When a patient is diagnosed with mesothelioma, the doctor will determine whether the cancer has spread. Knowing the stage of the cancer can help determine what treatment plan to follow.

The TNM system of mesothelioma consists of four stages. During Stage I, the cancer is contained to the pleura and has not spread to the lymph nodes. Mesothelioma in the second stage affects the pleura and has spread to the lung or diaphragm. If the cancer is in Stage III, it has spread into the first layer of the chest wall; the outer layer of the heart; the chest cavity behind the breastbone; the lymph nodes on one side of the chest; or a single place in the chest wall. Stage IV indicates the cancer has spread to other organs or to lymph nodes on both sides of the chest.

Mesothelioma also has another form of staging known as the Butchart System. In Stage I, mesothelioma is only found on one side of the chest pleura. During Stage II, the cancer can also be found in the chest wall, heart, esophagus, chest lymph nodes or the opposite pleura. If mesothelioma reaches Stage III, the cancer has invaded the diaphragm and peritoneum. Stage IV mesothelioma indicates that the cancer has spread to other organs.