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Mesothelioma Stages: Stage 1, 2, 3 & 4 Explained
Mesothelioma staging describes how far the cancer has grown and spread at the time it is assessed. For diffuse pleural mesothelioma, the current AJCC/UICC TNM Version 9 system groups disease into Stage I, Stage II, Stage IIIA, Stage IIIB, and Stage IV.
Mesothelioma staging describes how far the cancer has grown and spread at the time it is assessed. For diffuse pleural mesothelioma, the current AJCC/UICC TNM Version 9 system groups disease into Stage I, Stage II, Stage IIIA, Stage IIIB, and Stage IV based on the primary tumor (T), regional lymph nodes (N), and distant metastasis (M).
The current stage groupings are:
- Stage I: T1 N0 M0
- Stage II: T1 N1 M0 or T2 N0 M0
- Stage IIIA: T1 N2 M0, T2 N1/N2 M0, or T3 Any N M0
- Stage IIIB: T4 Any N M0
- Stage IV: Any T, Any N, M1
These categories matter because stage helps doctors describe disease extent, discuss prognosis and plan treatment. But stage does not determine treatment by itself. Histologic subtype, lymph-node status, resectability, cardiopulmonary fitness, performance status, overall health and patient preferences can also substantially affect management.
The formal TNM staging system described on this page applies to pleural mesothelioma. Peritoneal, pericardial and other uncommon mesotheliomas do not use this same pleural TNM stage-grouping system.
On This Page
- Mesothelioma Stages at a Glance
- What Are the Stages of Mesothelioma?
- How the TNM System Works
- AJCC/UICC Version 9 Stage Groupings
- Stage I Mesothelioma
- Stage II Mesothelioma
- Stage III Mesothelioma
- Stage IV Mesothelioma
- Stage 1 vs Stage 2 vs Stage 3 vs Stage 4
- How Doctors Determine Mesothelioma Stage
- How Stage Affects Treatment Planning
- How Stage Affects Prognosis
- Frequently Asked Questions
Mesothelioma Stages at a Glance
| Stage | AJCC/UICC Version 9 Grouping | Main Staging Feature |
|---|---|---|
| Stage I | T1 N0 M0 | Limited T1 pleural disease; no regional nodes or distant metastasis |
| Stage II | T1 N1 M0 or T2 N0 M0 | T1 disease with N1 nodes, or T2 disease without nodal spread |
| Stage IIIA | T1 N2 M0, T2 N1/N2 M0, or T3 Any N M0 | More advanced locoregional tumor and/or nodal disease |
| Stage IIIB | T4 Any N M0 | T4 locally advanced disease without distant metastasis |
| Stage IV | Any T, Any N, M1 | Distant metastatic disease |
A key point is that Stage III remains M0, while Stage IV requires M1. A very extensive T4 tumor can still be Stage IIIB if distant metastasis has not been identified.
What Are the Stages of Mesothelioma?
Cancer stage describes the anatomical extent of disease. For pleural mesothelioma, doctors first determine the T, N and M categories and then combine them into an overall stage group.
Lower stages generally represent less extensive anatomical disease, while higher stages reflect increasing local invasion, regional lymph-node involvement and, at Stage IV, distant metastatic disease. However, two patients with the same overall stage can still have different tumor biology, medical fitness and treatment options.
The stage should therefore be viewed as an important part of the clinical picture, not as a complete description of the patient or the cancer.
Which Mesothelioma Types Use the TNM Staging System?
The formal Stage I–IV TNM system is used for pleural mesothelioma, which begins in the pleura surrounding the lungs and lining the inside of the chest wall.
Other forms of mesothelioma include:
- peritoneal mesothelioma;
- pericardial mesothelioma;
- other rare primary sites.
These cancers still need an assessment of disease extent, but they are not assigned Stage I–IV using the same pleural TNM system discussed here.
This distinction matters because a reader should not apply a pleural Stage II or Stage IV definition to peritoneal mesothelioma.
