An unusual cytologic presentation of mesothelioma in serous effusions

Three cases of diffuse malignant mesothelioma in which samples of pleural fluid showed an unusual cytologic picture are presented. Instead of cells of an obvious mesothelial type forming organized clusters, the smears were dominated by foamy macrophage-like cells, with or without certain nuclear features suggesting malignancy. It is suggested that these cells were derived from neoplastic mesothelial cells by a process of differentiation.