Understanding cancer

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 old, they die, and new cells take their place.
Sometimes, this 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 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:
Benign tumors:
are rarely a threat to life
can be removed and usually don't grow back
don't invade the tissues around them
don't spread to other parts of the body
Malignant tumors:
may be a threat to life
often can be removed but sometimes grow back
can invade and damage nearby organs and tissues
can spread to other parts of the body
Stomach cancer usually begins in cells in the inner layer of the stomach. Over time, the cancer may invade more deeply into the stomach wall. A stomach tumor can grow through the stomach's outer layer into nearby organs, such as the liver, pancreas, esophagus, or intestine.
Stomach cancer cells can 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 may be found in lymph nodes near the stomach. The cancer cells may attach to other tissues and grow to form new tumors that may damage those tissues.
The spread of cancer is called metastasis. See the Staging section for information about stomach cancer that has spread.