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

Normal, healthy cells grow and divide to form new cells as the body needs them. When normal cells grow old or become damaged, 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 damaged cells do not die as they should. The build-up of extra cells often forms a mass of tissue called a growth or tumor.

Tumor cells can be benign (not cancer) or malignant (cancer). Benign tumor cells are usually not as harmful as malignant tumor cells:

Benign lung tumors

• are rarely a threat to life
• usually do not need to be removed
• do not invade the tissues around them
• do not spread to other parts of the body

Malignant lung tumors may be a threat to life

• may grow back after being removed
• can invade nearby tissues and organs
• can spread to other parts of the body
• Cancer cells spread by breaking away from the original tumor. They enter blood vessels or lymph vessels, which branch into all the tissues of the body. The cancer cells attach to other organs and form new tumors that may damage those organs. The spread of cancer is called metastasis.

Educational information provided by The National Cancer Institute (NCI) Internet site.