Certain organs such as the heart, the lungs, and the abdomen are surrounded by a thin tissue membrane. The pericardium is the membrane that surrounds the heart, the pleura surrounds the lungs, and the peritoneum surrounds the abdomen. These membranous layers are comprised of cells called mesothelium.
Cells within the body constantly divide and grow. An irregular or abnormal growth or division of the cells creates a marked mass (tumor). A tumor that develops in the mesothelium is called mesothelioma, or cancer of the mesothelium.
The most common type of mesothelioma is Malignant Pleural Mesothelioma (MPM). The rarest form of mesothelioma is pericardial mesothelioma. Mesothelioma is also referred celothelioma and/or asbestos cancer. (It is important to note that Asbestosis is not the same as Mesothelioma. Asbestosis is a non- malignant, chronic lung disease.)
Word Roots:
meso = middle or median
thelium = layer of cellular tissue
The stage of mesothelioma determines a patient's treatment options. The most common type of staging system is the TNM system. The TNM staging system is based on three variables: the size and location of the tumor, the involvement of the lymph nodes, and metastasis. The TNM system is made up of four stages. In Stage I the lymph nodes are not involved and mesothelioma involves either the right or left pleura. In Stage II the lymph nodes are affected on the same side as the mesothelioma on the pleura and/or lung. In Stage III the lymph nodes may or may not be affected but the chest wall, heart, and other organs in the chest are affected by mesothelioma. And in the final stage, Stage IV, mesothelioma has spread throughout the chest and into most of the chest lymph nodes. Distant metastases are also included in this stage.
Differentiation of mesothelioma from bronchogenic carcinoma is necessary to determine the optimal treatment strategy, and often requires histologic and/or histochemical examination of tissue samples collected by tumor biopsy. Mesothelioma can be primarily sarcomatoid or epithelial, and the epithelial type histology resembles that of bronchogenic adenocarcinoma. Acid mucopolysaccharide-positive, perinuclear keratin-positive, and CEA/Leu-M1-negative samples favor diagnosis of mesothelioma over adenocarcinoma. Clinical and radiologic presentation as well as gross appearance of the different tumor types can be markedly similar. While smoking history has a different impact on the risk for development of mesothelioma versus bronchogenic carcinoma, it is not helpful in distinguishing the two entities clinically.
Mesothelial cells are found on surfaces of the pleural, peritoneal cavities as well as the testis.
Malignant transformation is uncommon but when it does occur it usually arises on the pleural surfaces (80%) or peritoneal surfaces (18%).
Tumors usually begin as discrete plaques or nodules on the pleural or peritoneal lining and as they progress they will form sheets that cover the entire surface of the involved area. It is not unusual to see malignant cells grow along drainage or thoracotomy tracts.
Asbestos exposure is the most common cause of the disease (80%). There is approximately a 20 to 40 year latent period from the time of asbestos exposure to the development of the disease.
Men are more commonly affected due to work exposure, however women may develop it due to second hand exposure from asbestos fibers carried home on clothes or other objects.
Establishing a diagnosis of mesothelioma is difficult. At this point serum tumor markers have no role in the diagnosis or screening of mesothelioma. Having a high degree of suspicion due to history will help.
Chest CT is important to detect invasion of the chest wall, ribs and mediastinal structures.
Video-assisted thorascopic biopsy is the preferred method. Thoracentesis or closed pleural biopsy may help but may not provide enough tissue to differentiate between mesothelioma and adenocarcinoma.
Bronchoscopy and mediastinoscopy will also help in establishing the diagnosis.
There are 3 types of mesothelioma based on anatomical location (pleural, pericardial and peritoneal) and 3 sub-types of malignant mesotheliomas based on cell morphology (60% are epitheloid, 20% are sarcomatoid and about 25% are mixed). Sarcomatoid has the worst prognosis while epitheloid carries the best results.
Mesotheliomas are often discovered by finding excess fluid in the chest (chest x-ray), around the heart (echocardiography) and in the abdomen (ultrasound). Ultimately diagnosis comes from examination of mesothelial cells under the microscope. Treatment and prognosis are based on the type, sub-type and stage of the illness. .