How the TNM System Works
The TNM system uses three components.
Primary Tumor
Regional Lymph Nodes
Distant Metastasis
T — Primary Tumor
The T category describes the primary pleural tumor and its local anatomical extent.
The Version 9 clinical T classification includes quantitative CT measurements called Psum and Fmax, together with specific patterns of local invasion.
N — Regional Lymph Nodes
The N category describes regional lymph-node involvement:
- N0: no regional lymph-node metastasis;
- N1: ipsilateral intrathoracic nodal metastasis;
- N2: contralateral intrathoracic or ipsilateral/contralateral supraclavicular nodal metastasis.
M — Distant Metastasis
The M category describes distant metastatic disease:
- M0: no distant metastasis identified;
- M1: distant metastasis present.
M1 is especially important because any T and any N with M1 is Stage IV.
AJCC/UICC Version 9 Stage Groupings
Jump to a Stage
Stage I Mesothelioma
TNM Combination
T1 N0 M0
Stage I is the current earliest overall stage grouping.
Clinically, cT1 disease is limited to the ipsilateral pleura, with no fissural involvement, Psum ≤12 mm, and Fmax ≤5 mm. There is no regional lymph-node metastasis and no distant metastasis.
Current Version 9 resources list Stage I as one group, T1N0M0; they do not present current Stage IA and IB subdivisions for pleural mesothelioma.
Older articles may still use Stage IA/IB terminology from earlier staging frameworks, so the staging edition should always be checked when comparing information.
Stage II Mesothelioma
Stage II can occur in two distinct ways: T2 N0 M0 or T1 N1 M0.
This is a major reason Stage II should not simply be described as “Stage I that has grown a little further.”
T2 N0 M0
The primary tumor meets T2 criteria, but no regional lymph-node metastasis has been identified and there is no distant metastasis.
For clinical staging, T2 can include Psum greater than 12 mm but no more than 30 mm, or lower Psum with specified features such as qualifying fissural involvement, mediastinal fat invasion or a solitary focus of chest-wall soft-tissue invasion.
T1 N1 M0
The primary tumor remains T1, but cancer has spread to ipsilateral intrathoracic N1 lymph nodes. Distant metastasis is absent.
Therefore Stage II does not always mean lymph nodes are involved, and it does not always mean the primary tumor is T2.
Stage III Mesothelioma
Version 9 divides Stage III into Stage IIIA and Stage IIIB.
Both remain M0, meaning distant metastatic disease has not been identified.
Stage IIIA
Stage IIIA includes:
- T1 N2 M0
- T2 N1 M0
- T2 N2 M0
- T3 Any N M0
Clinical T3 includes pleural disease with Psum >30 mm, with or without specified local invasive features. N2 refers to contralateral intrathoracic nodal metastasis or supraclavicular nodal disease on either side.
Stage IIIB
Stage IIIB is: T4 Any N M0.
T4 can include features such as extensive chest-wall involvement, rib involvement, invasion of mediastinal organs, direct extension through the diaphragm or pericardium, direct extension to the contralateral pleura, or malignant pericardial effusion.
Stage IV Mesothelioma
Stage IV is defined by: Any T, Any N, M1.
The defining feature is distant metastatic disease.
The primary tumor does not need to be T4, and the patient does not need to have a particular regional lymph-node category. Once M1 disease is established, the overall stage is Stage IV.
This creates an important distinction: T4 Any N M0 = Stage IIIB, but Any T Any N M1 = Stage IV.
Distant metastatic disease can involve sites such as distant lymph nodes, bone, the peritoneum, or other distant organs and tissues. Contralateral thoracic findings require careful interpretation because direct extension to the contralateral pleura is a T4 feature, while discontinuous metastatic involvement can represent distant disease.
