Types of Mesothelioma

Mesothelioma is a cancer of the mesothelium – a membrane that surrounds organs in the chest and abdomen. The four types of mesothelia are:

Pleura – membrane surrounding the lungs
Pericardium – membrane surrounding the heart
Peritoneum – membrane surrounding the abdominal organs
Tunica vaginalis – membrane surrounding the testicles

Cancer can arise in any of these membranes and each type is named accordingly:

While most mesotheliomas are malignant and commonly fatal cancers, some tumors can be benign. Two types of benign mesothelioma are:

Benign fibrous mesothelioma (pleural)
Benign cystic mesothelioma (peritoneal)

A malignant mesothelioma can also be classified according to the type of cancer cell present. The cell type determines how aggressive the cancer is. Types of malignant mesothelioma cells are:

Pleural Mesothelioma

Pleural mesothelioma is sometimes referred to as lung cancer, but technically, it is not a cancer of the lung tissue; it is a cancer of the membrane surrounding the lung – the mesothelium.

Pleural mesothelioma – also often called asbestos cancer – is almost always caused by heavy exposure to asbestos dust in the air. The sharp asbestos fibers infiltrate deep into the lungs and over a period of decades can penetrate the lung’s cell walls. When the fibers work their way into the membrane surrounding the lungs, they can cause a chronic inflammation and eventually stimulate the growth of cancer cells. As these cancer cells reproduce and spread, mesothelioma becomes established.

Of the 2,000 to 3,000 annual cases of mesothelioma, roughly two-thirds are cancer of the pleura. The majority of those diagnosed survive only 12 to 18 months because it is a relatively aggressive cancer and is rarely diagnosed early. According to some recent studies, however, many patients diagnosed early and treated aggressively may survive as long as five years or more.

Peritoneal Mesothelioma

The peritoneum is the membrane surrounding the stomach, liver, pancreas, and intestines. Doctors are not certain how asbestos fibers can become lodged in the peritoneum. Some theorize that swallowing asbestos fibers can result in some sticking in the digestive tract and working their way into the abdominal membrane. Another theory is that asbestos in the lungs may be transported through the lymph system. These potential transport mechanisms can be considered rare because peritoneal mesothelioma is diagnosed in only about 20% to 25% of cases.

Though peritoneal tumors tend to be in more operable locations, the survival rate a short time ago was not that high – typically less than 12 months. But newer treatment strategies combining surgery, radiation and chemotherapy have lengthened median survival rates for this type of mesothelioma to more than five years.

Pericardial Mesothelioma

Asbestos fibers lodging in the pericardium surrounding the heart can also incite cancerous growth. It’s not clearly understood how asbestos fibers migrate from the lungs to the heart, however. One theory is that if asbestos fibers work their way from the lungs into the bloodstream, they may be carried to the heart where they can make their way into the pericardium. The fact that less than 5% to 10% of mesothelioma cases are pericardial mesothelioma indicates how rarely this migration occurs.

Pericardial mesothelioma is also rarely diagnosed early, and as the heart is involved, treatment options are fewer. Most patients with this type of mesothelioma will not survive twelve months.

Testicular Mesothelioma

This type of cancer is so rare only about 100 cases have been documented. Doctors are not certain what causes this mesothelioma of the membrane around the testicles (tunica vaginalis). In some cases they theorize that because the membrane surrounding the testicles is an extension of the peritoneum, primary cancer cells of the peritoneum can spread to the scrotum to create secondary tumors. But when testicular tumors are primary, they have no explanation for how they’re caused.

Because of testicular mesothelioma’s rarity, it can be difficult to generalize about treatments and prognosis. Though surgery to eliminate this type of tumor is easier and much less risky, it can be pointless if a primary tumor remains in the peritoneum. Even when the testicular tumor is primary and surgery is followed with chemotherapy or radiation treatment, residual cancer cells still commonly spread to other organs or lymph nodes often resulting in fatal recurrence within two years.

Benign Fibrous Mesothelioma

This non-cancerous tumor is diagnosed in about 5% of pleural mesothelioma cases. Its formation is completely unrelated to asbestos exposure. Because it is non-malignant, it is not a rapidly growing or spreading tumor and is not fatal. It is often very localized, well-defined, and easily removed by surgery. While surgery is curative for 90% of patients, 10% will have a recurring tumor up to ten years afterward.

Benign Cystic Mesothelioma

A non-fatal condition, benign cystic mesothelioma is similar to the benign pleural tumor except that it occurs in the peritoneum. At least one case has been reported in the tunica vaginalis, which is an extension of the peritoneum and surrounds the testicles. Nonetheless, benign cystic mesothelioma affects women more often than men. It also tends to occur more often in women of childbearing age.

As with benign fibrous mesothelioma, the peritoneal type is localized and responds well to surgery, though recurrence is still possible. Because it occurs more often in fertile women, studies are being conducted to determine if anti-estrogen drugs such as those used for breast cancer can be effective in preventing recurring tumors.

Sarcomatous Mesothelioma

Sarcomatous mesothelioma is a cancer tumor composed of sarcomatous cells. These irregular shaped cells are very aggressive – meaning they tend to grow and spread quickly. Any of the mesotheliomas – pleural, peritoneal, pericardial, or testicular – can be sarcomatous.

Sarcomatous mesothelioma occurs in only about 15% of cases. Because it is aggressive and responds poorly to treatment, it leads to a poor average survival rate.

Epithelial Mesothelioma

Mesotheliomas composed of epithelial cells are the most common, representing about two-thirds of all cases. Whether surrounding the lungs, heart, abdomen or testicles, any mesothelioma can be epithelial.

Epithelial cancer cells are not as aggressive as sarcomatous cells, and survival rates tend to be better than for either of the other two cancer cell types.

Biphasic Mesothelioma

Approximately 25% of malignant mesothelioma tumors – regardless of location – contain both epithelial and sarcomatous cell types. One often-quoted study found survival time for biphasic tumors to be worse than either of the other two types.