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By Mayo Clinic Staff. Cancer is caused by changes (mutations) to the DNA within cells. The DNA inside a cell is packaged into a large number of individual genes, each of which contains a set of instructions telling the cell what functions to perform, as well as how to grow and divide.
To explain what an unknown primary cancer is, you need to understand the difference between a primary and secondary cancer. A primary cancer is where the cancer first grows in your body.A secondary cancer is when a primary cancer spreads to another area of your body. A secondary cancer is also known as metastases.For example, if you are diagnosed with primary breast cancer and secondary bone cancer, it means that your cancer started in your breast and moved to your bones. A primary cancer is where the cancer first grows in your body. 2  A secondary cancer is when a primary cancer spreads to another area of your body. 3  If you are diagnosed with secondary cancer but your doctor is unsure where it began in your body, the first cancer is called an unknown primary cancer.