Sarcomatoid Mesothelioma

Sarcomatoid mesothelioma cells are the rarest of the three mesothelioma cell types.

Histopathology

When this cell type is viewed under an electron microscope, it is spindle-shaped, with a poorly defined nucleus. In addition, sarcomatoid mesothelioma cells may show characteristics that are similar to a leiomyoma, which is a benign mass of abnormal cells found in smooth muscles. They can also exhibit uncharacteristic elements that are similar to cells that maturing into chondrosarcomas, which are malignant cells made from cartilage; and cells that are maturing into osteosarcomas, which are malignant cells made from connective tissue. These variations in the normal appearance of these cells can sometimes cause sarcomatoid mesothelioma to be misdiagnosed as leiomyoma, chondrosarcoma, or osteosarcoma.

Histological examination of these cells has also shown that they have a subtype known as desmoplastic mesothelioma. Desmoplastic areas are areas of dense, fibrous tissue that connects cells together. They appear firm and rubbery looking, and can even be described as woody. They are sometimes mistaken for pleural plaque. This subtype of sarcomatoid mesothelioma represents between 2-10 percent of all mesotheliomas.

Sarcomatoid mesothelioma makes up approximately 10 percent of all pleural malignant mesotheliomas. Instances of sarcomatoid mesothelioma originating outside of the pleura are rare. About 2 percent do originate in the peritoneum, or abdominal cavity. However, they do not seem to have been any instances of sarcomatoid mesothelioma originating in the sac around the heart or in the testes.

Sarcomatoid mesotheliomas occur less frequently in women than men.

Diagnosis

When diagnosing sarcomatoid mesothelioma, one of the factors the pathologist will check for is the absence of epithelioid elements in the biopsy material or less than 10 percent of epithelioid tissue.

Diagnosing sarcomatoid cells is far more difficult than diagnosing epithelioid because:

  • Use of an electron microscope doesn’t give a clear indication that they are sarcomatoid cells because their structures mimic the structures of a number of other soft tissue sarcomas.
  • Use of immunohistochemistry isn’t effective because you don’t get a positive identification as often as you do with epithelioid cells. Immunohistochemistry uses proteins, called antibodies that the immune system sends out to inhibit antigens, or foreign bodies. Each protein only binds to a specific foreign body, so it is a marker for that foreign body. However, with sarcomatoid cells, the proteins that should bind to them only do so a limited number of times.

Study Demonstrates the Usefulness of Two Particular Markers

In a study titled “Sarcomatoid Mesothelioma: a Clinical–Pathologic Correlation of 326 Cases”, published online January 15, 2010 in Modern Pathology, researchers analyzed 326 cases of sarcomatoid mesothelioma to identify common characteristics.

Among these cases, there were 312 men and 14 women with an average age of 70 years old. Ninety-eight percent of the tumors were in the pleura, and two percent were in the abdominal cavity.

As part of this analysis, the researchers looked at markers that were helpful in identifying this cell type. They found that a particular marker called cytokeratin was observed in 93 percent of the cases. They also found that another marker called vimentin was observed in 91 percent of the cases.

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