Cancer starts when cells in the body change (mutate) and grow out of control. To help you understand what happens when you have cancer, let's look at how your body works normally. Your body is made up of tiny building blocks called cells. Normal cells grow when your body needs them and die when your body doesn't need them any longer.

Cancer is made up of abnormal cells that grow even though your body doesn't need them. In most cancers, the abnormal cells grow to form a lump or mass called a tumor. If cancer cells are in the body long enough, they can grow into (invade) nearby areas. They can also spread to other parts of the body (metastasis).

The place in your body where cancer cells first start to grow is known as the site of the primary tumor. Cancer of unknown primary (CUP) is cancer that starts in an unknown area and has already spread by the time it's diagnosed. Other names for CUP are unknown primary cancer (UPC) and occult primary tumor.

The places where CUP is often first found are the:

• Lymph nodes in your neck, arms, or groin.
• Lungs.
• Liver.
• Brain.
• Bones.

Cancers are named for the part of the body in which they start. For instance, breast cancer is cancer that first starts growing in cells of the breast. If breast cancer spreads to other parts of the body, it's still called breast cancer. So if it goes to the liver, it's called breast cancer with liver metastasis. The cancer cells in the liver look and respond to treatment like breast cancer cells. But with CUP, doctors can't tell which part of the body the cancer cells first started in.

CUP can affect any organ in the body. But the organ in which it first started may stay unknown. In most people with CUP, the only sign is a single site where the cancer cells have spread. This part of the body may be very far from where the cancer cells first grew.

Knowing the primary site helps doctors know the best way to treat the cancer. To try to find the source of the cancer, doctors might take out a sample of the cancer cells. This is called a biopsy. How the cells look under the microscope might help doctors find the primary site. Sometimes they are also able to figure out where the cancer started by tracking how it spreads. So, sometimes, after many tests, doctors are able to find where the cancer started. Then the cancer will be renamed for that site.

In some cases, doctors can't figure out where the cancer started. They may decide more testing wouldn't be helpful. If this happens, they'll treat the cancer based on the type of cancer that's most likely in your case.

Sometimes cancers can be broadly grouped by how the cells look and lab test results. This can help narrow down the places the cancer might have started. It can help your doctors decide on the best treatment to use.

Many people with CUP have a type of cancer called adenocarcinoma. These are cancers that start in the gland cells in the lining of organs. It may start in the glands in the linings of the lung, pancreas, stomach, liver, kidney, colon, breast, prostate, or other places.

Squamous cell carcinomas are also common. These start in flat cells on body surfaces, like the skin or linings of organs, like the mouth, throat, lungs, anus, cervix, and vagina.

Some cancer cells are so abnormal-looking that there's no way to tell what kind of cell they started in. These are called poorly differentiated cancers. They're not as common and tend to be lymphomas, melanomas, or sarcomas.

If you have questions about CUP, talk with your doctor. They can help you understand more about your cancer and what to expect.