Cause
Of Brain CancerIn the following I will briefly discuss what we know about
the cause of brain cancer and provide some data on the frequency of various brain
cancers. Brain cancer and spinal cord cancer originates from
the various cell types, which are present in that tissue. There seem to be genetic
factors on the one hand and environmental factors on the other hand, which trigger
normal cells to become cancerous and this process is not any different from other
cancers such as leukemia or solid tumors. There is an early peak of brain
cancer occurrence in the first 4 years of childhood (frequency of 3 new cases
per 100,000 per year). It is less frequent up to age 24, after which there is
a steady rise in the occurrence of brain cancer until the age of 65. At that age
there is a plateau of 18 new cases per 100,000 per year, which lasts until the
age of 80. Most brain tumors have an equal sex distribution except for meningiomas,
which are more common in females (Ref. 2). We learnt first from some familial
brain cancers that genetic factors are important in the causation of brain tumors.
Neurofibromatosis I is a familial condition where a child is born with a number
of skin pigmentations (cafe au lait spot) in combination with lumps (neurofibroma)
under the skin, a brain tumor called glioma attached to the optic nerve and other
brain tumors. This disease has been studied extensively and it has been found
that the problem originates from chromosome 17, where there is an altered gene
(NF1 gene). This disease is relatively common with an incidence of 1 in 3,500
children. Researches found that there is another gene (NF2 gene), which occurs
only with a frequency of 1 in 50,000 children and is located on chromosome 22.
These children develop acoustic neuromas (called schwannomas), attached to both
hearing nerves. They also develop various kinds of brain cancers (astrocytomas,
glioblastomas etc.). Since then geneticists have detected many other loci that
can trigger the development of brain cancer. Another line of research came from chemical exposure data to
carcinogens (cancer producing substances). Exposure to pesticides, herbicides,
fertilizers and various petrochemical products have been shown to lead to a higher
than expected incidence of brain cancer. The exact mechanism of how these chemicals
produce cancer is not known. The theories are that on the one hand some chemicals
are carcinogens, directly causing genetic changes in cells. On the other hand
there are likely some chemicals that destroy tumor suppressor genes that are located
on various chromosomes and whose function normally is to protect the body from
cancerous brain cell mutations. Another mechanism causing brain cancer
is through viral infections. We do know that there are clusters of gliomas (one
type of brain cancer) in schools and communities where an epidemic of mononucleosis
has occurred. Mononucleosis is caused from the Epstein-Barr virus and this virus
can penetrate into the brain cells. However, it remains a mystery why the majority
of patients have an uneventful recovery and a small percentage only comes down
with gliomas later in life. The patients who developed a glioma had Epstein-Barr
virus in the tumor tissue when the material removed by a neurosurgeon was sent
to the pathologist for analysis (Ref. 1 and 2).
| The popular belief that trauma to the
head would cause brain cancer is not supported by evidence. A recent prospective
study failed to show any increased incidence of meningiomas in patients who had
prior trauma to the head versus a control group of glioma patients who did not
have a trauma history. | Some
cancers such as acute lymphocytic leukemia were treated in the past with radiation
of the skull to prevent CNS induced relapse of the leukemia. Some investigators
described a higher incident of astrocytomas (one form of brain cancer) approximately
5 years after radiation was completed. The incidence appears to be low in the
order of 2% or 3 %, but since the introduction of intrathecal chemotherapy for
leukemia relapse prevention radiation of the skull has mostly been abandoned (Ref.
2). Another observation comes from using X-ray radiation for chronic cases
of the dermatological condition tinea capitis (a chronic yeast infection of the
scalp). This has lead to an incident of 15 to 20% of meningiomas later in life,
an unacceptable situation. Needless to say that the practice of radiation for
this condition has been abandoned.The following tables
summarize the frequency of various tumors, separated into brain and spinal cord
tumors. It is important to note that brain tumors are 7 times more common than
spinal cord tumors (modified from Ref. 1 and 2).
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