Stage 1 vs Stage 2 vs Stage 3 vs Stage 4
| Feature | Stage I | Stage II | Stage III | Stage IV |
|---|---|---|---|---|
| M status | M0 | M0 | M0 | M1 |
| Primary tumor | T1 | T1 or T2 | T1–T4 depending on subgroup | Any T |
| Nodes | N0 | N0 or N1 | N0–N2 depending on subgroup | Any N |
| Main distinction | T1N0 disease | T2N0 or T1N1 | More advanced locoregional disease | Distant metastasis |
| Subgroups | No current IA/IB grouping | None | IIIA and IIIB | None |
The stages are therefore not simply four progressively larger tumors. T, N and M interact to determine the final stage grouping.
What Changed With AJCC/UICC Version 9?
Version 9 uses revised pleural mesothelioma stage groupings and clinical T criteria. Current clinical T classification incorporates quantitative CT measurements, including Psum and Fmax.
Psum
Psum represents the sum of maximal pleural thickness measured on CT in the upper, middle and lower thirds of the affected hemithorax:
Psum = pmax1 + pmax2 + pmax3.
Fmax
Fmax is the maximum pleural tumor thickness measured along the fissures.
These measurements help define clinical T1, T2 and T3 disease, but the detailed criteria are better addressed in the individual Stage 1, Stage 2 and Stage 3 guides rather than repeated in full on this hub.
Version 9 also uses the current stage grouping in which Stage I = T1N0M0 and Stage II includes T1N1M0 as well as T2N0M0.
How Doctors Determine Mesothelioma Stage
Staging combines information from imaging, pathology and the overall clinical evaluation.
CT
Contrast-enhanced CT is central to defining pleural tumor extent, identifying suspicious lymph nodes, assessing nearby structures and looking for disease beyond the primary hemithorax. ASCO recommends CT of the chest and upper abdomen with IV contrast as the initial staging study.
FDG PET/CT
FDG PET/CT can help identify additional nodal or distant disease that might alter treatment planning. ASCO recommends it for initial staging, although it may be omitted when definitive surgical resection is not being considered.
Biopsy and Pathology
Adequate tissue is important to establish the diagnosis and histologic subtype when cancer-directed treatment is planned. Histology also affects prognosis and treatment decisions.
MRI
MRI can be used selectively when additional detail is needed regarding structures such as the chest wall, diaphragm or mediastinum.
Lymph-Node Assessment
Nodal status can change the overall stage substantially. For example:
T1 N0 → Stage I
T1 N1 → Stage II
T1 N2 → Stage IIIA.
This is why accurate staging involves more than measuring the primary pleural tumor.
Learn more about the full evaluation process in our mesothelioma diagnosis guide.
Clinical Stage vs Pathologic Stage
The clinical stage is determined before definitive surgery using information such as imaging, physical examination and biopsy.
When surgery is performed, examination of surgically obtained tissue can provide a pathologic stage. Pathologic staging may reveal more extensive disease than was visible before surgery.
Version 9 also uses different details for clinical and pathological T classification. For example, Psum and Fmax are part of the current clinical T framework, while pathologic T classification relies on direct anatomical findings in resected tissue.
Therefore: clinical T stage ≠ automatically pathologic T stage.
Does Stage Determine Whether Mesothelioma Is Resectable?
Not by itself.
The stage helps describe anatomical disease extent, but stage and resectability are not interchangeable concepts.
Current ASCO guidance advises against offering maximal surgical cytoreduction simply because a tumor appears anatomically resectable. Surgery should be considered only in highly selected patients after comprehensive staging and multidisciplinary assessment.
Important factors include:
- histologic subtype;
- nodal status;
- anatomical resectability;
- performance status;
- cardiopulmonary reserve;
- overall health;
- ability to tolerate multimodality treatment;
- patient goals.
How Stage Affects Treatment Planning
Stage is one component of treatment selection.
For some patients with favorable nonmetastatic disease, a multidisciplinary team may consider a surgery-based multimodality strategy. ASCO, however, reserves maximal cytoreduction for highly selected patients rather than recommending it routinely based on stage alone.
