C78-45 Male Breast Cancer With Pulmonary Metastases and Malignant Pleural Effusion: A Case of Advanced Endocrine-responsive Disease

Introduction Male breast cancer (MBC) is an uncommon malignancy, accounting for less than 1% of all breast cancers. Most tumors are invasive ductal carcinomas and hormone receptor-positive, often diagnosed at advanced stages due to low clinical suspicion. Although pulmonary metastases are a recognized feature of disseminated disease, malignant pleural effusion (MPE) is rarely reported in men. We describe a case of biopsy-proven metastatic MBC complicated by MPE after prolonged stability with sequential endocrine and targeted therapy, culminating in the development of MPE. Case Report An elderly man with a history of atrial fibrillation and type 2 diabetes mellitus presented with a retroareolar breast mass and nipple retraction. Imaging revealed a right breast lesion with axillary lymphadenopathy. Biopsy confirmed invasive ductal carcinoma, estrogen and progesterone receptor positive, HER2 and BRCA1/2 negative. The patient declined surgery and was started on tamoxifen, achieving metabolic response on follow-up PET-CT.After two years, disease progression was observed with a hypermetabolic pulmonary nodule. CT-guided biopsy confirmed metastatic breast carcinoma with glandular morphology, GATA3 positivity, and TTF-1 negativity, consistent with breast origin. Treatment was transitioned to fulvestrant combined with palbociclib, a CDK4/6 inhibitor. Despite hematologic toxicity necessitating dose adjustment, the patient maintained durable partial response and clinical stability.Approximately one year later, he developed progressive dyspnea and cough. Chest radiography demonstrated near-complete opacification of the left hemithorax. Thoracentesis drained over one liter of bloody exudative fluid. Cytology and immunostaining were GATA3 positive, confirming malignant pleural effusion. His course was complicated by recurrent pulmonary infections and frailty, prompting transition to hospice following multidisciplinary goals-of-care discussions. Discussion This case highlights the therapeutic complexity and evolving management of advanced MBC. Tamoxifen remains the cornerstone of treatment for hormone receptor-positive MBC and achieved durable disease control in this patient. The addition of CDK4/6 inhibition with fulvestrant reflects the expanding role of targeted therapy, which has demonstrated benefit in men despite limited trial inclusion. The subsequent development of MPE marked terminal progression, emphasizing both the limitations of systemic therapy and the importance of palliative integration. Few reports document MPE in MBC, underscoring the need to recognize pleural involvement as a late manifestation of disease.Documentation of such cases enhances understanding of MBC’s clinical course and supports continued refinement of evidence-based treatment strategies for this rare malignancy. This abstract is funded by: None