There are four different types of mesothelioma named for the location where the tumors develop: pleural (lungs), pericardial (heart), peritoneal (abdomen), and testicular. Mesothelioma is also defined by its cell structure. The three major cell types are epithelioid, sarcomatoid, and biphasic.
Key Points about Mesothelioma Types
- Pleural and peritoneal are the most common types of mesothelioma.
- Mesothelioma can also be categorized according to cell type: epithelial, sarcomatoid, and biphasic.
- Mesothelioma type significantly influences prognosis and treatment options.
Four Primary Mesothelioma Types by Location
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.
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 tissue biopsy through needle aspiration or thoracotomy.
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 needle biopsy is required for cell type diagnosis.
Asbestos has two routes to the abdominal region. One is by ingestion or simply having the fibers adhering to the inside of the mouth when breathing contaminated air. Subsequent swallowing of saliva and food will carry the fibers through the stomach and intestines. The fibers are absorbed by the peritoneal mesothelial cells and activate oncogenes. The second pathway to the peritoneal lining is via asbestos in the lungs traveling through the lymphatic ducts to the abdomen.
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. Once asbestos is inhaled, some fibers will pass through the lung tissue into the the bloodstream. The fibers will then travel through the blood vessels to the heart. Pericardial mesothelial cells may also absorb asbestos fibers through the thoracic wall or through lymph nodes from the lungs. Pericardial mesothelioma is extremely rare, but when there is evidence of a pericardial tumor, mesothelioma is the most common cause.
An echocardiography can rapidly determine the extent of thickening of the pericardial tissue and determine whether it is restricting heart function. However, the only way to diagnose pericardial mesothelioma is to obtain a tissue biopsy by means of a thoracotomy. Although pericardial effusion is often present, needle biopsies rarely obtain an adequate volume of fluid to make a cellular diagnosis.
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. The most commonly affected age group is 55-75 years olds, but a small number of patients are younger than 25. 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.
Scrotal ultrasound is most commonly used to image the area. If a hydrocele (accumulation of fluid) is present, a needle biopsy may provide cells for a diagnosis. Otherwise, surgical removal of the tumor with cell type analysis will provide the diagnosis.
Testicular mesothelioma is an aggressive cancer where over half will experience recurrence and 40% will die from the disease. Complete removal of the affected testicle has so far shown the best results. There is no well established protocol for the treatment of malignant testicular mesothelioma, because it is so rare.
There is not a consistent set of symptoms unique to testicular mesothelioma. Commonly individuals are treated for one or two wrong diagnosis before a diagnosis is finally made at surgery.
Mesothelioma Tumors Consist of Three Different Cell Types
The three cell types of mesothelioma are epithelial, sarcomatoid, and biphasic. The abnormalities in these cells provides information about the aggressiveness of the cancer and helps doctors predict a patient’s clinical course and provide the most appropriate treatment plan.
Classifying the cell type is determined by microscopically examining the tumor cells and describing the histology (structure) of the cancerous cells.
This common cell type accounts for 50 – 70% of all mesothelioma cases. The cells in this type respond best to treatment, and therefore patients with the epithelial cell mesothelioma typically have the highest survival rates. Epithelial cells are easily detected under a microscope because of their uniform, well-defined, and elongated shape. Epithelial mesothelioma is most common cell type in pleural mesothelioma.
Sarcomatoid cells occur in 10 – 20% of mesothelioma cases. It is the most aggressive form of the cancer, and it has the lowest rate of survival. Under a microscope, sarcomatoid cells appear long and spindle-shaped. Tumors with this cell type have similar characteristics of types of cancers, and diagnosis by microscopic examination is essential. Sarcomatoid mesothelioma can easily be confused with pulmonary sarcomatoid carcinoma, a form of lung cancer, due to similar symptoms and appearance.
When a mesothelioma tumor contains cellular characteristics of both epithelial and sarcomatoid cells, it is called biphasic. This cell type accounts for approximately 30% of all mesothelioma cases. Not all biphasic mesothelioma cells are the same: Some may resemble epithelial cell mesothelioma more, while others may resemble sarcomatoid mesothelioma cells more. This resemblance can influence the prognosis significantly. Biphasic tumors with a greater percentage of epithelial cells generally have a higher survival rate.
Determining Mesothelioma Type
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. The specialist will go through the following steps to try to accurately diagnose your disease:
Step 1: Imaging Tests – tests like a CT scan or MRI can reveal disease location information.
Step 2: Biopsy – invasive and minimally-invasive options obtain tumor cells for further testing.
Step 3: Staging – pleural mesothelioma uses the TNM staging system.
The Difference Between Malignant and Benign Mesothelioma
A third way to categorize mesothelioma is whether the tumor is malignant or benign. A malignant tumor indicates that the tumor cells are abnormal and may grow and invade surrounding tissues. A malignant tumor can lead to sickness and death if left untreated. A benign tumor, or nonmalignant tumor, may grow in size but typically does not invade surrounding tissue. Benign tumors can remain untreated as long as it does not interfere with other organs and vitals.
Most mesothelioma diagnoses are malignant, meaning the disease is deadly if left untreated. Malignant mesothelioma is also characterized as experiencing a long latency period and being difficult to diagnose, which can lead to its often poor prognosis. However, due to treatment advances through clinical trials, there is hope for mesothelioma patients to become mesothelioma survivors.
Benign mesothelioma consists of tumors that grow slowly and do not metastasize to other organs. These are typically a single tumor mass as opposed to the rapidly spreading, sheet-like formation seen in malignant mesothelioma. The condition is rare, with less than 200 cases reported. The cause of benign mesothelioma is not understood.
Many patients with benign mesothelioma have no symptoms. When symptoms are present, they pertain to the area of the tumor. Most patients do not have a history of asbestos exposure. Although benign mesothelioma tumors have been reported in all ages groups, it is most common in those age 60 – 75. Surgery alone is usually successful, since the benign type typically manifests as a single mass. There is a low rate of recurrence.