Malignant mesothelioma is cancer that starts in cells in the linings of certain parts of the body, most commonly the linings of the chest or abdomen (belly).

Cancer starts when cells start to grow out of control. Cells in nearly any part of the body can become cancer. To learn more about how cancers start and spread, see What Is Cancer?

A layer of specialized cells called mesothelial cells lines the inside of your chest, your abdomen, and the space around your heart. These cells also cover the outer surface of most of your internal organs. The lining formed by these cells is called the mesothelium.

The mesothelium helps protect your organs by making a special lubricating fluid that allows organs to slide against each other. For instance, this fluid makes it easier for your lungs to move (expand and contract) inside your chest when you breathe. The mesothelium has different names in different parts of the body:

Mesothelial tumors can start in any of these linings. These tumors can be cancer (malignant) or not cancer (benign).

A cancer tumor of the mesothelium is called a malignant mesothelioma. This is often shortened to just mesothelioma. Mesotheliomas can start in 4 main parts of the body.

Malignant mesotheliomas are grouped into 3 main types based on how the cancer cells look:

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What Is Malignant Mesothelioma? - American Cancer Society