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Stages of Mesothelioma
After someone is diagnosed with mesothelioma, doctors will try to figure out if it has spread, and if so, how far. This process is called staging.
The stage of a cancer describes how much cancer is in the body. It helps determine how serious the cancer is and how best to treat it. Doctors also use a cancer's stage when talking about survival statistics.
How is the stage determined?
Pleural mesothelioma, the most common type, is the only mesothelioma that has a formal staging system. These mesotheliomas start in the pleura, which includes the lining of the lungs and the inner lining of the chest wall.
Less common mesotheliomas, such as peritoneal mesothelioma (in the belly) and pericardial mesothelioma (in the heart sac), don’t have formal staging systems. But it’s still important for doctors to determine the extent of the cancer in the body. This can help determine treatment options.
The staging system most often used for pleural mesothelioma is the American Joint Committee on Cancer (AJCC) TNM system, which is based on 3 key pieces of information:
- The extent of the main (primary) tumor (T): How far has cancer spread in the pleura? Has it spread into other nearby pleura or structures? Can it be removed with surgery?
- The spread to nearby lymph nodes (N): Has the cancer spread to nearby lymph nodes?
- The spread (metastasis) to distant parts of the body (M): Has the cancer spread to distant organs, such as the bones, liver, the lungs or pleura on the other side of the body, or the peritoneum (the lining of the abdomen)?
Numbers or letters after T, N, and M provide more details about each of these factors. Higher numbers mean the cancer is more advanced.
Once a person’s T, N, and M categories have been determined, this information is combined in a process called stage grouping to assign an overall stage. For more information, see Cancer Staging.
Clinical stage vs. pathological stage
The clinical stage is based on the results of physical exams, biopsies, and imaging tests. The clinical stage is used to help decide on treatment options.
If surgery is done, the pathological stage, also called the surgical stage, can be determined. It’s based on the clinical stage, plus what’s learned about the extent of the cancer during the operation.
The pathological stage is often more accurate, because it is based on a firsthand view inside the body. The pathological stage might be more advanced than the clinical stage if surgery reveals that the cancer has spread farther than was seen on imaging tests.
TNM stages of pleural mesothelioma
The stages of pleural mesothelioma in the TNM system range from I (1) through IV (4). As a rule, the lower the number, the less the cancer has spread. Within a stage, an earlier letter means a lower stage.
Each person with cancer is unique, but cancers with similar stages tend to have a similar outlook and are often treated in much the same way.
Mesothelioma staging can be complex, so ask your doctor to explain it to you in a way you understand.
Stage grouping: T1, N0, M0
Mesothelioma is in the pleura lining the chest wall on one side of the chest. The total thickness of the pleural measured at 3 different spots on imaging (Psum) does not exceed 12 mm. It does not involve the fissure, where the lung lobes separate (T1).
The cancer has not spread to nearby lymph nodes (N0) or to distant parts of the body (M0).
Stage II pleural mesothelioma can have 2 possible stage groupings.
T2, N0, M0
Mesothelioma is in the pleura on one side of the chest. The total thickness of the pleura measured at 3 different spots on imaging (Psum) does not exceed 12 mm.
It involves the fissure (where the lung lobes separate), fatty tissue between the lungs (fat in the mediastinum), or a single place in the chest wall; OR mesothelioma is in the pleura on one side of the chest and the Psum exceeds 12 mm but does not exceed 30 mm (T2).
The cancer has not spread to nearby lymph nodes (N0) or to distant parts of the body (M0).
T1, N1, M0
Mesothelioma is in the pleura on one side of the chest. The total thickness of the pleura measured at 3 different spots on imaging (Psum) does not exceed 12 mm. It does not involve the fissure, where the lung lobes separate (T1).
The cancer has spread to nearby lymph nodes on the same side of the body as the main tumor (N1). It has not spread to distant parts of the body (M0).
Stage IIIA
Stage IIIA can have 3 possible stage groupings.
