Cancer is a group of many related diseases that begin in
cells, the body's basic unit of life. To understand cancer, it is helpful to
know what happens when normal cells become cancerous.
The body is made up of many types of cells. Normally,
cells grow and divide to produce more cells only when the body needs them. This
orderly process helps keep the body healthy. Sometimes, however, cells keep
dividing when new cells are not needed. These extra cells form a mass of tissue,
called a growth or tumor.
Tumors can be benign or malignant.
Benign tumors are not cancer. They can often be removed and, in
most cases, they do not come back. Cells from benign tumors do not spread to
other parts of the body. Most important, benign tumors are rarely a threat
Malignant tumors are cancer. Cells in these tumors are abnormal
and divide without control or order. They can invade and damage nearby
tissues and organs. Also, cancer cells can break away from a malignant tumor
and enter the bloodstream or the lymphatic system. That is how cancer
spreads from the original cancer site to form new tumors in other organs.
The spread of cancer is called metastasis.
Leukemia and lymphoma are cancers that arise in
blood-forming cells. The abnormal cells circulate in the bloodstream and
lymphatic system. They may also invade (infiltrate) body organs and form
Most cancers are named for the organ or type of cell in
which they begin. For example, cancer that begins in the lung is lung cancer,
and cancer that begins in cells in the skin known as melanocytes is
When cancer spreads (metastasizes), cancer cells are often
found in nearby or regional lymph nodes (sometimes called lymph glands).
If the cancer has reached these nodes, it means that cancer cells may have
spread to other organs, such as the liver, bones, or brain. When cancer spreads
from its original location to another part of the body, the new tumor has the
same kind of abnormal cells and the same name as the primary tumor. For example,
if lung cancer spreads to the brain, the cancer cells in the brain are actually
lung cancer cells. The disease is called metastatic lung cancer (it is not brain
The more we can learn about what causes cancer, the more
likely we are to find ways to prevent it. In the laboratory, scientists explore
possible causes of cancer and try to determine exactly what happens in cells
when they become cancerous. Researchers also study patterns of cancer in the
population to look for risk factors, conditions that increase the chance
that cancer might occur. They also look for protective factors, things that
decrease the risk.
Even though doctors can seldom explain why one person gets
cancer and another does not, it is clear that cancer is not caused by an injury,
such as a bump or bruise. And although being infected with certain viruses may
increase the risk of some types of cancer, cancer is not contagious; no one can
"catch" cancer from another person.
Cancer develops over time. It is a result of a complex mix
of factors related to lifestyle, heredity, and environment. A number of factors
that increase a person's chance of developing cancer have been identified. Many
types of cancer are related to the use of tobacco, what people eat and drink,
exposure to ultraviolet (UV) radiation from the sun, and, to a lesser
extent, exposure to cancer-causing agents (carcinogens) in the
environment and the workplace. Some people are more sensitive than others to
factors that can cause cancer.
Still, most people who get cancer have none of the known
risk factors. And most people who do have risk factors do not get the disease.
Some cancer risk factors can be avoided. Others, such as
inherited factors, are unavoidable, but it may be helpful to be aware of them.
People can help protect themselves by avoiding known risk factors whenever
possible. They can also talk with their doctor about regular checkups and about
whether cancer screening tests could be of benefit.