Primary Cancer vs Secondary Cancer

diagram of cancer cells Photo©Ugreen

A primary cancer is defined as the original site (organ or tissue) where a cancer began.

For example, a cancer starting in the lungs is called primary lung cancer. If lung cancer 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.

Second or Secondary Cancer

The terms second and secondary cancer are sometimes used interchangeably but can mean different things. The term secondary cancer may refer to either metastasis from a primary cancer, or a second cancer unrelated to the original cancer. When the term second cancer is used, it usually refers to a second primary cancer, in other words, a different cancer than the first cancer, arising in a different organ or tissue.

This can be confusing if you hear that secondary cancers may occur as a result of treatment for a primary cancer. Some chemotherapy drugs, as well as radiation therapy, can increase the risk of developing cancer by damaging DNA as they battle cancer cells. When this occurs the new tumor is referred to as a second primary cancer.

Cancer of Unknown Origin

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. With improved diagnostic tests and molecular profiling, a diagnosis of cancer of unknown origin is less common than in the past, but this does still occur. The reason is often because a tumor is very "undifferentiated." Cancer cells can differ significantly from normal cells, sometimes making them relatively indistinguishable under a microscope. Even if a primary site is unable to be determined for a cancer, doctor s are still able to treat that cancer. It's estimated that 2 to 5 percent of all cancer patients have metastases for which the primary site cannot be determined.

Cancers do not always have a primary site. An example of this is lymphoma. Yet while the primary site where a lymphoma begin may not be known, it is not considered to be a cancer of unknown origin.

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