For unresectable or metastatic disease, systemic treatments can include contemporary immunotherapy, chemotherapy and chemoimmunotherapy strategies depending on histology and individual circumstances. Current treatment planning also considers previous therapy, medical contraindications, performance status and patient preferences.
The role of radiation, pleural procedures, clinical trials and palliative care also depends on the specific clinical situation rather than the stage number alone.
Considering Your Next Step?
Connect with an experienced mesothelioma treatment center to discuss staging results and treatment planning.
How Stage Affects Prognosis
Stage is an important prognostic factor, but it cannot predict an individual outcome by itself.
The NCI lists other relevant factors including:
- tumor extent;
- whether disease can be completely resected;
- pleural fluid burden;
- age;
- performance status;
- general health;
- heart and lung health;
- histologic subtype;
- selected blood-count parameters;
- whether disease is newly diagnosed or recurrent.
For this reason, assigning a fixed life expectancy to each mesothelioma stage would be medically misleading.
Population-level survival statistics describe groups, not individual outcomes, and older statistics may also reflect different staging systems and treatment eras.
SEER Stage Is Not the Same as AJCC Stage
Readers may also encounter survival statistics divided into:
- localized;
- regional;
- distant.
These are SEER Summary Stage categories, not direct replacements for AJCC Stage I, II, III and IV.
Therefore: SEER regional ≠ AJCC Stage II or III, and SEER distant should not automatically be labeled AJCC Stage IV survival.
This distinction is important when comparing mesothelioma survival statistics online.
Can Mesothelioma Stage Change?
The extent of the cancer can change over time, but technically the original stage assigned at diagnosis remains part of the medical record.
If disease progresses or recurs, doctors may perform new imaging and other tests to reassess how far it has spread. This process can be called restaging. A new assessment is added to the original stage rather than replacing the original diagnosis-stage classification.
In everyday clinical conversation, people may still say that a cancer “has become Stage 4” after later distant spread. Technically, however, survival statistics generally refer to the stage present at initial diagnosis, so this distinction matters.
Why Getting the Stage Right Matters
Correct staging can influence:
- treatment planning;
- whether additional nodal evaluation is needed;
- assessment of surgical candidacy;
- systemic-treatment decisions;
- clinical-trial eligibility;
- prognosis discussions.
Pleural mesothelioma is uncommon and staging can be complex, particularly when distinguishing clinical tumor extent, nodal disease and suspected distant metastasis.
A second opinion can be useful when there is uncertainty about pathology, stage or the proposed treatment strategy. The NCI notes that another specialist can review pathology, scans and the treatment plan and may confirm or modify the interpretation.
Questions to Ask After Learning Your Mesothelioma Stage
Consider discussing questions such as:
- Which TNM edition was used to determine my stage?
- What are my exact T, N and M categories?
- Is my stage clinical or pathologic?
- What imaging was used to determine the stage?
- Were my Psum and Fmax measurements assessed?
- Which regional lymph nodes have been evaluated?
- Is additional nodal sampling needed?
- Has distant metastatic disease been ruled out?
- What is my histologic subtype?
- Is my disease considered technically resectable?
- Does my stage make me a candidate for surgery, or are other factors more important?
- What systemic treatment options fit my histology and overall health?
- Should my pathology or scans be reviewed at a mesothelioma center?
- Are there appropriate clinical trials?
- What factors besides stage most affect my prognosis?
Frequently Asked Questions
How many stages of mesothelioma are there?
Diffuse pleural mesothelioma is grouped from Stage I through Stage IV, with Stage III divided into IIIA and IIIB under the current Version 9 system.
What is Stage 1 mesothelioma?
Stage I pleural mesothelioma is T1 N0 M0. There is no regional lymph-node metastasis and no distant metastasis.
Does current Stage 1 have Stage 1A and 1B?
The current AJCC/UICC Version 9 stage grouping shown in the official Version 9 resources lists Stage I as T1 N0 M0 without current IA and IB stage groups.
What is Stage 2 mesothelioma?
