Most people who develop mesothelioma have worked in jobs where they inhaled or ingested asbestos fibers, or were exposed to airborne asbestos dust and fibers in other ways. Washing clothes of a family member who worked with asbestos also creates a risk for developing mesothelioma. Unlike lung cancer, there seems to be no association between mesothelioma and tobacco smoking, but smoking greatly increases the risk of other asbestos-induced cancers.
Signs and symptoms of mesothelioma include shortness of breath due to pleural effusion (fluid between the lung and the chest wall), chest wall pain, and constitutional signs such as unexplained weight loss. The diagnosis may be suspected based on chest X-ray and CT scan findings, but must be confirmed either by examining serous effusion cytology or with a biopsy (removing a sample of the suspicious tissue). A thoracoscopy (inserting a tube with a camera into the chest) can be used to acquire biopsy material, and allows the introduction of substances such as talc to obliterate the pleural space (a procedure called pleurodesis), preventing more fluid from accumulating and pressing on the lung. Despite treatment with chemotherapy, radiation therapy or sometimes surgery, mesothelioma carries a poor prognosis. Research about screening tests for the early detection of mesothelioma is ongoing.
Signs and symptoms
Symptoms or signs of mesothelioma may not appear until 20 to 50 years (or more) after exposure to asbestos. Shortness of breath, cough, and pain in the chest due to an accumulation of fluid in the pleural space (pleural effusion) are often symptoms of pleural mesothelioma.
Symptoms of peritoneal mesothelioma include weight loss and cachexia, abdominal swelling and pain due to ascites (a buildup of fluid in the abdominal cavity). Other symptoms of peritoneal mesothelioma may include bowel obstruction, blood clotting abnormalities, anemia, and fever. If the cancer has spread beyond the mesothelium to other parts of the body, symptoms may include pain, trouble swallowing, or swelling of the neck or face.
These symptoms may be caused by mesothelioma or by other, less serious conditions.
Mesothelioma that affects the pleura can cause these signs and symptoms:
- Chest wall pain
- Pleural effusion, or fluid surrounding the lung
- Shortness of breath
- Fatigue or anemia
- Wheezing, hoarseness, or cough
- Blood in the sputum (fluid) coughed up (hemoptysis)
In severe cases, the person may have many tumor masses. The individual may develop a pneumothorax, or collapse of the lung. The disease may metastasize, or spread, to other parts of the body.
Tumors that affect the abdominal cavity often do not cause symptoms until they are at a late stage. Symptoms include:
- Abdominal pain
- Ascites, or an abnormal buildup of fluid in the abdomen
- A mass in the abdomen
- Problems with bowel function
- Weight loss
In severe cases of the disease, the following signs and symptoms may be present:
- Blood clots in the veins, which may cause thrombophlebitis
- Disseminated intravascular coagulation, a disorder causing severe bleeding in many body organs
- Jaundice, or yellowing of the eyes and skin
- Low blood sugar level
- Pleural effusion
- Pulmonary emboli, or blood clots in the arteries of the lungs
- Severe ascites
A mesothelioma does not usually spread to the bone, brain, or adrenal glands. Pleural tumors are usually found only on one side of the lungs.
Cause
Working with asbestos is the major risk factor for mesothelioma. In the United States, asbestos is the major cause of malignant mesothelioma and has been considered "indisputably" associated with the development of mesothelioma. Indeed, the relationship between asbestos and mesothelioma is so strong that many consider mesothelioma a “signal” or “sentinel” tumor.A history of asbestos exposure exists in most cases. However, mesothelioma has been reported in some individuals without any known exposure to asbestos. In rare cases, mesothelioma has also been associated with irradiation, intrapleural thorium dioxide (Thorotrast), and inhalation of other fibrous silicates, such as erionite. Some studies suggest that simian virus 40 (SV40) may act as a cofactor in the development of mesothelioma. This has been confirmed in animal studies, but studies in humans are inconclusive.
Asbestos was known in antiquity, but it was not mined and widely used commercially until the late 19th century. Its use greatly increased during World War II. Since the early 1940s, millions of American workers have been exposed to asbestos dust. Initially, the risks associated with asbestos exposure were not publicly known. However, an increased risk of developing mesothelioma was later found among shipyard workers, people who work in asbestos mines and mills, producers of asbestos products, workers in the heating and construction industries, and other tradespeople. Today, the official position of the U.S. Occupational Safety and Health Administration (OSHA) and the U.S. EPA is that protections and "permissible exposure limits" required by U.S. regulations, while adequate to prevent most asbestos-related non-malignant disease, they are not adequate to prevent or protect against asbestos-related cancers such as mesothelioma. Likewise, the British Government's Health and Safety Executive (HSE) states formally that any threshold for exposure to asbestos must be at a very low level and it is widely agreed that if any such threshold does exist at all, then it cannot currently be quantified. For practical purposes, therefore, HSE assumes that no such "safe" threshold exists. Others have noted as well that there is no evidence of a threshold level below which there is no risk of mesothelioma. There appears to be a linear, dose-response relationship, with increasing dose producing increasing disease.Nevertheless, mesothelioma may be related to brief, low level or indirect exposures to asbestos.The dose necessary for effect appears to be lower for asbestos-induced mesothelioma than for pulmonary asbestosis or lung cancer. Again, there is no known safe level of exposure to asbestos as it relates to increased risk of mesothelioma.
The duration of exposure to asbestos causing mesothelioma can be short. For example, cases of mesothelioma have been documented with only 1–3 months of exposure. People who work with asbestos wear personal protective equipment to lower their risk of exposure.
Latency, the time from first exposure to manifestation of disease, is prolonged in the case of mesothelioma. It is virtually never less than fifteen years and peaks at 30–40 years. In a review of occupationally related mesothelioma cases, the median latency was 32 years. Based upon the data from Peto et al., the risk of mesothelioma appears to increase to the third or fourth power from first exposure.
Environmental exposures
