What is the most common type of stomach cancer?

Nearly all stomach cancers are adenocarcinomas. NCI's summary for clinicians puts the share at "90% to 95% of all gastric malignancies." Adenocarcinoma of the stomach "begins in the mucus-producing cells in the innermost lining of the stomach."

That lining is called the mucosa. It touches food and stomach acid directly, so it takes the most wear. It is also where the damage that leads to cancer usually begins.

Doctors then split gastric adenocarcinoma two ways, and both show up on pathology reports.

The first split is by location. Gastric cardia cancer "begins in the top inch of the stomach, just below where it meets the esophagus." Non-cardia gastric cancer "is cancer that begins in all other sections of the stomach." The two have moved in opposite directions over the decades. NCI notes that non-cardia cancers have become less common overall. Cardia cancers rose sharply in the 1970s and 1980s, then held fairly steady.

The second split is by how the cells look under a microscope. Intestinal adenocarcinomas "are well differentiated." The cells still form glands and look much like normal tissue. Diffuse adenocarcinomas "are undifferentiated or poorly differentiated." They do not form glands, and NCI says they "tend to grow and spread more quickly." The diffuse form can also creep through the stomach wall instead of building a single lump. That stiffens the whole organ. That pattern has a name: linitis plastica. It is harder to spot on a scan and harder to remove with surgery. So the word diffuse on a report is worth asking your team about.

Most stomach adenocarcinoma traces back to one bacterium. Helicobacter pylori lives in the stomach lining, and NCI's fact sheet reports that the Centers for Disease Control and Prevention "estimates that about two-thirds of the world's population harbors the bacterium." Most carriers never get cancer. But the infection can sit there for decades. NCI explains that "the long-term presence of an inflammatory response predisposes cells in the stomach lining to become cancer." The World Health Organization's International Agency for Research on Cancer classified H. pylori as a human carcinogen in 1994. Clearing it helps. NCI cites a study in which two weeks of antibiotics "significantly reduced the incidence of gastric cancer by nearly 50% over 22 years."

Rarer stomach cancers exist. They are not treated like adenocarcinoma at all. They include gastrointestinal neuroendocrine tumors, gastrointestinal stromal tumors (usually shortened to GIST), and primary gastric lymphoma. Each starts in a different cell type and follows its own treatment path. That is why the exact name on the pathology report drives everything that comes after it.

Stomach cancer is not common in the United States. The American Cancer Society's 2026 estimates are 31,510 new US cases and 10,740 deaths. Part of what makes it dangerous is how ordinary it feels early. NCI notes that symptoms "may include common conditions like indigestion and stomach pain." Indigestion that keeps returning, gets worse, or stops responding to the usual remedies is worth an appointment. Another antacid is not the answer.

If you or someone close to you has this diagnosis, two lines on the pathology report are worth reading out to the team. Where in the stomach did it start, and is it called intestinal or diffuse? It is also fair to ask whether H. pylori was tested for, and whether treating it belongs in the plan.