Myelomatous Pleural Effusion: A Case Report.

Multiple myeloma is a relatively common hematologic malignancy; however, serous cavity effusions occur infrequently, and bona fide myelomatous effusions are exceedingly rare, typically representing an advanced, biologically aggressive variant associated with dismal outcomes. We report a compelling case of a 50-year-old patient with established multiple myeloma who developed leukemic transformation with concurrent pleural cavity infiltration while receiving systemic chemotherapy. The patient exhibited rapidly progressive dyspnea, a nonproductive cough, and debilitating fatigue in the days preceding admission. Cross-sectional imaging demonstrated a massive pleural effusion accompanied by partial ipsilateral pulmonary collapse. Cytopathologic examination of the pleural fluid revealed a predominance of atypical plasma cells and plasmablasts with marked dysmorphism, unequivocally confirming a myelomatous pleural effusion. Although hematologic neoplasms are well-recognized etiologies of body cavity effusions, the possibility of myelomatous involvement-albeit rare-must be judiciously entertained, as it heralds a distinct biologic trajectory and necessitates tailored therapeutic strategies that diverge substantially from the management of more common etiologies. This case underscores the need for heightened clinical vigilance and timely cytologic assessment in patients with multiple myeloma who present with new or unexplained serous effusions, as early identification of pleural involvement may influence both therapeutic decision-making and prognostication.