A primary lung cancer is a cancer that begins in the lungs. If a primary lung cancer spreads to other regions of the body (known as metastasis) it is referred to primary lung cancer metastatic to the location of the spread. For example, a cancer that begins in the lungs and spreads to the brain would be termed “primary lung cancer metastatic to the brain.”
Cancers that begin in other regions of the body and spread to the lungs are not primary lung cancers. Sometimes they are described as “secondary cancers” but most often they are defined first by the site of the cancer followed by “metastatic to the lungs.” For example, a breast cancer that spreads to the lungs may be called “breast cancer metastatic to the lungs” or “metastatic lung cancer from the breast.” On the other hand, if someone with breast cancer developed another unrelated cancer that began in the lungs, that tumor would be considered a primary lung cancer.
A second area of cancer in the lung resulting from the spread of the first tumor would be referred to as a secondary cancer, or “primary lung cancer metastatic to another region in the lungs.”
Likewise, sometimes people who have had lung cancer develop another lung cancer at a later date. If the second tumor is a “new cancer” and unrelated to the original cancer, it would be called a primary lung cancer. If instead the second tumor was related to spread of the first tumor it may be called a secondary cancer, or more accurately, “lung cancer metastatic to another region of the lungs.”
Sometimes two separate cancers develop in the lungs at the same time. In this scenario both tumors would be considered primary lung cancers.
On rare occasions, a tumor is found in the lungs and doctors can’t be certain where it originated – the original site of the cancer is unknown. Some cancers are only discovered after they have spread to many parts of the body, including the lungs. In this case, the cancer would be called “metastatic cancer to the lungs of unknown origin.”