Definition:
A primary cancer is the original site (organ or tissue) where a cancer started.
For example, a cancer starting in the lungs is called primary lung cancer. If it spreads to the brain, it would be called primary lung cancer metastatic to the brain.
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.
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