A primary cancer is the point where the cancer first grew in the body. When a cancer spreads, the same cells grown by the primary cancer site begin to grow in other parts of the body. For example, if skin cancer metastasizes to the lungs, one would have growths of skin cancer cells in the lungs. The following article describes the terms primary and secondary, as well as unknown primaries.
https://www.cancer.ie/cancer-information/unknown-primary-cancer#sthash.U20BqUPC.dpbs
We've heard a few stories of melanoma metastasizing without knowing where it began. This would be considered an unknown primary, which are usually rare. It can be especially disturbing to find that your skin cancer spread to organs undetected.
- Nina, SkinCancer.net Team