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A risk factor is anything that affects a person’s chance of getting a disease such as cancer. Different cancers have different risk factors. Some risk factors, like smoking, can be changed. Others, like a person's age or family history, can't be changed.

But having a risk factor, or even several, does not mean that a person will get the disease, and many people get cancer without having any known risk factors.

Since the exact type of cancer is not known, it’s hard to identify factors that might affect risk for cancer of unknown primary (CUP). These cancers are also a very diverse group, making this issue even more complicated. But there is research that provides some information about CUP risk factors.

Smoking is probably an important risk factor for CUP. More than half of patients with CUP have a history of smoking. When autopsy studies are done, many cancers of unknown primary are found to have started in the pancreas, lungs, kidneys, throat, larynx, or esophagus. Smoking increases the risk for all of these cancers.

Some other cancers of unknown primary are eventually found to have started in the stomach, colon, rectum, or ovaries. Diet, nutrition, and weight are factors that have been linked to these cancers. For more information, see the specific type of cancer.

Melanoma (an aggressive type of skin cancer) is another source of cancer of unknown primary. An important melanoma risk factor is exposure to ultraviolet radiation in sunlight.

Overall, however, there are no factors that specifically increase the risk of cancer of unknown primary. Even in people who may have one or more of the risk factors above, it’s not possible to know for sure if these factors contributed to the cancer.