Understanding the Stages of Mesothelioma

The condition known as mesothelioma is a type of lung cancer that affects the encasing layers (mesothelial membrane) of many of the body’s organs. It is caused exclusively by exposure to the dangerously harmful fibres contained in asbestos materials.

Knowing the stage of progression of any type of cancer is crucial, since it tells doctors how far the cancer has spread so that they can prescribe treatment accordingly.

During the diagnostic process (see ‘Guide to Diagnosing Mesothelioma’ for more information on this), your doctor will be able to determine the stage that the condition has advanced to. Unfortunately, since it can take as many as forty to sixty years for mesothelioma symptoms to appear after exposure to asbestos fibres, by the time most people are diagnosed, it has already progressed to an advanced stage of cancer. That being said, there are treatments available which can ease your discomfort, and in some cases it can be operated on.

The following guide will outline the stages of mesothelioma that have been identified for the two main areas where this type of cancer occurs – in the lungs (pleural) and the abdomen (peritoneal).

Stages of Pleural (Lung) Mesothelioma

In the UK, the most common staging system for pleural mesothelioma is known as the International Mesothelioma Interest Group (IMIG) system. This is based on the TNM staging system which is used to pin-point the progression of cancer in general.

The TNM staging system works by looking at:

  • The location and size of the primary tumour (T).

  • Whether the cancer cells have spread to nearby lymph nodes (N).

  • Whether the cancer cells have spread to other parts of the body, known as metastases (M).

Once these TNM factors have been clarified, the information is then used to determine one out of four possible stages for mesothelioma (1 being the earliest stage, 4 being the most advanced stage):

  • Stage 1a – mesothelioma has affected the outer layer of the pleura around the chest wall on one side only, and it may also have grown into the pleural tissue that covers the diaphragm.

Stage 1b – mesothelioma has begun to spread to the inner pleural layer that is closest to the lungs on one side of the chest only, but it has not spread into the lung tissue itself or the diaphragm.

  • Stage 2 – mesothelioma has spread to both inner and outer layers of the pleura on one side of the chest. It has also either enlarged to form a tumour on the pleural tissue around the lungs or has begun to spread into lung tissue itself or the diaphragm muscle.

  • Stage 3 – mesothelioma has spread to the chest wall or the covering of the pericardium (heart), but it can still potentially be removed by surgery. Alternatively, the cancer has spread to the lymph nodes on the same side of the chest.

  • Stage 4 – mesothelioma has progressed to the stage where it cannot be operated on for removal. It has spread to one of the following: different parts of the chest wall; the peritoneum (having grown through the diaphragm); the pleura on the other side of the chest; the chest organs; the inner layer of the pericardium; the lymph nodes on the other side of the chest; above the collarbone; or to other parts of the body.

Stages of Peritoneal Mesothelioma

At present, no specific staging system for peritoneal mesothelioma has been established, so it will purely be diagnosed according to the TNM system for cancer in general, as described above.


Establishing the stages of mesothelioma via the TNM and IMIG staging systems, which pin-point exactly how far the cancer has spread, has been highly beneficial in helping doctors treat their patients. Knowing what stage the cancer has progressed to will also assist in reaching an understanding of where and when you were exposed to asbestos materials, which will help you to better target your compensation claim.

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