Cancer is a class of diseases in which a group of cells display the traits of uncontrolled growth, invasion, and sometimes metastasis. These three malignant properties differentiate cancer from benign tumors, which are self-limited, do not invade or metastasize. Most forms of cancers form a tumor but some, like leukemia, do not.
Cancer originates in cells, the building blocks that make up tissues, which in turn make up human organs. Normally, cells only generate new cells when the body needs them. When cells grow old, they die, and the new cells take their place. Sometimes, this process goes wrong, and new cells form when the body does not need them, or old cells do not die when they should. This is what is referred to as uncontrolled growth.
The second malignant property, invasion, is when a tumor has formed, due to uncontrolled growth, and cells break away from the tumor and crawl through surrounding tissues. This property enables the cancer cells to move into a blood vessel and be transported through the body, possibly establishing a secondary tumor. The creation of this second tumor is also the third malignant property of cancer cells, the metastasis.
Once breast cancer cells spread, the cancer cells are often found in lymph nodes near the breast. But, breast cancer can spread to almost any other part of the body. The most common places where breast cancer spreads to are the bones, liver, lungs, and brain. The new tumors that are a result of the metastasis, have the same kind of abnormal cells and the same name as the original tumor in the breast. For example, if breast cancer spreads to the lung, the cancer cells in the lung are actually breast cancer cells. The disease is metastatic breast cancer, not lung cancer. For that reason, it is treated as breast cancer, not lung cancer. Doctors call the new tumor "distant" or metastatic disease.