What is cancer?


What is cancer?
The body is made up of trillions of living cells. Normal body cells grow, divide into new
cells, and die in an orderly fashion. During the early years of a person's life, normal cells
divide faster to allow the person to grow. After the person becomes an adult, most cells
divide only to replace worn-out or dying cells or to repair injuries.
Cancer begins when cells in a part of the body start to grow out of control. There are
many kinds of cancer, but they all start because of out-of-control growth of abnormal
cells.
Cancer cell growth is different from normal cell growth. Instead of dying, cancer cells
continue to grow and form new, abnormal cells. Cancer cells can also invade (grow into)
other tissues, something that normal cells cannot do. Growing out of control and invading
other tissues is what makes a cell a cancer cell.
Cells become cancer cells because of damage to DNA. DNA is in every cell and directs
all its actions. In a normal cell, when DNA gets damaged the cell either repairs the
damage or the cell dies. In cancer cells, the damaged DNA is not repaired, but the cell
doesn't die like it should. Instead, this cell goes on making new cells that the body does
not need. These new cells will all have the same damaged DNA as the first cell does.
People can inherit damaged DNA, but most DNA damage is caused by mistakes that
happen while the normal cell is reproducing or by something in our environment.
Sometimes the cause of the DNA damage is something obvious, like cigarette smoking.
But often no clear cause is found.
In most cases the cancer cells form a tumor. Some cancers, like leukemia, rarely form
tumors. Instead, these cancer cells involve the blood and blood-forming organs and
circulate through other tissues where they grow.
Cancer cells often travel to other parts of the body, where they begin to grow and form
new tumors that replace normal tissue. This process is called metastasis. It happens when
the cancer cells get into the bloodstream or lymph vessels of our body.  No matter where a cancer spreads, it is always named for the place where it started. For
example, breast cancer that has spread to the liver is still called breast cancer, not liver
cancer. Likewise, prostate cancer that has spread to the bone is metastatic prostate cancer,
not bone cancer.
Different types of cancer can behave very differently. For example, lung cancer and
breast cancer are very different diseases. They grow at different rates and respond to
different treatments. That is why people with cancer need treatment that is aimed at their
particular kind of cancer.
Not all tumors are cancerous. Tumors that aren't cancer are called benign. Benign tumors
can cause problems – they can grow very large and press on healthy organs and tissues.
But they cannot grow into (invade) other tissues. Because they can't invade, they also
can't spread to other parts of the body (metastasize). These tumors are almost never life
threatening.

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