Stomach cancer begins when cancer cells form within the inner lining of your stomach. These cells can grow into a tumor. Also called gastric cancer, the disease usually grows slowly over many years.. A stomach cancer or gastric cancer almost always is an adenocarcinoma. These cancers develop from the cells that form the innermost lining of the stomach (the mucosa). These are cancers of the system tissue that are sometimes found within the wall of the stomach. The treatment and outlook depend upon the sort of lymphoma. ome of these tumors are non-cancerous (benign); others are cancerous. Although GISTs can be found anywhere in the digestive tract, most are found in the stomach. Most of those tumors don't spread to other organs. These tumors are discussed in additional detail in Gastrointestinal Carcinoid Tumors. Other sorts of cancer, like epithelial cell carcinoma, small cell carcinoma, and leiomyosarcoma, also can start within the stomach, but these cancers are very rare.