The specimen is composed mostly of malignant plasma cells with abundant cytoplasm and eccentrically placed nuclei as well as binucleated forms (examples indicated by arrows) in a background of reactive mesothelial cells. The malignant cells are diffusely and strongly immunoreactive for CD138 and Mum1, with Lambda light chain restriction by ISH, confirming monoclonal proliferation; negative for epithelial markers (BER-EP4 and MOC31) and mesothelial marker (Calretinin). Flow cytometry results demonstrate the presence of a clonal population of cells which are immunophenotypically consistent with plasma cells neoplasm.

In view of the patient’s history of high-risk IgG Lambda multiple myeloma with multifocal bone involvement, the cytomorphology, immunoprofile, and flow cytometry are compatible with myelomatous pleural effusion (MPE).

Plasma cell myeloma/multiple myeloma (PCM/MM) is a bone marrow-based, multifocal neoplastic proliferation of plasma cells. The etiology of PCM/MM is unknown but is believed to represent a combined effect of genetic susceptibility and environmental exposures. PCM/MM rarely involves the serous cavities. The pleura is involved more commonly than the peritoneum or pericardium. Pleural effusion (PE) is not an uncommon finding, present in 6-14% of patients with PCM/MM, but only <1% of patients present as MPE. The most common cause of PCM/MM associated PE are congestive heart failure, renal failure with or without nephrotic syndrome, parapneumonic effusion, and amyloidosis. The diagnostic workup of MPE includes cytologic examination with ancillary studies (Immunohistochemistry, Kappa/Lambda ISH, etc.), flow cytometry, protein electrophoresis with immunofixation, pleural biopsy, and clinical features.