What is the difference between primary and secondary cancer?

Most cancers start in one place. The place where a cancer starts is called the primary site. The cancer that starts there is the primary tumour. For example, if you have cancer that starts in the breast, you have primary breast cancer.

Cancers can spread to distant parts of the body. To do this, cancer cells usually get into the bloodstream or the lymphatic system. (The lymphatic system makes and stores cells that fight infection. There are lymph channels and lymph nodes all over the body.) The blood or lymph carries the cancer cells around the body until they get stuck. They may get stuck in a small blood vessel, or in a lymph node.

Next, cancer starts to grow in this new place. The place where it now grows is called the secondary site. The cancer is called a secondary tumour or metastasis. If your breast cancer spreads to the lungs, for example, you have secondary breast cancer. Your secondary cancer is made from breast cancer cells, not lung cancer cells. So you still have breast cancer, even though it is now in your lungs.

The most common places for secondary cancers to grow are the lungs, liver, lymph nodes, bones, brain and skin. Where a secondary cancer is most likely to develop depends on where the primary cancer was.

Sometimes a patient may get a secondary cancer, but doctors can’t find out where the primary cancer is. This type of cancer is called an unknown primary tumour.