Histology

Malignant mesothelioma is a relatively rare but unique neoplasm of the pleura and other serosal surfaces. During the past 50 years, mesothelioma has been the subject of numerous epidemiologic, clinical, experimental, and pathologic studies. Pathologic diagnosis of mesotheliomas can be accomplished at several levels during the pathologic evaluation, beginning with gross and microscopic examination and extending through immunohistochemical, molecular, and electron microscopic confirmation. Malignant mesotheliomas are classified based on their histomorphologic features. The most common is the epithelial type with its morphologic variants like tubulopapillary, solid, and adenomatoid (microglandular) patterns. Sometimes, one pattern predominates but several different patterns are commonly seen in the same tumor. Less common patterns include small cell, clear cell, and the deciduoid. Sarcomatous mesothelioma is an aggressive type of malignant mesothelioma; histologically it may show a wide range of architectural complexity from hypocellular quiet bland spindle cells to densely cellular areas with obviously high-grade cytology. Two histologic variants are recognized; lymphohistiocytoid and desmoplastic diffuse malignant mesothelioma (DMM). Biphasic malignant mesothelioma is characterized by the coexistence of epithelial and sarcomatous types. The differential diagnosis of malignant mesothelioma includes pleural diffuse or localized neoplastic or non-neoplastic processes. The distinction between these entities demands the integration of clinical, radiographic, and immunohistochemical studies, and several key differentiating features are discussed when enough tissue is available for evaluation by the pathologist. The most important and frequent differential diagnosis includes metastatic adenocarcinoma and sarcomas and chronic fibrous pleuritis.