Cancer is an abnormal growth of cells. Cancer cells rapidly reproduce despite restriction of space, nutrients shared by other cells, or signals sent from the body to stop reproduction. Cancer cells are often shaped differently than healthy cells, they do not function properly, and they can spread to many areas of the body.
Oncology is the study of cancer and tumors.
The number of cells in each body tissue is tightly controlled. The number of new cells made equals the number needed to replace dying cells plus the number needed for growth and development. Unlike normal cells, tumors are clusters of cells that are capable of growing and dividing uncontrollably. Their growth is not regulated. Benign tumors tend to grow slowly and do not spread.