There are four primary types of mesothelioma based on location.
Pleural and peritoneal mesothelioma are the two most common types.
Mesothelioma can also be categorized by cell type: epithelial, sarcomatoid & biphasic.
Mesothelioma type significantly influences prognosis and treatment options.
Four Primary Mesothelioma Types by Location
Mesothelioma, also referred to as diffuse malignant mesothelioma, is a rare form of cancer that originates in the mesothelium, a thin layer of cells that protect the body’s internal organs. The location of the mesothelioma tumor is the primary way the type of mesothelioma is determined. The name of the mesothelioma is based on the location too. To determine the type of mesothelioma you have, your primary care physician will refer you to an oncologist or mesothelioma specialist who will perform a more detailed evaluation.
Pleural Mesothelioma (Lungs)
Pleural mesothelioma is the most common form of the cancer, accounting for approximately 75% of all cases. This type of mesothelioma develops in the lining of the lung cavity, known as the pleura, and is often revealed by a CT or MRI of the chest. Obtaining a true diagnosis usually requires a surgical or image-guided percutaneous tissue biopsy.
Peritoneal Mesothelioma (Abdomen)
The second-most prevalent type is peritoneal mesothelioma, responsible for about 10% of all cases. The abdominal cavity and its organs are all lined with mesothelial cells. In peritoneal mesothelioma, a CT or MRI of the abdomen typically will reveal thickening of the peritoneal lining. Less commonly, small nodules or a single larger tumor may be present. Surgical or image-guided percutaneous biopsy is required for cell type diagnosis.
Pericardial Mesothelioma (Heart)
Pericardial mesothelioma accounts for about 1% of all mesothelioma cases. The pericardium is the lining of the heart cavity, the outer layer of which consists of mesothelial cells. Pericardial mesothelioma is extremely rare, but when there is evidence of a primary pericardial tumor (originating in the pericardium, as opposed to a metastatic tumor to the pericardium from another organ), mesothelioma is the most common cause. The only way to diagnose pericardial mesothelioma is to obtain a tissue biopsy, most commonly surgically.
Testicular Mesothelioma (Testes)
Testicular mesothelioma originates in the tissue of the tunica vaginalis and is extremely rare, with only about 100 cases total reported. Because it is so rare, it is difficult to fully understand this type of mesothelioma and the most effective way to treat it. As with all cases of mesothelioma, there is a strong association with occupational asbestos exposure. The asbestos pathway to the testicular region is not understood. Diagnosis is typically done through ultrasound and a biopsy.
Mesothelioma Tumors: Benign vs. Malignant
Mesothelioma tumors can also be identified as malignant or benign. A malignant, or cancerous, tumor is considered dangerous as the cells can grow and divide quickly and uncontrollably, destroying tissue and spreading to other parts of the body. Benign tumors are slow-growing and generally won’t begin to metastasize, or grow and spread in other areas of the body. Benign tumors are also often in the form of a single mass, rather than the spreading or sheet-like formation generally associated with malignant mesothelioma.
Benign mesothelioma is not well understood, as fewer than 200 cases have been reported. Some benign tumors have the potential to become malignant and are often removed in surgery. There is a low rate of recurrence for those diagnosed with benign mesothelioma.
Most mesothelioma cases are malignant. Malignant mesothelioma has a long latency period and is difficult to diagnose. Patients with malignant mesothelioma typically face a rather poor prognosis and may have limited treatment options.
Mesothelioma Cell Types
In addition to the primary types of mesothelioma, the disease can be further categorized by the tumors’ cell type: epithelioid, sarcomatoid and biphasic. Cell typing provides information on the tumor’s growth rates, likelihood to metastasize or spread, as well as providing expectations for a patient’s prognosis and chances for responding to certain treatment methods. Some researchers even believe mesothelioma cell types can be a better predictor of survival than the stage of the disease.
Epithelioid mesothelioma is the most common cell type, usually associated with pleural mesothelioma. Patients with epithelioid mesothelioma may experience shortness of breath, chest pain and fluid buildup in their lungs. Compared to the other two types of malignant mesothelioma, this cell type responds well to treatment, and many patients will undergo a multimodal approach. Epithelioid mesothelioma has the best prognosis of the three cell types.
Sarcomatoid mesothelioma is the least common cell type, and also the most aggressive. Sarcomatoid cells are most often found in pleural mesothelioma cases, but has also been associated with peritoneal mesothelioma. Patients with sarcomatoid mesothelioma may experience shortness of breath, abdominal pain and weight loss. Because of the cell’s aggressive nature, this type grows much more quickly compared to epithelioid cells and will frequently metastasize. As such, this type is the most difficult to treat and has the worst prognosis.
Biphasic mesothelioma is a mix of epithelial and sarcomatoid cells, and is the second most common cell type. Biphasic is also largely associated with pleural and peritoneal mesothelioma, but has also been found in some pericardial mesothelioma cases. The symptoms are generally the same as patients with epithelioid and sarcomatoid mesothelioma. The prognosis and responsiveness to treatment will vary, largely dependent on how many cells are epithelial versus sarcomatoid. Patients with more sarcomatoid cells, for instance, may not be candidates for surgery and will likely face a poorer prognosis.
Classifying Mesothelioma Cell Type
When identifying cell type, doctors have to be careful in their classification so as to not confuse the tumors with another kind of cancer. All of these cell types, and their rare subtypes, can be very difficult to identify, which results in misdiagnoses for many patients. For instance, epithelioid mesothelioma can be easily misidentified as adenocarcinoma, which consists of glandular formations in epithelial cells, while sarcomatoid mesothelioma is often mistaken for other forms of sarcomas or lung cancer.
After initial imaging scans, a mesothelioma specialist will run several other tests to identify the type of mesothelioma. To come to a diagnosis, the following steps are generally involved:
Steps To Diagnose Cell Type
Tissue or fluid is collected for examination.
Scientists run the tissue through a chemical process, called pathology, that reveals its microscopic structures.
The sample is then studied and classified as a specific cell type based on its observable characteristics.
Pathologists rely on immunohistochemistry for the diagnosis of mesothelioma. Immunohistochemistry involves analyzing the cells for certain proteins expressed by the cell by detecting interaction between antibodies and these proteins; various antibodies are applied to tissue samples in this process.
How Cell Type Impacts Life Expectancy
Mesothelioma cell type is only one of many factors that can impact a patient’s prognosis. Though the location and stage of the disease impacts the mesothelioma treatment plan most directly, the cell structure will help doctors identify how aggressively to treat the cancer and provides more insight into a patient’s life expectancy.
Studies show that each cell type responds quite differently to treatments and can significantly impact prognosis, making accurate diagnosis of cell type highly important. Cell type can account for a difference of up to 6 to 7 months of survival.
Ultimately, there are a number of unique factors at play, all of which influence one another and play a role in determining an effective treatment plan and patient outcome. It’s important to talk with your doctor and to get a second opinion from a mesothelioma specialist to ensure highest accuracy in diagnosis, which will only improve treatment efficacy and prognosis.