Mesothelioma is an asbestos-caused cancer of the membranes that surround many of the body’s vital organs. This membrane, known as mesothelium, secretes a lubricating fluid that provides easy movement of the organs within the body. When the mesothelium becomes cancerous, it is called mesothelioma. Surrounding the lung, the mesothelium is called the pleura and the form of mesothelioma is called pleural mesothelioma; in the abdomen, it is the peritoneum and the cancer is called peritoneal mesothelioma; the lining around the heart is the pericardium and there mesothelioma is identified as pericardial mesothelioma. There is also mesothelial tissue in the reproductive organs: in males, this tissue is called the tunica vaginalis testis and in women the tunica serosa uteri. Mesothelioma of the reproductive tissues has only presented in men, so it is known as mesothelioma of the tunica vaginalis.
Mesothelioma is a diffuse malignancy and spreads itself throughout a tissue area. Some forms of cancer are characterized by a single, solid tumor mass, whereas malignant mesothelioma presents as an invasion of a large number of smaller masses throughout an entire area. The diffuse nature of mesothelioma has important implications for how mesothelioma is treated, as complete surgical extraction of the cancerous tissue is difficult.
Types of Mesothelioma
There are multiple forms of mesothelioma. The differences involve the location in which the tumor begins, known as its origin site, and the type of cells that the tumor invades, known as its histological subtype.
Location of Disease
The most common form of mesothelioma is pleural mesothelioma, where the cancer attacks the pleural tissue surrounding the lung. As many as eighty percent of all mesothelioma diagnoses are for pleural mesothelioma. The pleura’s proximity to the lung is the reason many people mistakenly think of mesothelioma as lung cancer, which it is not. Pleural mesothelioma can spread (“metastasize”) to the lung, but the origin site is the actual pleural tissue surrounding the lung—not the lung itself. There are many differences between mesothelioma and lung caner.
After pleural mesothelioma, the next most common type is peritoneal mesothelioma, which develops in the majority of other cases. There is some evidence to suggest that diagnoses of mesothelioma are on the rise. While pericardial mesothelioma and mesotheliomas of the tunica vaginalis are also possible forms of the disease, they are very rare tumors and occur less often.
Histological Subtype
Besides the location in which the tumors form, the other identifying factor in the diagnosis of mesothelioma is the histological subtype of the cancer. This refers to the type of cells that the tumors invade. There are three histological subtypes of mesothelioma: epitheloid mesothelioma, which attacks epithelial cells, sarcomatoid mesothelioma, which attacks sarcomatous cells, and bi-phasic mesothelioma, where the tumor attacks both epithelial and sarcomatous cells. The identification of the proper subtype is an important diagnostic factor because it greatly affects the disease’s prognosis. Epitheloid mesothelioma has the best response rate for treatment, while sarcomatoid mesothelioma has the worst.
