Stomach cancer is a malignant tumour that starts in cells of the stomach. Malignant means that it can spread, or metastasize, to other parts of the body.
The stomach is part of the digestive system. It is a muscular, sac-like organ in the upper abdomen. In the stomach, the food is mixed with digestive juices. These juices are made by glands in the lining of the stomach. They help break the food down into a semi-solid mixture that then passes into the small intestine.
Cells in the stomach sometimes change and no longer grow or behave normally. These changes may lead to non-cancerous, or benign, tumours such as gastric polyps, benign gastrointestinal stromal tumours (GISTs) or lipomas.
Changes to cells in the stomach can also cause precancerous conditions. This means that the cells are not yet cancer but there is a higher chance these abnormal changes will become cancer. Precancerous conditions that can develop in the stomach are gastric adenoma, or adenomatous polyps, and gastric epithelial dysplasia.
In some cases, changes to stomach cells can cause cancer. Most often, cancer starts in gland cells in the inner layer of the stomach wall, which is called the gastric mucosa. This type of cancer is called adenocarcinoma of the stomach. It makes up about 95% of all stomach cancers.
Rare types of stomach cancer can also develop. These include gastric non-Hodgkin lymphomas and soft tissue sarcomas like malignant GISTs.