Staging is a method of describing a cancer. It illustrates where the cancer is located, where it has spread and what part of the body’s organ it affects. Doctors apply diagnostic tests to verify the cancer's stage. It is based on the results of the physical exam, biopsies, and imaging tests (CT scan, PET scan, etc.).Therefore, staging may not be comprehensive until all the tests are completed. Determining the stage helps the doctor to make a decision on what kind of treatment is best to cure a patient. Given that pleural mesothelioma occurs most frequently and has been studied the most, it is the only mesothelioma wherein a staging system is present. Currently, there is no standard staging system for peritoneal mesothelioma.

The most common tool that doctors draw on to explain the stage is the TNM system. This system uses three criterions to evaluate the stage of the cancer which are the tumor itself, the lymph nodes around the tumor, and if the tumor has spread to the rest of the body. The results are combined to determine the stage of cancer for each person. TNM is an abbreviation for tumor (T), node (N), and metastasis (M). Doctors come across at these three factors to determine the stage of cancer:

- The size and position of the mesothelioma primary tumor (T)
- Whether the mesothelioma cells have spread to lymph nodes (N)
- Whether the mesothelioma cells have spread to other parts of the body - metastases (M)

Tumor- by means of the TNM system, the "T" plus a letter and number (0 to 4) is used to describe the stage of the tumor.

Specific tumor stage detail is listed below:

TX: The main tumor cannot be evaluated.
T0: There is no tumor.
T1: The tumor has attacked the thin membrane that lines the lung and inner chest walls on one side of the body, called the ipsilateral pleura.
T1a: The tumor involves the ipsilateral parietal pleura.
T1b: The tumor has invaded the ipsilateral parietal pleura and the visceral pleura.
T2: The tumor involves any of the ipsilateral pleural surfaces and invades either the diaphragm or the lung.
T3: The tumor involves any of the ipsilateral pleural surfaces, with no less than one of the following: invasion of the membrane that surrounds the thorax; invasion to the area of the chest between the lungs; a single area of invasion into the muscles of the chest wall; or minimal invasion into the lining around the heart.
T4: The tumor involves any of the ipsilateral pleural surfaces, with at least one of the following:

- Spread in several areas of the chest wall with or without involvement of rib
- Spread through the diaphragm to the abdominal cavity
- Spread of any mediastinal organs
- Extension to the pleura on the opposite side of the body
- Spread into the spine
- Invasion of the heart.

Node. The “N” in the TNM staging system stands for lymph nodes, the tiny, bean-shaped organs that help fight  against infection.

NX: The regional lymph nodes cannot be evaluated.
N0: Cancer has not spread to the regional lymph nodes.
N1: Cancer has spread to the bronchopulmonary lymph nodes on one side of the body and/or hilar lymph node.
N2: Cancer has spread to the subcarinal lymph node and/or the mediastinal lymph node on one side of the body, including the internal mammary lymph nodes on one side of the body and the peridiaphragmatic lymph node.
N3: Cancer has spread to the mediastinal lymph nodes on both sides of the body, the internal mammary lymph nodes, and/or supraclavicular lymph node on one or both sides of the body.
Distant metastasis. The “M” in the TNM system specifies whether the cancer has spread to other parts of the body.
M0: Cancer has not spread to other parts of the body.
M1: Cancer has spread to other parts of the body.

Mesothelioma Cancer Stages

Stage I: The tumor has invaded the ipsilateral parietal pleura, with or without involvement of the visceral pleura, but has not spread to the lymph nodes or other parts of the body (T1, N0, M0).
Stage IA: The tumor has invaded the ipsilateral parietal pleura, without involvement of the visceral pleura, but has not spread to the lymph nodes or other parts of the body (T1a, N0, M0).
Stage IB: The tumor has invaded the ipsilateral parietal pleura, with involvement of the visceral pleura, but has not spread to the lymph nodes or other parts of the body (T1b, N0, M0).
Stage II: The tumor is identified as T2, and cancer has not spread to the lymph nodes or all over the body (T2, N0, and M0).
Stage III: Some of the following situation:

- The tumor is defined as T1 or T2. Cancer has spread to the lymph nodes described  as N1 or N2 but not to other parts of the body (T1 or T2, N1 or N2, M0).
- The tumor is defined as T3 (see above), and cancer may or may not have spread to lymph nodes and has not spread to other parts of the body (T3, N0-2, M0).
Stage IV: One of the following circumstances:

- The tumor is defined as T4 (see above). Cancer may or may not have spread to the lymph nodes and has not spread to other parts of the body (T4, any N, M0).
- Cancer has spread to lymph nodes described as N3 but not to other parts of the body (any T, N3, M0).
- Cancer may or may not have spread to the lymph nodes and has spread to other parts of the body (any T, any N, M1).