T2, N1, M0
Mesothelioma is in the pleura on one side of the chest. The total thickness of the pleura measured at 3 different spots on imaging (Psum) does not exceed 12 mm. It involves the fissure (where the lung lobes separate), fatty tissue between the lungs (fat in the mediastinum), or single place in the chest wall; OR mesothelioma is in the pleura on one side of the chest and the Psum exceeds 12 mm but does not exceed 30 mm (T2).
The cancer has spread to nearby lymph nodes on the same side of the body as the main tumor (N1). It has not spread to distant parts of the body (M0).
T3, N0/N1, M0
Mesothelioma is in the pleura on one side of the chest. The total thickness of the pleura measured at 3 different spots on imaging (Psum) exceeds 30 mm (T3).
The cancer has not spread to nearby lymph nodes (N0) OR has spread to nearby lymph nodes on the same side of the body as the main tumor (N1). It has not spread to distant parts of the body (M0).
T1-T3, N2, M0
Mesothelioma might or might not have grown into nearby structures, and it might still possibly be removed (resected) with surgery (T1 to T3).
The cancer has spread to nearby lymph nodes on the other side of the body, or to lymph nodes above the collarbone (supraclavicular lymph nodes) on either side (N2). It has not spread to distant parts of the body (M0).
Stage IIIB
T4, any N, M0
Mesothelioma has grown too far to be removed completely with surgery (T4). The tumor has grown into at least 1 of the following:
- Into a bone in the chest wall, such as a rib
- Into an organ in the mediastinum (heart, spine, esophagus, trachea, or blood vessels)
- Through the diaphragm (the thin muscle separating the chest from the abdomen) or sac around the heart (pericardium)
- Across to the pleura on the other side of the chest
- In the fluid surrounding the heart (pericardial effusion)
The cancer might or might not have spread to nearby lymph nodes (any N). It has not spread to distant parts of the body (M0).
Any T, any N, M1
Mesothelioma may or may not have grown into nearby structures (any T). It might or might not have spread to nearby lymph nodes (any N). It has spread to distant organs, like the bones, the lung or pleura on the other side of the body, or the peritoneum (the lining of the abdomen) (M1).
The following categories might also be used:
- TX: Main tumor cannot be assessed due to lack of information.
- T0: There's no evidence of a primary tumor.
- NX: Nearby lymph nodes cannot be assessed due to lack of information.
Resectable vs. unresectable mesothelioma
The TNM system groups pleural mesotheliomas into stages that help give doctors an idea about a person’s prognosis (outlook).
But for treatment purposes, doctors often use a simpler system, based on whether the cancer is likely to be resectable, where all visible tumors can be removed by surgery, or unresectable, where all of the cancer cannot be removed.
Most stage I and II and some stage III mesotheliomas are resectable, but there are exceptions. Whether the cancer can be removed depends on how far the tumor has grown, its subtype, where it’s located, and if a person is healthy enough to have surgery.
Even for resectable mesotheliomas, cancer cells that can’t be seen are often left behind after surgery. Because of this, doctors also often use other treatments, such as chemotherapy, immunotherapy, and radiation therapy when possible.
Other prognostic factors for mesothelioma
Stage is an important factor in predicting a person’s prognosis (outlook), but other factors also play a role. Some factors linked to longer survival times include:
- Still being able to carry out normal daily tasks
- Being younger
- Being female
- Having the epithelioid subtype of mesothelioma
- Having normal levels of red blood cells, white blood cells, and platelets
- Written by
- References
Developed by the American Cancer Society medical and editorial content team with medical review and contribution by the American Society of Clinical Oncology (ASCO).
American Joint Committee on Cancer. Malignant Pleural Mesothelioma. In: AJCC Cancer Staging System. Version Nine. American College of Surgeons; 2024.
National Comprehensive Cancer Network. NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines): Mesothelioma: Pleural. Version 2.2026. Accessed at www.nccn.org on November 13, 2025.
Last Revised: February 3, 2026
American Cancer Society medical information is copyrighted material. For reprint requests, please see our Content Usage Policy.
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