Cancer begins in cells, the building blocks that make up 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, 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 or weak cells do not die when they should. These extra cells can form a mass of tissues called a growth or tumor.
Not all tumors are cancer. Tumors can be benign or malignant.