Cancer starts when cells in a part of the body start to grow out of control. 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, a function that normal cells cannot perform. Growing out of control and invading other tissues are what makes a cell a cancer cell.
Cells become cancer cells because of changes to their DNA (deoxyribonucleic acid). DNA is in every cell and it directs all its actions. In a normal cell, when DNA is damaged the cell either repairs the damage or dies. In cancer cells, the damaged DNA is not repaired, but the cell doesn’t die like it should. Instead, the cell goes on making new cells that the body doesn’t need. These new cells all have the same damaged DNA as the first cell does.
People can inherit abnormal or faulty DNA (it’s passed on from their parents), but most DNA damage is caused by mistakes that happen while a normal cell is reproducing or by something in the environment. Sometimes DNA damage may be caused by something obvious like cigarette smoking or sun exposure. But it’s rare to know exactly what caused any one person’s cancer.
In most cases, the cancer cells form a tumour. Over time, the tumours can invade nearby normal tissue, crowd it out, or push it aside. Some cancers, like leukemia, rarely form tumours. Instead, these cancer cells involve the blood and blood-forming organs and circulate through other tissues where they grow.