Straight Talk about Mesothelioma, a blog series created by Michael T. Milano, M.D., Ph.D., a radiation oncology specialist, as a resource for mesothelioma patients and their loved ones.

Despite the fact that asbestos is not yet banned in the U.S., it’s well established that malignant mesothelioma is caused exclusively by exposure to asbestos.

Mesothelioma is not a common cancer and, as such, it is often misdiagnosed as lung cancer, lymphoma, or lung infection. The reason for this confusion is that there are several types of mesothelial cells, and some may resemble other cancer cells. Whenever mesothelioma is suspected, further testing is necessary to determine the location of the cancer and the sub-type of mesothelioma.

Today, mesothelioma is classified based on the actual cell type seen under the microscope.

How Many Types of Mesothelioma Are There?

There are essentially 4 sub-types of malignant mesothelioma cells when analyzed under a microscope. These include the following:

  • Epithelioid
  • Sarcomatoid
  • Biphasic
  • Poorly differentiated

What Are the Features of an Epithelial Type Mesothelioma?

Epithelioid mesothelioma is the most common mesothelioma sub-type, accounting for about 50 percent of all cases. Epithelioid mesothelioma tends to spread locally; it is usually associated with a large amount of fluid in the chest cavity and also spreads to the nearby lymph nodes.

What Are the Features of a Sarcomatoid Type Mesothelioma?

Sarcomatoid mesothelioma accounts for about 20 percent of cases, and is the third-most common type of mesothelioma. These cancer cells in particular tend to be quite aggressive. By the time the diagnosis is made, the cancer has often spread to distant parts of the body. Unlike the epithelioid mesothelioma, the sarcomatoid subtype is not always associated with fluid collection in the chest.

What Are the Features of a Biphasic Type Mesothelioma?

Biphasic means that the mesothelioma contains both epithelioid and sarcomatoid cells. Some people refer to biphasic mesothelioma as “mixed” mesothelioma. These types of mesothelioma tend to grow locally and can also spread. They also secrete fluid in the chest cavity. They account for about 25 percent of all forms of mesothelioma.

What Is a Poorly Differentiated Mesothelioma?

In about 5 percent of all cases of mesothelioma, the cells are poorly differentiated, which means that the pathologist is not able to determine the specific cell type(s) because the cells have become so aggressive.

What Is the Importance of All of These Classifications?

The chief reason to classify the forms of mesothelioma is for prognostic and treatment purposes. Overall, epithelioid mesothelioma has the best prognosis because these cancers more often remain localized to the chest-area such that they can be surgically removed (in healthy patients. The undifferentiated forms of mesothelioma have the worse prognosis because they have an unpredictable behavior and the cells usually do not respond to any type of treatment.

Doctors can make predictions on how the tumor will respond, based upon the specific cell type, with epithelioid mesothelioma having the best response rates and the sarcomatoid and undifferentiated tumors having the worst response rates. In general, mesothelioma is a very difficult cancer to treat, and, unfortunately, many people who develop mesothelioma have a poor prognosis. Having an epithelioid mesothelioma only increases the average life span by a few months. Research is ongoing – and, more importantly, drugs are being developed that will target each of these specific cell types.