Staging is a term used in the diagnosis and treatment of cancer, including malignant mesothelioma. Different cancers have different staging systems; staging is used to provide a verbal shortcut to describing how far a cancer has progressed in a patient. Staging is also used to group patients who have similar treatment options and similar prognosis, for purposes of evaluating the success of therapies (for example, a therapy might be established to work very well in Stage I patients, less well in Stage II, and not at all in Stage III). Staging is a convenient and effective tool for analysis, but patients and doctors should remember that all patients are individuals, and that stating is only one tool among many for use in determining the proper course of treatment.

Mesothelioma can be staged under two different systems. The older system is known as the Butchart system, which uses Roman numerals to divide mesothelioma patients into four groups. In the Butchart system, stage I mesothelioma has a tumor in either the right or left pleura but not both. Stage I mesothelioma may involve the lung, the pericardium or the diaphragm on the same side as the primary tumor. A Stage II mesothelioma has invaded the chest wall, the esophagus, or the heart itself. Mesotheliomas are also considered to be Stage II if both sides of the chest are cancerous, or if local lymph nodes are affected. Stage III mesothelioma has grown into the peritoneum, or has spread into lymph nodes not immediately local to the primary tumor. Stage IV mesothelioma has metastasized through the body. The Butchart system is less descriptive but is still used by some clinicians.

The newer system is known as the TNM system, standing for Tumor, Nodes, Metastasis. Within each category, the mesothelioma is assigned a number from 0 to 4 indicating the severity of the condition. T classifies the size and spread of the primary tumor; T0 is a tumor that is completely localized to one spot, while T4 is a tumor which has grown enormously through the body. N classifies the involvement of the lymph nodes; N0 is a mesothelioma which has not affected the lymph nodes at all, while N4 mesothelioma is extensive involvement in the lymph system. M classifies the amount of metastasis that has already occurred; M0 is a mesothelioma which is entirely confined to the primary tumor, while M1 means metastasis has occurred. There is no M2 through M4, since metastasis is basically a yes or no question; the mesothelioma has either metastasized, or it hasn’t.

The TNM system is newer than the Butchart system; many physicians prefer it because it provides more detail about the nature of the mesothelioma and its progression in the individual patient.

These systems apply only to pleural mesothelioma. Pericardial mesothelioma and peritoneal mesothelioma have not yet had staging systems designed for them, although many doctors will informally stage them according to the TNM system.