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Quick Facts

Development and Spread of Cancer


The Manual's Editorial Staff

Last full review/revision Jul 2021| Content last modified Jul 2021
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How does cancer start?

Cancer starts when a healthy cell changes. This change can happen:

  • Without any known cause

  • Because you were exposed to a carcinogen

A carcinogen is a substance that can cause cancer.

Some carcinogens include:

  • Sunlight (both outside and inside using a tanning bed)

  • Tobacco

  • Certain chemicals

  • Certain viruses

  • Radiation

You usually have to be exposed to a carcinogen for a long time to get cancer. Lots of people who are exposed never get cancer.

Some cells are more likely than others to become cancerous. These cells may have a flaw in their genes. Genes are the set of instructions inside cells that tells them what to do. When cells with flaws in their genes are exposed to a carcinogen, they're more likely to turn into cancer. This is why a certain kind of cancer might run in your family.

How does cancer spread?

Cancer cells can spread:

  • By expanding directly into nearby tissues or organs

  • By traveling through the lymphatic system or blood vessels to other organs

The lymphatic system is part of your body's defenses. It's a network of lymph nodes and lymph vessels that carry lymph fluid through your body. Lymph fluid carries away the cells and substances left over from your body's fight against disease. Your body's defenses fight and kill cancer cells too, but sometimes living cancer cells get into lymph fluid. Cancer cells may travel in lymph fluid or in blood to reach other organs. Sometimes cancer cells land in organs far away from the original cancer. The original cancer is called the primary cancer. Cancer that has spread to other organs is called metastatic cancer.

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