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:
Malignant tumors are cancer:
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.