Gastric adenocarcinoma is the third most common cause of cancer death worldwide. However, its incidence has declined rapidly over the last 70 years, especially in Western countries, which may be attributable to changes in diet (more fruits and vegetables), food refrigeration (allowing more fresh foods and reduced salted, smoked, and preserved foods), reduced toxic environmental exposures, and a decline in Helicobacter pylori infections. The incidence of gastric cancer remains high (62/100,000 males) in Japan and many developing regions, including eastern Asia, Eastern Europe, Chile, Colombia, and Central America. In the United States, there were an estimated 27,510 new cases and 11,140 deaths in 2019. The incidence is higher in Asian Americans, Hispanics, African Americans, and American Indian/Alaska Natives.
There are two main histologic variants of gastric cancer: “intestinal-type” (which resembles intestinal cancers in forming glandular structures) and “diffuse” (which is poorly differentiated, has signet-ring cells, and lacks glandular formation). The incidence of intestinal-type gastric cancer has declined significantly, but it is still the more common type (70–80%); it occurs twice as often in men as women, primarily affects older people (mean age 68 years), and is more strongly associated with environmental factors. It is believed to arise through a gradual, multi-step progression from inflammation (most commonly due to H pylori), to atrophic gastritis, to intestinal metaplasia, and finally dysplasia or cancer. Chronic H pylori gastritis is the strongest risk factor for gastric carcinoma, increasing the relative risk 3.5- to 20-fold. It is estimated that 60–90% of cases of gastric carcinomas may be attributable to H pylori. Other risk factors for intestinal-type gastric cancer include pernicious anemia, a history of partial gastric resection more than 15 years previously, smoking, and diets that are high in nitrates or salt and low in vitamin C. Diffuse gastric cancer accounts for 20–30% of gastric cancer cases. In contrast to intestinal-type cancer, it affects men and women equally, occurs more commonly in young people, is not as strongly related to H pylori infection, and has a worse prognosis than intestinal-type cancer with early metastasis. Most diffuse gastric cancers are attributable to acquired or hereditary mutations in the genes regulating the E-cadherin cell adhesion protein. Hereditary diffuse gastric cancer accounts for 1–3% of gastric cancers. The cancer may arise at a young age, is often multifocal and infiltrating with signet ring cell histology, and confers poor prognosis. Many of these families have a germline mutation of E-cadherin CDH1, which is inherited in an autosomal dominant pattern and carries a greater than 60% lifetime risk of gastric cancer. Prophylactic gastrectomy should be considered in patients known to carry this mutation.
In addition to the hereditary diffuse gastric cancer, there are other hereditary cancer predisposition syndromes that account for 3–5% of gastric cancers. These include Lynch syndrome, juvenile polyposis syndrome, Peutz-Jeghers syndrome, and familial adenomatous polyposis. There are specific guidelines for how often screening endoscopies should be done for these syndromes (see Chapter 15).
Most gastric cancers arise in the body and antrum. These may occur in a variety of morphologic types: (1) polypoid or fungating intraluminal masses(eFigure 39–4); (2) ulcerating masses; (3) diffusely spreading (linitis plastica), in which the tumor spreads through the submucosa, resulting in a rigid, atonic stomach with thickened folds (prognosis dismal)(eFigure 39–5); and (4) superficially spreading or “early” gastric cancer—confined to the mucosa or submucosa (with or without lymph node metastases) and associated with a favorable prognosis. HER2 amplification and overexpression is seen in 10–25% of gastric adenocarcinoma cases and is more commonly observed in intestinal histology and moderately differentiated disease. The prognostic significance of HER2 status is uncertain. Testing for microsatellite instability (MSI), deficiency in mismatch repair proteins (dMMR), and PD-L1 is recommended in advanced disease to identify tumors that may respond to immunotherapy. For gastric adenocarcinoma, MSI-H/dMMR is found in 8–16% of cases.
Large fungating mass of gastric body with spontaneous oozing. Biopsies confirmed adenocarcinoma. (Used, with permission, from R. McClean.)
Diffusely spreading gastric adenocarcinoma with submucosal spread resulting in disruption of normal gastric folds inferiorly. (Used, with permission, from Y. Chen.)
In contrast to the dramatic decline in cancers of the distal stomach, a rise in incidence of tumors of the gastric cardia has been noted. These tumors have demographic and pathologic features that resemble Barrett-associated esophageal adenocarcinomas (see Esophageal Cancer).