The mass of extra tissue, called a tumor, can be benign or malignant. Benign tumors are not cancer. They can usually be removed, and in most cases, they don't come back. Most important, 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.
They can invade and damage nearby tissues and organs. Also, cancer cells can break away from a malignant tumor and enter the bloodstream or Iymphatic system. That is how breast cancer spreads and forms secondary tumors in other parts of the body. The spread of cancer is called metastasis. There are more than 100 different types of cancer, including several types of breast cancer.