This extra tissue, called a tumor, can be benign or malignant. Eighty percent of all breast tumors are benign. Benign tumors are not cancer. The cells in benign tumors do not invade other tissues and do not spread to other parts of the body. Benign breast tumors are not a threat to life.
Malignant tumors are cancer.
The cancer cells grow and divide out of control, invading and damaging nearby tissues and organs. Cancer cells can also break away from the original tumor and enter the bloodstream or lymphatic system. This is how breast cancer spreads and forms secondary tumors in other parts of the body. This spread is called metastasis. The breast consist of lobes, lobules, and bulbs that are connected by ducts.