Malignant tumours of the mesothelium are cancerous growths that have the potential to spread (metastasize) to other parts of the body. Malignant mesothelioma is an uncommon cancer that starts in the mesothelial cells. Mesothelial cells form a lining called the mesothelium, which covers the outer surface of most internal organs.
The most common places for mesothelioma to develop are the pleura (the membrane that covers the lungs and lines the chest), followed by the peritoneum (the membrane that covers the abdominal organs and lines the abdomen).
Most people who develop mesothelioma have a history of exposure to asbestos.
Cell subtypes of mesothelioma
Mesothelioma can be grouped into 3 main subtypes based on how the cells look under the microscope (histology). The 3 subtypes of mesothelioma are:
- epithelioid (epithelial)
- This is the most common subtype and accounts for about 50%–60% of all mesotheliomas.
- It has the most favourable prognosis of all the subtypes.
- Sometimes it can be difficult to tell an epithelioid pleural mesothelioma from lung cancer.
- sarcomatoid (fibrous)
- This subtype accounts for about 10% of all mesotheliomas.
- Desmoplastic mesothelioma is often considered to be a variation of sarcomatoid mesothelioma.
- mixed (biphasic)
- These tumours contain both epithelioid and sarcomatoid cells.
- They account for the remaining cases of mesothelioma.
Although the different subtypes of mesothelioma are generally treated the same, knowing the subtype helps doctors predict prognosis and how the disease will respond to treatment.
Mesothelioma most often develops in the pleura and accounts for about 70%–80% of all cases of malignant mesothelioma. Mesothelioma causes thickening of the pleura and most people develop a pleural effusion (abnormal buildup of fluid in the pleural space).
Mesothelioma typically grows along the pleura and extends to the muscles, ribs and diaphragm. Pleural mesothelioma does not usually spread beyond the chest until late in the course of the disease. Spread to the lymph nodes is more likely to occur with pleural mesothelioma than with peritoneal mesothelioma.
It can be difficult to tell pleural mesothelioma from other lung conditions, such as lung cancer.
Peritoneal mesothelioma is the second most common type of mesothelioma. It accounts for up to 25% of all cases of malignant mesothelioma. Most people develop ascites or peritoneal effusion (abnormal buildup of fluid in the abdomen).
Peritoneal mesothelioma often remains confined to the abdomen. However it is usually widespread throughout the peritoneal cavity. Peritoneal mesothelioma can spread to the pleural cavity. Spread (metastasis) to distant sites is not common with peritoneal mesothelioma and it does not usually spread to lymph nodes.
Very rarely, mesothelioma can develop in the pericardium and the testicle.
- pericardial mesothelioma
- The pericardium is the sac or membrane that surrounds the heart.
- Pericardial mesothelioma may cause heart rhythm problems, pericardial effusion (a buildup of fluid around the heart) and congestive heart failure (inability of the heart to pump blood efficiently).
- Pericardial mesothelioma can occur at any age, but most often occurs in people in their 40s to 70s.
- Surgery can be done to remove a mesothelioma from the pericardium or to relieve symptoms. A procedure called pericardiocentesis can be done to remove fluid from around the pericardium. Pericardiocentesis is used to collect a sample of fluid to be examined under a microscope or to reduce the amount of fluid around the pericardium.
- mesothelioma of the tunica vaginalis testis
- The tunica vaginalis testis is the outer lining of a man’s testicle.
- Most men with mesothelioma of the tunica vaginalis testis are 50 years of age or older.
- This type of mesothelioma may look like a hydrocele (collection of fluid in the scrotum) or a hernia (a loop of bowel that pushes into the groin or scrotum) at the time of diagnosis.
- Treatment is surgical removal of the mesothelioma, the testicle (orchiectomy) and lymph nodes in the groin (retroperitoneal lymph node dissection).