Sometimes, this orderly process goes wrong. New cells form when the body does not need them, and old cells do not die when they should. These extra cells can form a mass of tissue called a growth or tumor.

Tumors can be benign or malignant:

  • Benign tumors are not cancer:
    • Benign tumors are rarely life-threatening.
    • Generally, benign tumors can be removed. They usually do not grow back.
    • Cells from benign tumors do not invade the tissues around them.
    • Cells from benign tumors do not spread to other parts of the body.
  • Malignant tumors are cancer:
    • Malignant tumors are generally more serious than benign tumors. They may be life-threatening.
    • Malignant tumors often can be removed. But sometimes they grow back.
    • Cells from malignant tumors can invade and damage nearby tissues and organs.
    • Cells from malignant tumors can spread (metastasize) to other parts of the body. Cancer cells spread by breaking away from the original (primary) tumor and entering the bloodstream or lymphatic system. The cells invade other organs and form new tumors that damage these organs. The spread of cancer is called metastasis.

    • When breast cancer cells spread, the cancer cells are often found in lymph nodes near the breast. Also, breast cancer can spread to almost any other part of the body. The most common are the bones, liver, lungs, and brain. The new tumor has the same kind of abnormal cells and the same name as the primary tumor. For example, if breast cancer spreads to the bones, the cancer cells in the bones are actually breast cancer cells. The disease is metastatic breast cancer, not bone cancer. For that reason, it is treated as breast cancer, not bone cancer. Doctors call the new tumor "distant" or metastatic disease.