopsy and partial debulking of the right main bronchus mass confirmed SCLC. Staging imaging revealed a suspicious mass in the left breast. Ultrasound of the left breast showed an irregular, hypoechoic, solid lesion with lobulated margins measuring 2.5 x 2.4 cm, without associated axillary lymph node involvement. MRI of the brain showed no additional sites of metastases, indicating the breast as an isolated extrathoracic site of disease. Core needle biopsy of the breast lesion confirmed metastatic small cell carcinoma, with immunohistochemical staining positive for TTF-1, synaptophysin, and chromogranin, supporting lung origin. The patient was subsequently started on systemic chemotherapy. This case highlights a rare instance of isolated breast metastasis from SCLC. Given the unusual site, new breast lesions in patients with lung cancer should raise the suspicion for metastases, even in the absence of widespread disease. Early biopsy is essential to delineate a (second) synchronous primary vs unusual site of metastases to help guide therapy, preventing misdiagnosis as a primary breast malignancy. This abstract is funded by: None