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Mesothelioma
Treatment
 What
are the risk factors for malignant
mesothelioma?
A
risk factor is anything that increases a
person's chance of getting a disease such
as cancer. Different cancers have
different risk factors. For example,
unprotected exposure to strong sunlight is
a risk factor for skin cancer and smoking
is a risk factor for lung cancer as well
as other types of cancer. Scientists have
found several risk factors that make a
person more likely to develop
mesothelioma.
Asbestos:
The main risk factor for developing
mesothelioma is exposure to asbestos.
Asbestos refers to a family of
magnesium-silicate mineral fibers. In the
past, asbestos was used widely for
insulation because it does not conduct
heat well and it is resistant to melting
or burning. As the link between asbestos
and mesothelioma has become well known,
the use of this material has decreased.
However, up to 8 million Americans may
already have been exposed to
asbestos.
According
to the United States 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.
There
are two 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 of which 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 have been
associated with malignant (cancerous)
mesotheliomas and should be considered
dangerous as well.
It
may be that asbestos causes cancer by
physically irritating cells rather than by
a chemical effect. When 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
three 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
have also been linked to asbestos
exposure, but the increased risk is not as
great as with lung cancer.
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 exposure to
asbestos and diagnosis of mesothelioma is
usually between 20 and 40 years.
Although
the risk of developing mesothelioma rises
with the amount of asbestos exposure, it
is clear that genetic factors also play a
role in determining who develops the
disease. This explains why not all persons
exposed to high levels of asbestos dust
develop mesothelioma.
Radiation:
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.
Zeolite:
This is a silicate mineral, chemically
related to asbestos, common in the soil of
the Anatoli region of Turkey. A few cases
of mesothelioma have been described in
this region and may have been caused by
this mineral.
Simian
Virus 40 (SV40):
This virus has recently been identified by
researchers in human mesothelioma cells,
and has been shown to induce mesothelioma
in the animal model. Polio vaccines
administered as a primary prevention
measure during 1955 - 1961 have been shown
to be contaminated with SV40. However the
implications of these facts are not
totally understood and further research
will be needed to clarify the link between
malignant mesothelioma and a viral
etiology.
Tobacco:
Although tobacco smoking has not been
associated with the development of
mesotheliomas, 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.
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