Doctors classify malignant mesothelioma by where the tumor is. If the cancer has metastacized, the classification derives from the place where the cancer started.
Pleural mesothelioma makes up 70% to 80% of all mesothelioma cases. The pleura is the serous membrane around the lungs and inner walls of the chest, which is why you sometimes see people refer to mesothelioma as a form of lung cancer. It is not lung cancer – the two are separate diseases – but because they are both cancers in the chest and because lung cancer is much more common, this confusion arises.
Peritoneal mesothelioma, sometimes called abdominal mesothelioma, is 20% to 30% of mesothelioma cases. The peritoneum is below the chest, roughly in the abdominal area.
Pericardial mesothelioma affects the membrane that surrounds the heart. Like pleural mesothelioma, this disease can be very difficult for doctors to identify and diagnose. It is often mistaken for other more common heart ailments and a tissue biopsy is required to make a definitive diagnosis.
An even rarer form of mesothelioma is testicular mesothelioma, which forms in the tunica vaginalis membrane around the testicles.
Malignant mesothelioma just means the mesothelioma is a cancer. All forms of cancer are malignant. There is a condition called benign mesothelioma, where the tumor – a swelling caused by abnormal tissue – cannot metasticize and grows slowly or not at all. If you have a benign mesothelioma, your doctor will be able to advise you what, if any, treatments are needed. This is usually not considered a form of cancer.
Another form of classification is histological. The medical field developed this way of distinguishing pathologies from laboratory examinations of affected tissue. Doctors remove a sample of tissue from the body and look at it under a microscope. In a mesothelioma biopsy, doctors look for one of three types of disease: epithelioid, sarcomatoid or biphasic. Epithelioid mesothelioma is the most common form of the illness, accounting for 50 to 70 percent of all diagnosed cases. Epithelioid cells exhibit a tubular pattern, uniform shape and a defined cell nucleus.
Sarcomatoid mesothelioma is the least common form of the illness, comprising approximately 10 to 15 percent of all cases. Sarcomatoid cells are more irregular and oval in shape than epithelioid cells, and also exhibit a less defined cell nucleus.
Biphasic mesothelioma accounts for 20 to 40 percent of all mesothelioma cases. In this form of mesothelioma, both epithelioid and sarcomatoid cancer cells are present.
Does it matter which histological form of mesothelioma you have? Your oncologist will want to know as this will inform decisions with regard to treatment. Epithelial mesothelioma generally has a better prognosis.