Microcystic variant malignant mesothelioma presenting as a localized paraspinal mass.

A 58-year-old man presented with productive cough and fever. Computed tomography (CT) scan of the chest showed an upper right paraspinal mass. CT-guided fine-needle aspiration biopsy showed lobules of vacuolated cells against a background of myxoid material. The cells demonstrated moderate to severe nuclear atypia and occasional mitoses. Immunohistochemistry revealed tumor cells to be immunoreactive for calretinin, WT-1, D2-40, cytokeratin (CK) 7, AE1/AE3, high molecular weight keratin, vimentin and epithelial membrane antigen, and negative for thyroid transcription factor-1, Ber-EP4, carcinoembryonic antigen, S100 protein, CK20, and CDX2. The combined morphologic and immunohistochemical findings confirmed the diagnosis of microcystic variant of localized malignant mesothelioma. The subsequent lung resection showed a pleural-based mass in the right upper lobe and confirmed the diagnosis. Awareness of the existence of unusual morphologic variants and localized forms of mesothelioma are necessary to avoid misdiagnosis of fine needle biopsy samples. Recognition of characteristic cytomorphologic features along with optimal use of panel of immunohistochemistry studies is crucial for making a specific diagnosis.