How Does Cancer Develop?

How cancer starts

When genes work properly, they tell cells when it is the right time to grow and divide. When cells divide, they make exact copies of themselves. One cell divides into 2 identical cells, then 2 cells divide into 4, and so on.

In adults, cells normally grow and divide to make more cells only when the body needs them, such as to replace aging or damaged cells. But cancer cells are different. Cancer cells have gene mutations that turn the cell from a normal cell into a cancer cell.

3d illustration of X chromosomes

These gene mutations may be inherited, develop over time as we get older and genes wear out, or develop if we are around something that damages our genes, like cigarette smoke, alcohol or ultraviolet (UV) radiation from the sun.

How cancer grows

Gene mutations in cancer cells interfere with the normal instructions in a cell and can cause it to grow out of control or not die when it should. A cancer can continue to grow because cancer cells act differently than normal cells. Cancer cells are different from normal cells because they:

  • Divide out of control
  • Are immature and don’t develop into mature cells with specific jobs
  • Avoid the immune system
  • Ignore signals that tell them to stop dividing or to die when they should
  • Don’t stick together very well and can spread to other parts of the body through the blood or lymphatic system
  • Grow into and damage tissues and organs
Closeup of brain MRI scan result

As cancer cells divide, a tumour will develop and grow. Cancer cells have the same needs as normal cells. They need a blood supply to bring oxygen and nutrients to grow and survive. When a tumour is very small, it can easily grow, and it gets oxygen and nutrients from nearby blood vessels.

How cancer spreads

As a tumour gets bigger, cancer cells can spread to surrounding tissues and structures by pushing on normal tissue beside the tumour. Cancer cells also make enzymes that break down normal cells and tissues as they grow. Cancer that grows into nearby tissue is called local invasion or invasive cancer.

Cancer can also spread from where it first started to other parts of the body. This process is called metastasis. Cancer cells can metastasize when they break away from the tumour and travel to a new location in the body through the blood or lymphatic system.

Where cancer can spread and staging

Most cancers have a tendency to spread to certain areas of the body. This has helped doctors develop staging systems that are used to classify cancers based on information about where the cancer is in the body and if it has spread from where it started.

Sad young woman with cancer sitting on the hospital bed

Many cancers follow a staging system from 1 to 4 that is usually given in Roman numerals I, II, III or IV. Knowing how a cancer spreads and where a cancer may spread helps doctors predict how the cancer will grow. This also helps them plan treatment and give appropriate supportive care.


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