What Is Cancer?

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 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 to life.
• 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 (primary) cancer site to form new tumor in other organs.  The spread of cancer is called metastasis.
• Leukemia and lymphoma are cancers that arise in blood forming organs.  The abnormal cells circulate in the bloodstream and lymphatic system.  They may also invade (infiltrate) body organs and form tumors.

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 called melanoma.

When cancer spreads , 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 (primary) 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.  This process of spreading is called metastasis.  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 cancer).  The secondary or metastatic cancer will be treated as if it were the primary tumor—lung cancer.