Stage II is either T2 N0 M0 or T1 N1 M0. It therefore does not always involve lymph-node metastasis.
What is Stage 3 mesothelioma?
Stage III is nonmetastatic M0 disease divided into Stage IIIA and Stage IIIB. Stage IIIA includes selected T/N combinations, while Stage IIIB is T4 Any N M0.
What is Stage 4 mesothelioma?
Stage IV is Any T, Any N, M1. Distant metastatic disease is what defines Stage IV.
Is T4 always Stage 4?
No. T4 Any N M0 is Stage IIIB, not Stage IV. Stage IV requires M1.
Does lymph-node involvement always mean Stage 3?
No. T1 N1 M0 is Stage II. Other nodal combinations can produce Stage IIIA depending on the T category.
Which mesothelioma types use Stage 1 through Stage 4?
The formal TNM Stage I–IV framework described here applies to pleural mesothelioma. Less common mesotheliomas do not use this same formal pleural staging system.
Does mesothelioma stage determine treatment?
Not by itself. Stage is important, but histology, nodal status, resectability, performance status, cardiopulmonary fitness, other medical conditions and patient preferences also affect treatment planning.
Does stage determine life expectancy?
No single life-expectancy number can accurately predict an individual outcome from stage alone. Prognosis also depends on histology, age, performance status, general health, resectability and other clinical factors.
Can the stage be different after surgery?
Yes. A pathologic stage determined from surgical findings can reveal more extensive disease than the preoperative clinical stage.
Can mesothelioma be restaged?
Cancer can be reassessed after progression or recurrence, but the original stage at diagnosis is generally retained and new restaging information is added to it.
Key Takeaways
- The current formal staging system applies to diffuse pleural mesothelioma.
- Stage I = T1 N0 M0.
- Stage II = T1 N1 M0 or T2 N0 M0.
- Stage IIIA = T1 N2 M0, T2 N1/N2 M0, or T3 Any N M0.
- Stage IIIB = T4 Any N M0.
- Stage IV = Any T, Any N, M1.
- Current Version 9 resources do not subdivide Stage I into current IA/IB groups.
- Stage II does not always involve lymph nodes.
- Stage III remains M0.
- T4 disease is not automatically Stage IV.
- Distant metastasis, M1, defines Stage IV.
- Version 9 clinical T classification uses Psum and Fmax measurements.
- Clinical and pathologic stages can differ.
- Stage does not determine surgical candidacy by itself.
- Histology, nodal status, resectability, overall fitness and other clinical factors also influence treatment and prognosis.
- SEER localized/regional/distant categories should not be treated as direct equivalents of AJCC Stage I–IV.
Related Mesothelioma Resources
Medical References
- American Joint Committee on Cancer. Diffuse Pleural Mesothelioma. AJCC Cancer Staging System, Version 9. American College of Surgeons; 2024.
- NHS England / National Disease Registration Service. Pleural Mesothelioma — TNM 9th Edition Staging Sheet. Updated January 16, 2026.
- American Cancer Society. Stages of Mesothelioma. Last revised February 3, 2026.
- Kindler HL, Ismaila N, Bazhenova L, et al. Treatment of Pleural Mesothelioma: ASCO Guideline Update. Journal of Clinical Oncology. 2025;43:1006–1038. doi:10.1200/JCO-24-02425.
- National Comprehensive Cancer Network. NCCN Clinical Practice Guidelines in Oncology: Mesothelioma: Pleural. Version 2.2026.
- National Cancer Institute. Mesothelioma Diagnosis & Prognosis. Updated May 16, 2025.
- National Cancer Institute. Malignant Mesothelioma Treatment (PDQ®) — Health Professional Version.
- National Cancer Institute. Cancer Staging.
Medical Disclaimer: This article is intended for general educational purposes only. It does not provide medical advice, diagnosis, or treatment and should not replace consultation with a qualified healthcare professional. Treatment and staging decisions must be made by a patient and their medical team based on the individual diagnosis.
