How is mesothelioma staged?
Staging is the process of finding out how far the cancer has spread. Staging of mesothelioma is based on imaging studies such as x-rays, CT scans, and MRI scans. The treatment and outlook for patients with mesothelioma largely depends on the stage (extent of spread) of their cancer. Since pleural mesothelioma occurs most frequently and has been studied the most, it is the only mesothelioma for which a staging classification exists.
The staging system most often used for mesothelioma is the Butchart system. This system is based mainly on the extent of the primary tumor mass, and divides mesotheliomas into stages I through IV.
Butchart Staging System
Stage I: Mesothelioma is present within the right or left pleura, and may also involve the lung, pericardium, or diaphragm (the muscle separating the chest from the abdomen) on the same side.
Stage II: Mesothelioma invades the chest wall or involves the esophagus (food passage connecting the throat to the stomach), heart, or pleura on both sides. The lymph nodes in the chest may also be involved.
Stage III: Mesothelioma has penetrated through the diaphragm into the peritoneum (lining of the abdominal cavity). Lymph nodes beyond those in the chest may also be involved.
Stage IV: There is evidence of distant metastases (spread through the bloodstream to other organs).
Another staging system has recently been developed by the American Joint Committee on Cancer (AJCC). This is a TNM system, similar to staging systems used for most other cancers. T stands for tumor (its size and how far it has spread to nearby organs), N stands for spread to lymph nodes and M is for metastasis (spread to distant organs). In TNM staging, information about the tumor, lymph nodes, and metastasis is combined in a process called stage grouping to assign a stage described by Roman numerals from I to IV. Minor differences exist between the AJCC TNM staging system and the Butchart staging system.
TNM Staging System
Stage I: Mesothelioma involves the right or left pleura. It may also have spread into the lung, pericardium, or diaphragm on the same side. It has not yet spread to the lymph nodes.
Stage II: Mesothelioma has spread from the pleura on one side to the nearby peribronchial and/or hilar lymph nodes next to the lung on the same side. It may also have spread into the lung, pericardium, or diaphragm on the same side.
Stage III: Mesothelioma has spread into the chest wall muscle, ribs, heart, esophagus, or other organs in the chest on the same side as the primary tumor, with or without spread to subcarinal and/or mediastinal lymph nodes on the same side as the main tumor. Subcarinal nodes are located at the point where the windpipe branches to the left and right lungs. Mediastinal lymph nodes are located in the space behind the chest bone in front of the heart. Mesotheliomas with the same extent of local spread as in stage II that have also spread to subcarinal and/or mediastinal lymph nodes on the same side are also included in stage III.
Stage IV: Mesothelioma has spread into the lymph nodes in the chest on the side opposite that of the primary tumor, or directly extends to the pleura or lung on the opposite side, or directly extends into the peritoneum, or directly extends into organs in the abdominal cavity or neck. Any mesothelioma with evidence of distant metastases (spread to other organs through the bloodstream) or spread to organs beyond the chest or abdomen is included in this stage.
Although the recently developed TNM classification is the most detailed and precise, the original Butchart staging system is still used most often to describe the spread of pleural mesotheliomas. Understanding these staging systems for mesothelioma is important both for estimating and better understanding prognosis, and also for assessing therapeutic options.
Because pleural mesothelioma has been better studied than peritoneal mesothelioma we know more about factors associated with prognosis for pleural mesothelioma. Younger age at diagnosis, performance status (functional status) and absence of weight loss are associated with a more favorable prognosis.
Mesotheliomas are usually of three different cell types (histological analysis): 1) epithelial cell type - has the most favorable prognosis; 2) fibrosarcomatous cell type - carries the worst prognosis and 3) mixed cell type - has an intermediate prognosis.