The short answer
Cancer spreads when cells break away from the first tumor, travel through blood or lymph, and grow in a new place. This is called metastasis, and it is what makes cancer dangerous.
Metastasis is when cancer cells break away and grow in a new part of the body.
Cells usually travel through the bloodstream or the lymph system.
A metastatic tumor is made of the original cancer's cells, not new local cells.
Where a cancer spreads follows patterns doctors use to guide tests and treatment.
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The full explanation.
The simple version
Cancer starts in one place — the primary tumor. Sometimes cells break away from it, travel to a new part of the body, and start growing there. That process is called metastasis, and the new tumors are called metastatic tumors.
How the cells travel
Cancer cells usually spread in one of two ways. They can enter the bloodstream, or they can enter the lymph system — the network of vessels and nodes that carries immune cells. Once traveling, most cancer cells do not survive, but a few can settle in a new organ and grow.
A metastatic tumor is made of the same kind of cells as the original cancer.
Why the "same cancer" rule matters
If colon cancer spreads to the liver, the new tumor is still colon cancer cells — it is called metastatic colon cancer, not liver cancer. This matters because treatment is chosen based on where the cancer began, since that is what the cells "act like."
Why spread changes treatment
When cancer is only in one place, treatment can often focus there — surgery or radiation to that spot. Once cancer has spread, doctors usually add treatments that travel through the whole body, such as chemotherapy, targeted therapy, or immunotherapy, because there may be cells in more than one place.
Common patterns
Different cancers tend to spread to certain organs — for example, bone, liver, lung, and brain are common sites. Knowing these patterns helps a care team decide which scans and tests to order.
Words to know
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Common questions
▸What does metastasis mean?
Metastasis is the spread of cancer cells from the place where they first formed to another part of the body. The new tumors are called metastatic or secondary tumors.
▸How do cancer cells travel?
They can enter the blood or the lymph system, move to a new area, and begin to grow there. Most cells that break away do not survive the journey, but some do.
▸Is metastatic breast cancer a new cancer?
No. If breast cancer spreads to the lung, it is still breast cancer under the microscope and is treated as breast cancer, not lung cancer.
▸Can spread be found before symptoms?
Sometimes. Imaging and blood tests can find spread before a person feels it, which is one reason follow-up scans are used.
Questions to ask your doctor
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