Detailed Guide: Malignant Mesothelioma
Detailed Guide: Malignant Mesothelioma
What Are the Risk Factors for Malignant Mesothelioma?
Last Revised: 10/19/2006
American Cancer Society
A risk factor is anything that increases your chance of getting a disease such as cancer. Different cancers have different risk factors. For example, exposing skin to strong sunlight is a risk factor for skin cancer. Smoking is a risk factor for cancers of the lung, mouth, larynx, bladder, kidney, and several other organs. Individuals exposed to asbestos should be encouraged to avoid tobacco exposure because together the risk for lung cancer is significantly higher than from smoking without a history of asbestos exposure. But having a risk factor, or even several, does not mean that you will get the disease.
The main risk factor for developing mesothelioma is exposure to asbestos. Asbestos refers to a family of fibrous minerals made of silicate. Asbestos was once used in many products such as insulation, floor tiles, door gaskets, soundproofing, roofing, patching compounds, fireproof gloves and ironing board covers, and even brake pads. As the link between asbestos and mesothelioma has become well known, the use of this material has almost stopped. Most use stopped after 1989, but it is still used in some products. Experts have linked this drop in asbestos use to the fact that the rate of development of mesothelioma is no longer increasing.
Still, up to 8 million Americans may already have been exposed to asbestos. Exposure to asbestos particles suspended in air and building materials is much less hazardous except when they are being removed.
Since asbestos is a naturally occurring mineral, it can also be found in dust and rocks in certain parts of the United States as well as the world.
According to the U.S. Environmental Protection Agency, as many as 733,000 schools and public buildings in the country today contain asbestos insulation. As many as 10% to 15% of schools in the United States may contain asbestos insulation. People who may be at risk for occupational asbestos exposure include some miners, factory workers, insulation manufacturers, railroad workers, ship builders, gas mask manufacturers, and construction workers, particularly those involved with installing insulation. Several studies have shown that family members of people exposed to asbestos at work have an increased risk of developing mesothelioma, because asbestos fibers are carried home on the clothes of the workers.
The incidence rate for mesothelioma in men is dropping, probably because they are no longer being exposed directly to asbestos in their work. But the incidence rate for mesothelioma in women is steady, which suggests that they are being exposed in a way that is not directly tied to work, but more to their environment either at home or work. One example would be asbestos in buildings where they work or live. A study from California also links mesothelioma to naturally occurring asbestos deposits in mountains.
Another important point about asbestos and mesothelioma is that the risk of mesothelioma does not drop with time after exposure to asbestos. The risk appears to be lifelong and undiminished.
There are 2 main forms of asbestos -- serpentine and amphiboles.
Serpentine fibers are curly and pliable. Chrysotile is the only type of serpentine fiber and it is the most widely used form of asbestos.
Amphiboles are thin, rod-like fibers. There are 5 main types — crocidolite, amosite, anthrophylite, tremolite, and actinolyte. Amphiboles (particularly crocidolite) are considered to be the most carcinogenic (cancer-causing).
However, even the more commonly used chrysotile fibers are associated with malignant (cancerous) mesotheliomas and should be considered dangerous as well.
When asbestos fibers are inhaled, most are cleared in the nose, throat, trachea (windpipe), or bronchi (large breathing tubes of the lungs). Fibers are cleared by sticking to mucus inside the air passages and being coughed up or swallowed. The long, thin, fibers are less readily cleared, and they may reach the ends of the small airways and penetrate into the pleural lining of the lung and chest wall. These fibers may then directly injure mesothelial cells of the pleura, and eventually cause mesothelioma.
Asbestos fibers can also damage cells of the lung and result in asbestosis (formation of scar tissue in the lung), and/or lung cancer. The risk of lung cancer among people exposed to asbestos is increased by 7 times, compared with the general population. Indeed, asbestosis, mesothelioma, and lung cancer are the 3 most frequent causes of death and disease among people with heavy asbestos exposure. Peritoneal mesothelioma, which forms in the abdomen, may result from coughing up and swallowing inhaled asbestos fibers. Cancers of the larynx, pancreas, esophagus, colon, and kidney may also come from asbestos exposure, but the increased risk is small.
The risk of developing a mesothelioma is related to how much asbestos a person was exposed to and how long this exposure lasted. People exposed at an early age, for a long period of time, and at higher levels are most likely to develop this cancer. Mesotheliomas take a long time to develop. The time between first exposure to asbestos and diagnosis of mesothelioma is usually between 20 and 50 years.
There have been a few published reports of pleural and peritoneal mesotheliomas that developed following exposure to thorium dioxide (Thorotrast). This material was used in the past by doctors for certain x-ray tests. Because Thorotrast was found to cause cancers, it has not been used for many years.
This is a silicate mineral, chemically related to asbestos, common in the soil of the Anatoli region of Turkey. Many cases of mesothelioma have been described in this region and may have been caused by this mineral.
Although tobacco smoking has not been associated with developing mesothelioma, the combination of smoking and asbestos exposure greatly increases the risk of lung cancer. Asbestos workers who also smoke have a lung cancer risk 50 to 90 times greater than that of the general population. More asbestos workers die of lung cancer than of mesothelioma.
Some recent studies have raised the possibility that infection with simian virus 40 (SV40) might increase the risk of developing mesothelioma. Some injectable polio vaccines prepared between 1955 and 1963 were contaminated with SV40. About 10 to 30 million people were probably exposed to the virus.
Intentional infection with SV40 of some laboratory animals, such as hamsters, causes mesotheliomas to develop. Researchers also have noticed that SV40 can cause mouse cells grown in dishes to become cancerous, and that asbestos increases the cancer-causing effect of SV40 on these cells. Other researchers have studied biopsy specimens of human mesotheliomas and detected SV40 DNA. However, similar fragments of SV40 DNA can also be found in noncancerous human tissues and some researchers think the SV40 viruses found are contaminants.
Another study did find SV40 virus in tissues from mesothelioma patients that did not appear to be contaminants. In this study, which also looked at tissue from healthy people, the SV40 virus wasn’t linked to mesothelioma unless the person was also exposed to asbestos. The researchers in this study thought the SV40 infection was not caused by the polio immunization, but occurred naturally as do other viral infections.
So far, the largest studies addressing this issue in humans have not found any increased risk for mesothelioma or other cancers among people who received the contaminated vaccines as children. But, the peak age range for diagnosis of mesothelioma is 50 to 70 years. Some researchers have pointed out that this issue may remain unresolved until more of the people accidentally exposed to SV40 between 1955 and 1963 reach that age range. Research into this important topic is still underway.
A recent study by the Institute of Medicine concluded that we still don’t know whether SV40 is responsible for some mesotheliomas and more research needs to be done. Last Revised: 10/19/2006
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