Causes of Cancer
What causes cancer?
There is no one single cause for cancer. Scientists believe that it is the interaction
of many factors together that produces cancer. The factors involved may be genetic,
environmental, or constitutional characteristics of the individual.
Diagnosis, treatment, and prognosis for childhood cancers are different than for adult
cancers. The main differences are the survival rate and the cause of the cancer. The
overall five-year survival rate for childhood cancer is about 83%, while in adult
cancers the survival rate is 69%. This difference is thought to be because childhood
cancer is more responsive to therapy and a child can tolerate more aggressive therapy.
Survival rates may vary depending on the person's age, cancer type, and stage at diagnosis.
Childhood cancers often happen or begin in the stem cells, which are cells capable
of producing other types of specialized cells that the body needs. A sporadic (happens
by chance) cell change or mutation is usually what causes childhood cancer. In adults,
the type of cell that becomes cancerous is usually an epithelial cell. Epithelial
cells line the body cavity and cover the body surface. Cancer happens from environmental
exposures to these cells over time. Adult cancers are sometimes referred to as acquired for this reason.
What are the risk factors for cancer?
As mentioned, some cancers, particularly in adults, have been associated with repetitive
exposures or risk factors. A risk factor is anything that may increase a person's
chance of developing a disease. A risk factor does not necessarily cause the disease,
but it may make the body less resistant to it. The following risk factors and mechanisms
are considered to contribute to cancer:
Lifestyle factors. Smoking, a high-fat diet, and working with toxic chemicals are examples of lifestyle choices
that may be risk factors for some adult cancers. Most children with cancer, however,
are too young to have been exposed to these lifestyle factors for any extended time.
Family history, inheritance, and genetics. These may play an important role in some childhood cancers. It is possible for cancer
of varying forms to be present more than once in a family. It is unknown in these
circumstances if the disease is caused by a genetic mutation, exposure to chemicals
near a family's residence, a combination of these factors, or simply coincidence.
Some genetic disorders. For example, Wiskott-Aldrich syndrome is known to alter the immune system. The immune
system is a complex system that functions to protect our bodies from infection and
disease. The bone marrow makes cells that later mature and function as part of the
immune system. One theory suggests that the cells in the bone marrow, the stem cells,
become damaged or defective, so when they reproduce to make more cells, they make
abnormal cells or cancer cells. The cause of the defect in the stem cells could be
related to an inherited genetic defect or exposure to a virus or toxin.
Exposures to certain viruses. Epstein-Barr virus and HIV, the virus that causes AIDS, have been linked to an increased
risk of developing certain childhood cancers. These include Hodgkin and non-Hodgkin
lymphoma. Possibly, the virus alters a cell in some way. That cell then reproduces
an altered cell and, eventually, these alterations become a cancer cell that reproduces
more cancer cells.
Environmental exposures. Pesticides, fertilizers, and power lines have been researched for a direct link to
childhood cancers. There has been evidence of cancer happening among nonrelated children
in certain neighborhoods and/or cities. Whether prenatal or infant exposure to these
agents causes cancer, or whether it is a coincidence, is unknown.
Some forms of high-dose chemotherapy and radiation. In some cases, children who have been exposed to these agents may develop a second
malignancy later in life. These strong anticancer agents can alter cells and/or the
immune system. A second malignancy is a cancer that appears as a result from treatment
of a different cancer.
How do genes affect cancer growth?
The discovery of certain types of genes that contribute to cancer has been an extremely
important development for cancer research. The majority of cancers are observed to
have some type of genetic alteration. Some of these alterations are inherited. Others
are sporadic, which means they happen by chance or happen from environmental exposures
(usually over many years). There are three main types of genes that can affect cell
growth, and are altered (mutated) in certain types of cancers, including the following:
Oncogenes. These genes regulate the normal growth of cells. Scientists commonly describe oncogenes
as similar to a cancer "switch" that most people have in their bodies. What "flips
the switch" to make these oncogenes suddenly become unable to control the normal growth
of cells and allowing abnormal cancer cells to begin to grow, is unknown.
Tumor suppressor genes. These genes are able to recognize abnormal growth and reproduction of damaged cells,
or cancer cells, and can interrupt their reproduction until the defect is corrected.
If the tumor suppressor genes are mutated, however, and they do not function properly,
tumor growth may happen.
Mismatch-repair genes. These genes help recognize errors when DNA is copied to make a new cell. If the DNA
does not "match" perfectly, these genes repair the mismatch and correct the error.
If these genes are not working properly, however, errors in DNA can be sent to new
cells, causing them to be damaged.
Usually the number of cells in any of our body tissues is tightly controlled so that
new cells are made for normal growth and development, as well as to replace dying
cells. Ultimately, cancer is a loss of this balance due to genetic alterations that
"tip the balance" in favor of excessive cell growth.