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Cancer begins when normal cells change and grow uncontrollably, forming a mass called a tumor. A tumor can be cancerous or benign. A cancerous tumor is malignant, meaning it can spread to other parts of the body. A benign tumor means the tumor will not spread.
Cancer is a group of more than 100 different diseases. Most of the time it is fairly easy for doctors to figure out where a cancer began, known as the primary site, and identify any secondary or metastatic site if the cancer has spread.
No matter where the cancer spreads, it is still named for the area of the body where it began. For example, breast cancer that has spread to the brain is called metastatic breast cancer, not brain cancer.
For about 2% of people diagnosed with cancer, though, the cancer is found at a secondary site, but routine testing cannot help doctors find where the cancer began. These cancers are called carcinoma of unknown primary site or cancer of unknown primary (CUP).
For some people, specialized testing can eventually help identify the primary site; however, sometimes it cannot. This may be because the primary tumor is still very small, the body caused the primary tumor to shrink or disappear, and/or the primary tumor was removed during previous surgery for another condition, such as the removal of a mole on the skin or surgery to remove a woman’s uterus, known as a hysterectomy.
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