On biopsy, a characteristic finding of malignant mesothelioma is -

Most malignant mesothelioma (50% to 60%) are of the epithelial type, approximately 10% are sarcomatoid and the rest are biphasic malignant mesotheliomas. The diagnosis of malignant mesothelioma should be confirmed by immunohistochemistry, which shows that the tumor cells are diffusely positive for pankeratin, keratin 5/6, calretinin and WT1, and negative for the epithelial markers CEA, CD15, Ber-EP4, Moc-31, TTF- 1, B72.3 and so forth. For epithelial mesotheliomas, positive staining for pankeratin and calretinin and negative staining for three epithelial markers are considered sufficient for diagnosis. Occasionally, however, more stainings are required, because some carcinomas are positive for calretinin or are negative for some of the epithelial markers. In these difficult cases, electron microscopy showing the classic long-branching microvilli of human mesothelial cells compared to the short nonbranching microvilli of carcinomas can still be considered the gold standard for correct diagnosis.