Put quite simply, cancer is the out-of-control growth and spread of abnormal cells. These cells accumulate and form tumors that compress, invade and destroy normal tissue. Breast cancer is a tumor that has developed from the cells of the breast. The cause of breast cancer is unknown.

There are several types of breast cancer. The most common is ductal carcinoma, which begins in the lining of the milk ducts of the breast. Another type, lobular carcinoma, begins in the lobules where breast milk is produced. If a malignant tumor invades nearby tissue, it is known as infiltrating or invasive cancer.

A malignant tumor can invade surrounding tissue and destroy it. Cancer cells can also break away from a malignant tumor and enter the bloodstream or the lymphatic system. This is how cancer spreads within the body. When breast cancer spreads outside the breast, cancer cells often are found in the lymph nodes under the arm. Cancer cells may spread beyond the breast such as to other lymph nodes, the bones, liver, or lungs. (Although it is not common, some patients whose underarm lymph nodes are clear of breast cancer may still have cancer cells which have spread to other parts of the body)

 

Cancer that spreads to other parts of the body is the same disease and has the same name as the original cancer. When breast cancer spreads, it is called metastatic breast cancer even though it is found in another part of the body. For example, breast cancer that has spread to the bones is called metastatic breast cancer, not bone cancer.