Cellular Classification of Malignant Mesothelioma
Histologically, these tumors are composed of spindle cells (sarcomatoid) or epithelial elements or both (biphasic). Desmoplastic mesothelioma, consisting of bland tumor cells between dense bands of stroma, is a subtype of sarcomatoid mesothelioma. The epithelioid form is occasionally confused with lung adenocarcinoma or metastatic carcinomas. Epithelioid tumors account for approximately 60% of mesothelioma diagnoses. Attempts to diagnose by cytology or needle biopsy of the pleura are often unsuccessful. It can be especially difficult to differentiate mesothelioma from adenocarcinoma on small tissue specimens. Thoracoscopy can be valuable in obtaining adequate tissue specimens for diagnostic purposes.
Examination of the gross tumor at surgery and use of special stains or electron microscopy can often help to determine diagnosis. Pancytokeratin stains are positive in nearly all mesotheliomas. Particularly useful immunohistochemical stains for the differential diagnosis of epithelioid mesothelioma include cytokeratin 5 and 6, calretinin, WT-1, and D2-40. Calretinin and D2-40 positivity in combination with pancytokeratin positivity is most useful to distinguish sarcomatoid mesothelioma from sarcoma and other histologies. Histologic appearance seems to be of prognostic value, and most clinical studies show that patients with epithelial mesotheliomas have a better prognosis than those with sarcomatoid or biphasic mesotheliomas.[3-5]References
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