Mesothelioma is a rare type of cancer, commonly linked to exposure to asbestos, that often forms on the thin protective tissues that cover the lungs (pleural mesothelioma) or the abdomen (peritoneal mesothelioma). This lining covers all internal organs, and is composed of  specialized cells called mesothelial cells. The disease is also called malignant mesothelioma because there are benign forms of mesothelioma, but the cancerous form is best-known and often referred to simply as mesothelioma.

Although the use of asbestos has decreased significantly in the U.S. and elsewhere in past decades, there are about 3,000 new cases of mesothelioma every year in the United States alone. The majority of patients were exposed to asbestos in the workplace, and the disease can develop some 20 and even 50 years after exposure.

For this reason, mesothelioma is more commonly found in men then in women, and pleural mesothelioma — believed to be typically caused by inhaling small asbestos fibers — is the most common of the four types of this disease. This disease is also most commonly found in older people because of its long latency, than in younger people. According to the American Cancer Society, the average age at diagnosis of pleural mesothelioma is 69.

There is currently no cure for mesothelioma, and its prognosis is usually poor, often because it is already in an advanced stage when diagnosed. But a combination of therapies, usually chemotherapy, radiotherapy and surgery, can ease the symptoms and improve survival. Research efforts conducted over the past decades have made significant improvements in understanding the disease, and in developing treatment options.

Likely Causes of Malignant Mesothelioma

In 70 percent to 80 percent of mesothelioma cases, patients were exposed to asbestos in their workplace, which can include industrial settings, shipyards, auto repair shops, power stations, old houses, schools, and public buildings. The risk is usually related to long-term exposure, but in some cases short-term (or even one-time) exposure can cause mesothelioma. Asbestos is a group of minerals that naturally occur as bundles of fibers, with properties attractive to many types of construction, like versatility, heat resistance, tensile strength, and insulation. It was found to be highly toxic in the 1960s, and more than 50 countries have banned or restricted its use since the 1970s.

Tiny asbestos fibers can be inhaled or swallowed, which the body is unable to expel. Trapped fibers can trigger biological alterations that result in inflammation, scarring, and genetic changes that can lead to cancer. Mesotheliomas related to asbestos exposure, again, have a long latency period and can take years to develop. The disease can also be hard to diagnose because its symptoms, like shortness of breath, pain in the chest or stomach, and chronic cough, are similar to other diseases.

Mesothelioma is also associated with high doses of radiation, possibly given as a treatment for other cancers, but such cases are rare. Some studies have also suggested that simian virus 40, a virus that contaminated some polio vaccines in the U.S. given in the mid-1950s to mid-1960s, may be a cause of mesothelioma.

Four Types of Mesothelioma

There are four different types of mesothelioma, defined by where in the body the cancer has developed. Pleural mesothelioma is a malignant cancer in the lining of the lungs, known as the pleura. This mesothelioma accounts for about 75 percent of all cases. The second most common type is peritoneal mesothelioma, which develops in lining of the abdomen, or peritoneum. About 250 to 500 new cases of peritoneal mesothelioma are diagnosed annually in the U.S. Together, these two types count for about 90% of all mesothelioma cases.

Pericardial mesothelioma occurs when the cancer develops in the thin membrane surrounding the heart, known as the pericardium. Treatment options for this very rare cancer are few. Testicular mesothelioma, or mesothelioma of the tunica vaginalis, develops in the membrane that covers the testicles. To date, only about 200 cases of pericardial mesothelioma have been reported, and number of cases of testicular mesothelioma is similar.

Mesothelioma is also distinguished by the arrangement of cancer cells. There are three main types here: epithelioid (50 percent of all cases and with best prognosis of these three types), sarcomatoid or fibrous (about 10 percent), and mixed or biphasic, with areas of both epithelioid and fibrous cells.

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