Up to 95 percent of stomach cancers are adenocarcinomas. These adenocarcinoma cancers are formed in the cells that line the stomach and can be divided into three types.

Non-cardia stomach cancer

This type of stomach cancer develops from prolonged periods of inflammation and irritation. It is commonly associated with chronic infection with Helicobacter pylori (H. pylori) bacteria.

H. pylori is a spiral-shaped bacterium commonly found in the stomach. The bacteria's shape and the way they move allow them to penetrate the stomach's protective mucous lining, where they produce substances that weaken the lining and make the stomach more susceptible to damage from gastric acids.

The bacteria can also attach to cells of the stomach, causing stomach inflammation, and can stimulate the production of excess stomach acid. Over time, infection with the bacteria can also increase the risk of stomach cancer.

Although it is not known how H. pylori infection is spread, scientists believe it may be contracted through food and water. According to the National Institutes of Health, approximately 20 percent of people under the age of 40 and half of adults over 60 in the United States are infected, with higher rates in developing countries.

Having H. pylori infection doesn't necessarily mean you'll have ulcers or develop stomach cancer.

Diffuse stomach cancer

This type of cancer grows within the stomach wall as individual scattered cells rather than forming a lump or tumor. It can be very difficult to detect using standard endoscopic techniques and often multiple biopsies throughout the stomach are needed. Diffuse stomach cancer sometimes has a genetic cause.

Proximal or gastroesophageal (GE) junction stomach cancer

This type of stomach cancer affects the first part of the stomach and often extends into the area where the stomach and esophagus meet, known as the gastroesophageal junction. Risk factors for this type of cancer include obesity and gastroesophageal reflux disease. This is rapidly increasing in the United States.

Other less common types of stomach cancer include:

  • Lymphoma: Cancer that begins in the cells of the immune system
  • Gastrointestinal stromal tumor (GIST): Cancer of the muscle or connective tissue of the stomach wall
  • Carcinoid tumors: Cancer of the hormone-producing cells of the stomach