Most people use the word “stomach” to refer to the body below the chest and above the pelvis. The medical word for this area, though, is “abdomen.” What you might call a stomachache, doctors call abdominal pain because the stomach is only one of many organs in the abdomen, including the small intestine, large intestine or colon, and pancreas.

Stomach cancer should not be confused with other cancers that can occur in the abdomen—these cancers have different symptoms, different outlooks, and different cancer treatments.

The stomach

The stomach is a sac-like organ that holds food and starts to digest it by secreting gastric juice. The food and gastric juice are mixed and then emptied into the first part of the small intestine called the duodenum.

The stomach has five parts:

• Cardia. The part closest to the esophagus, where chewed food comes from the mouth through the chest.
• Fundus. The upper part of the stomach next to the cardia.
• Body or corpus. The main part of the stomach, between the upper and lower parts.
• Antrum. The lower portion near the small intestine, where food mixes with gastric juice.
• Pylorus. The last part of the stomach, which acts as a valve to control emptying of the stomach contents into the small intestine.

The stomach wall has five layers, which are important in determining the stage of stomach cancer, plus treatment and prognosis:

• Mucosa. The innermost layer, where stomach acid and digestive enzymes are made. Most stomach cancers start in this layer.
• Submucosa. A supporting layer.
• Muscularis propria. A thick layer of muscle that moves and mixes the stomach contents.
• Subserosa. One of two outer layers that wrap the stomach.
• Outermost serosa. The other outer layer.

Stomach cancer occurs when cells in these layers—most often the mucosa—begin growing out of control. This happens because of genetic mutations in these cells. We still don’t know the exact cause of stomach cancer, but research has identified several risk factors, including a stomach infection from the H. pylori bacteria.

As a cancer grows from the mucosa into deeper layers, the stage becomes more advanced. Stomach cancers typically develop slowly over many years. Before a true cancer develops, pre-cancerous changes often occur in the mucosa—these early changes rarely cause symptoms and often go undetected.