Mesotheliomas have 3 basic histologic forms: epithelioid (the most frequent), sarcomatoid or mixed (biphasic). More often, areas showing features and admixtures of these three types may be encountered within a single tumor; a sarcomatoid component is observed in 25% of cases,[31,32] but a pure sarcomatoid variety is extremely rare and only 32 cases have been reported in the literature since 2006 ( Table 1 ).
The epithelioid MPM can grow with 4 different patterns: tubular, papillary (the most common, often found in association with other patterns), diffuse, and deciduoid (cells with abundant glassy eosinophilic cytoplasm). Atypia is a frequent feature but is typically mild; only a few cases have moderate or severe atypia. Unusual histologic features include lymphoid follicles, striking myxoid stroma, prominent foamy histiocytes, and marked vascular proliferation. Multicystic mesotheliomas and well-differentiated papillary mesothelioma are histological types associated with a long survival in the absence of treatment.
Sarcomatoid mesothelioma is extremely rare: Neumann and colleagues in a series of 53, and Deraco and colleagues in a series of 49 MPM reported no sarcomatoid histotypes. It is composed of a fascicular proliferation of spindle cells with oval nuclei, scant amphophilic cytoplasm and occasionally prominent nucleoli. In general, sarcomatoid mesotheliomas show more atypia than their epithelioid counterparts, and often display mitotic activity and foci of necrosis. The tumor cells can display a fibrosarcoma-like appearance; therefore, sarcomatoid MPM must be differentiated from the rare variant of extra-intestinal gastrointestinal stromal tumors (GISTs) with sarcomatoid features or from true retroperitoneal sarcomas.
A large number of immunohistochemical markers have been suggested for diagnostic aid, but none of the markers alone is diagnostic. However, they become very useful when used as a panel. Malignant MPM is characterized by positive staining for EMA, calretinin, WT1, cytokeratin 5/6, antimesothelial cell antibody-1, and mesothelin. Cytokeratins help to confirm invasion and to distinguish mesothelioma from sarcoma and melanoma. Mesotheliomas are characterized by the absence of antigens such as carcinoembryonic antigen (CEA), thyroid transcription factor-1, B72.3, MOC-31, Ber-EP4, and BG8. Immunohistochemistry is also useful to distinguish peritoneal mesotheliomas from primary papillary serous carcinoma of peritoneum, serous ovarian carcinomas, colorectal adenocarcinoma diffusely involving the peritoneum, and borderline serous tumors. In particular, calretinin, cytokeratin, and thrombomodulin are typically positive in patients with mesotheliomas and negative in those with serous carcinomas.
Detecting characteristic ultrastructural features by electron microscopy may help the diagnosis of malignant mesothelioma. Typical mesothelioma show tall and thin microvilli on the cell surface. It has been suggested that only microvilli whose length exceeds the width by a margin of 15:1 are diagnostic of mesothelioma. Because the microvilli are often poorly developed in the sarcomatoid variety, electron microscopy is generally not useful in their diagnosis.
Peritoneal mesothelioma usually remains confined to the peritoneal cavity for most of its natural history. Typical growth pattern of peritoneal mesothelioma is locally expansive masses. Hematogenous or lymphatic metastasis is unusual. However, parasternal, retroperitoneal, mediastinal,[39,40] axillary, supraclavicular, and cervical lymph nodes; lung, bone,[41,43] liver, and umbilical ('Sister Mary Joseph's nodule') metastases have all been reported.
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Cite this: Peritoneal Mesothelioma: A Review - Medscape - May 10, 2007.