Cancer begins in cells, the building blocks that form tissues. Tissues make up the organs of the body.

Normally, cells grow and divide to form new cells as the body needs them. When cells grow old, they die, and new cells take their place.

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, and 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 can invade other organs, forming new tumors that damage these organs. The spread of cancer is called metastasis.

Most cancers are named for where they start. For example, lung cancer starts in the lung, and breast cancer starts in the breast. Lymphoma is cancer that starts in the lymphatic system. And leukemia is cancer that starts in white blood cells (leukocytes).

When cancer spreads and forms a new tumor in another part of the body, the new tumor has the same kind of abnormal cells and the same name as the primary tumor. For example, if prostate cancer spreads to the bones, the cancer cells in the bones are actually prostate cancer cells. The disease is metastatic prostate cancer, not bone cancer. For that reason, it is treated as prostate cancer, not bone cancer. Doctors sometimes call the new tumor "distant" or metastatic disease.

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