What is gastric cancer? Gastric cancer develops from malignant cells in the lining of the stomach, the part of the digestive system which processes nutrients and helps to pass waste material out of the body. The wall of the stomach is made up of 3 layers of tissue: the mucosal (innermost) layer, the muscularis (middle) layer, and the serosal (outermost) layer. Gastric cancer begins in the cells lining the mucosal layer and spreads through the outer layers as it grows. Stomach cancer was the 5th most diagnosed cancer in 2012, with almost a million new cases, accounting for 6.8% of all cancer cases.