Cancer develops when cells in a part of the body begin to grow out of control. Although there are many kinds of cancer, they all start because of out of control growth of abnormal cells.
Our bodies are made up of millions of cells, grouped together to form tissues or organs such as muscles and bones, the lungs, or the liver. Genes inside each cell order it to grow, work, reproduce, and die.
Normally, our cells obey these orders and we remain healthy. Very occasionally, the controls that regulate a cell break down and, although the body has no need for further cells of its type, a cell begins to grow and divide. This loss of control in the cell develops because of damage to DNA. DNA is in all cells and directs all activities. Most of the time, when DNA becomes damaged the body is able to repair it. In cancer cells, the damaged DNA has not been repaired.
People can inherit damaged DNA, which accounts for inherited cancers. More often, though, a person's DNA becomes damaged by exposure to something in the environment, like smoking or UV rays.
When such a damaged cell has descendants that inherit the ability to grow without responding to regulation, the result is a group of cells able to grow and expand indefinitely. Ultimately, a mass called a tumour may be formed by this cluster of unwanted cells.
Figure 1. Stages in Tumour Growth and Metastasis
Tumours can be either benign (non-cancerous) or malignant (cancerous). Benign tumour cells stay in one place in the body and are not usually life threatening. Some cancers, like leukemia, do not form tumours. Instead, these cancer cells involve the blood and blood forming organs and circulate through other tissues where they grow.
Often, malignant tumour cells are able to invade the tissues around them and spread to other parts of the body where they begin to grow and replace normal tissue. This process is called metastasis. Regardless of where a cancer may spread, it is always named for the place it began. For example, colon cancer that spreads to the liver is still colon cancer.