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What is mesothelioma?

Malignant mesothelioma is the most serious of all asbestos-related diseases. Although uncommon, mesothelioma is no longer considered rare. Mesothelioma is a cancer that is particulary difficult for doctors to diagnose and one that does not respond well to therapy.

The chest cavity, abdominal cavity, and the cavity around the heart are surrounded by a a layer of specialized mesothelial cells. Most other internal organs are also covered by these cells. The tissue formed by these cells is called mesothelium.

A special protective lubricating fluid that allows organs to move around is produced by the mesothelium. This protective fluid helps the lungs to move without resistance inside the chest cavity while breathing. The medical name for the mesothelium of the chest is the pleura and the medical name for the mesothelium of the abdomen is the peritoneum. The mesothelium of the "sac-like" space around the heart is called the pericardium.

A malignant (cancerous) tumor of the mesothelium is called a malignant mesothelioma. As most mesothelial tumors are cancerous, malignant mesothelioma is often simply called mesothelioma. Tumors of the mesothelium can also be benign (noncancerous) but benign mesotheliomas are much more rare.

Mesothelioma was recognized as a tumor of the pleura, peritoneum and pericardium in the late 1700's. However it was not until much later, in 1960, that this particular type of tumor was described in more detail and even more importantly, its association with asbestos exposure was recognized. The first report linking mesothelioma to asbestos exposure was written by J.C.Wagner, and described 32 cases of workers in the "Asbestos Hills" in South Africa. Since than the relationship between mesothelioma and asbestos exposure has been confirmed in studies around the world.

The incidence of mesothelioma in the United States remains very low, with 14 cases occurring per million people per year. Despite these numbers the noticed threefold increase in mesothelioma in males between 1970 and 1984, is directly associated with environmental and occupational exposure to asbestos, mostly in areas of asbestos product plants and shipbuilding facilities.

Although the disease is much more commonly seen in 60-year old men, it has been described in women and early childhood as well. The cause of the disease is not so well understood in these latter two groups, but there is some evidence of possible asbestos exposure for some of these cases as well.

Malignant mesotheliomas are divided into three main types. About 50% to 70% of mesotheliomas are the epithelioid type. This type has the best prognosis (outlook for survival). The other two types are the sarcomatoid type (7%-20%), and the mixed/biphasic type (20%-35%). Treatment options for all three types are the same.

About three-fourths of mesotheliomas start in the chest cavity. They are known as pleural mesothelioma. Another 10% to 20% begin in the abdomen. These are called peritoneal mesothelioma. Pericardial mesothelioma, starts in the cavity around the heart, and is very rare. The covering layer of the testicles is actually an outpouching of peritoneum into the scrotum. Mesotheliomas that affect this covering of the testicles are quite rare.

Glossary of Terms

asbestos: combination of several minerals that separate into long, threadlike fibers. Because they do not burn, do not conduct heat or electricity, and are very resistant to chemicals, these minerals are often used for making fireproof materials, electrical insulation, roofing, filters, etc. benign: doing no harm, good incidence: the frequency with which an event occurs (usually in a group at risk) pericardium: a thin membrane surrounding the heart and the roots of the great blood vessels.

peritoneum: a thin membrane that covers the abdominal cavity and partially covers some of the abdominal organs.

pleura: a thin membrane that covers the lungs (visceral pleura) and lines the chest cavity (parietal pleura) malignant: harmful, dangerous (a malignant tumor is a cancer).

median: middle number in a series of numbers (for example: median survival of 10 months means that for that specific group of patients the survival varied from probably 2 months to 30 months).

mesoderm: the middle layer of cells in an embryo, from which the muscular, skeletal, vascular, connective etc. tissues develop.

mesothelioma: a tumor of the mesothelium, that can be benign (localized) or malignant (diffusely spread), and that is most commonly caused by the ingestion of asbestos particles.

mesothelium: the thin layer of mesodermal epithelial cells that forms the pleura, peritoneum, pericardium.

metastases: the spread of tumor cells from one part of the body to another unrelated part of the body by the way of the bloodstream or lymphatics.

prognosis: prediction of the probable course of the disease in an individual. Prognostic factors are factors associated with prognosis.

TNM staging: assigning a stage to the tumor based on size, local versus disseminated growth, lymph node involvement and presence or absence of distant metastases.

tumor: a mass of tissue, a growth independent of its surrounding structures and having no physiological function/ a neoplasm. A tumor can be benign or malignant.