Cytopathology of malignant mesothelioma of the pleura in fine-needle aspiration biopsy

The cytologic findings in fine‐needle aspiration biopsy (FNAB) of 9 histologically, immunohistochemically, and ultrastructurally confirmed malignant mesotheliomas of the pleura were reviewed. There were 4 epithelial malignant mesotheliomas (EMM), 3 sarcomatous mesotheliomas (SM), 1 mixed malignant mesothelioma (MMM), and 1 poorly differentiated malignant mesothelioma (PDMM). In FNAB, three EMMs yielded polygonal malignant cells with oval nuclei and prominent nucleoli, singly, in small sheets, and in tridimensional clusters with smooth or lobulated contours. In one EMM, only atypical mesothelial cells with prominent nucleoli were seen singly and in loose aggregates. In FNAB, two SM showed malignant spindle‐shaped cells (MSC) with scant, ill‐defined cytoplasm singly and in loose clusters. Cohesive clusters of elongated and polygonal cells with ill‐defined, clear cytoplasm and pleomorphic nuclei containing small or prominent nucleoli were observed in one SM, with extensive clear‐cell changes. In needle aspirates, the MMM showed polygonal malignant cells in clusters admixed with MSC. The PDMM displayed single and clustered large pleomorphic malignant cells, with prominent single or multiple nucleoli. Electron microscopic examination of minute tissue fragments retrieved from the FNAB in 2 cases of EMM revealed epithelial mesothelial cells with well‐formed desmosomes and long slender microvilli (LSM) on the free tumor cell surfaces. In one case, LSM in direct contact with collagen fiber bundles in the tumor tissue matrix were observed, supporting the diagnosis of an invasive EMM. Diagn. Cytopathol. 1999;21:253–259. © 1999 Wiley‐Liss, Inc.