Cancer starts when cells in the body change (mutate) and grow out of control. Your body is made up of tiny building blocks called cells. Normal cells grow when your body needs them and die when your body doesn't need them any longer.

Cancer is made up of abnormal cells that grow even though your body doesn't need them. In most cancers, the abnormal cells grow to form a lump or mass called a tumor. If cancer cells are in the body long enough, they can grow into (invade) nearby areas. They can also spread to other parts of the body (metastasize).

Malignant mesothelioma, often just called mesothelioma, is a rare kind of cancer. It starts in the mesothelium. This is a thin lining that covers organs and lines the inside of certain parts of the body. It most often starts in the lining of the chest and belly (abdomen).

The mesothelium is a lining that protects the outer surface of some organs. These include the lungs, stomach, and heart. It also lines the inside of your chest and abdomen.

The mesothelium makes fluid that helps the organs move against each other inside your body. For instance, it helps your lungs to slide inside your chest when you breathe.

The mesothelium has different names in different parts of the body:

• The lining in the chest cavity and covering the lungs is the pleura.
• The lining in the abdomen is the peritoneum.
• The lining around the heart is the pericardium.
• The lining around the testicles is the tunica vaginalis .

Mesothelioma can start in any of these linings. It's most common in the chest. This is called pleural mesothelioma. The next most common place is the abdomen. There it's called peritoneal mesothelioma.

Mesotheliomas can be grouped based on how the cancer cells look under a microscope:

• The epithelioid type is the most common. It also tends to have a better outlook than the other types.
• The sarcomatoid or fibrous type tends to be harder to treat. It's not common.
• The mixed (biphasic) type has both epithelioid and sarcomatoid areas. It tends to have an outlook in between the other two types.

Mesothelioma starts in the thin lining around certain organs. Unlike many other kinds of cancer, it usually doesn't grow as a single large tumor. Instead, it often grows along the lining at first. Over time, it can affect how well the lining works, which can lead to symptoms. For instance, if the cancer starts in the lining around the lungs, it can spread and affect their ability to expand. This can cause symptoms, such as shortness of breath or chest pain. As it grows, the cancer can spread into nearby organs or other parts of the body, such as lymph nodes, bones, or the liver.

If you have questions about mesothelioma, talk with your doctor. Your doctor can help you understand more about this cancer.