A primary cancer is the original site (organ or tissue) where a cancer started.
Conversely, a cancer that starts in the breast and spreads to the lungs would be called primary breast cancer metastatic to the lungs.
In some cases, the original site where a cancer starts is unknown. Some cancers are only discovered after they have spread to several regions of the body, including the lungs. In this case, the tumor would be called a metastatic cancer of unknown primary origin.