Cancer begins in cells, the building blocks that make up tissues. Tissues make up the stomach and other organs of the body.
Normal cells grow and divide to form new cells as the body needs them. When normal cells grow old or get damaged, they die, and new cells take their place.
Sometimes, this process goes wrong. New cells form when the body doesn't need them, and old or damaged cells don't die as they should. The buildup of extra cells often forms a mass of tissue called a growth, polyp, or tumor.
Tumors in the stomach can be benign (not cancer) or malignant (cancer). Benign tumors are not as harmful as malignant tumors: