Stomach cancer begins when the cancer cells start to form in the mucus-producing inner lining of the stomach. The abnormal growth of cancer cells turns out to form a mass in the stomach. Stomach cancer is also called Gastric cancer, and is a slow-growing cancer which can take years to grow. It's still not completely known what makes the stomach cancer start growing in the stomach. Since it grows at a slow-rate, stomach cancer isn’t usually found at early stages.