It starts in the mesothelium, the tissue surrounding the lungs, abdomen, heart and reproductive organs. The cancer cells grow by dividing themselves and then start to invade between healthy cells, growing deeper into surrounding healthy tissue.

Mesothelioma tends to spread to nearby tissue (local spread). It is possible for the cancer cells to get into the lymphatic system by spreading to the lymph glands, or get into the bloodstream. The lymph and blood systems cover the whole body, and so the cancer cells may travel to further away parts and start growing there too.

When cancer cells from a tumour settle in other parts of the body and start growing there, they are called secondary cancers (metastases).