According to the National Cancer Institute, metastasis is the process by which cancer spreads from the place of origin to other parts of the body. Advanced stages of breast cancer can spread to any number of different tissues and organs, where a secondary cancer can begin to develop. Secondary cancers are specifically cancers caused by the metastasis of a pre-existing cancer. However, it is possible for someone with cancer to develop an entirely separate cancer, one that has no connection to the original disease. How can medical professionals differentiate between the two, and know when one cancer causes another?

When cancers spread, cells gradually break off of the tumor and then disperse to nearby tissues or organs. If the cells reach the bloodstream or lymphatic system, then spreading to distant tissues and organs becomes much more likely. If breast cancer in an advanced stage spreads to a distant part of the body, doctors can tell if the cancer originated there or in the breast.

This is done by examining cancerous cells microscopically. For example, if the breast cancer were to spread to the liver, the cells which make up the tumor in the liver would be cancerous breast cells. If the cancerous cells from the liver resemble liver cells, it is likely that the liver cancer is an entirely separate entity from the breast cancer, and would not be classified as a secondary cancer.