Most stomach cancers form a tumor (lump) or ulcer in the inner lining of the stomach. Less commonly, the cancer spreads along the stomach wall without forming a lump. In the early stages, stomach cancer stays within the stomach lining or stomach wall. However, the cancer cells can spread into neighboring organs. This is called localized spread. Sometimes the cancer cells can break away and travel to other parts of the body in the bloodstream and lymph system (the lymphatic system which makes up part of your body's immune system). This is called metastasis.
Attribution: BUPA