Cytomorphology of metastatic mesothelioma in fine-needle aspiration specimens

The morphologic features of mesothelioma occurring in serous fluids, as well as fine-needle aspiration (FNA) of primary tumors, have been extensively described. Descriptions of the FNA findings in metastatic lesions of mesothelioma, however, are limited to single cases. In this report, we illustrate the cytomorphologic features of four cases of metastatic mesothelioma which underwent FNA, and one additional cytologic preparation taken from a surgically-excised metastatic lesion. The cytomorphologic features of metastatic mesothelioma varied greatly among individual cases with respect to the degree of nuclear pleomorphism, amount of cytoplasm, and degree of cellular cohesion. However, one feature which was consistently seen was the presence of numerous distinct, uniformly small intracytoplasmic vacuoles, believed to represent intracellular fat and glycogen. These morphologic features are illustrated, and the differential diagnosis of intracytoplasmic vacuoles within epithelioid tumor cells in primary and metastatic lesions is discussed.