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Classification and Staging
Several classification systems exist to further define gastric cancer and predict prognosis. As mentioned earlier (see Fig. 54-2), gastric cancers can be subdivided into intestinal and diffuse types. Gastric cancer can also be divided into early and advanced lesions. Early gastric cancer is defined as a cancer that does not invade beyond the submucosa, regardless of lymph node involvement. This form of cancer has a much higher prevalence in the Far East, especially Japan, and carries a very favorable prognosis, with 5-year survival rates greater than 90% being reported in Asia and greater than 80% in Western countries.329-332
The most commonly used clinical staging classification system for gastric cancer is the TNM system, used by the International Union Against Cancer (UICC) and the American Joint Committee on Cancer (AJCC).333,334 In the TNM staging system, T (Tumor) indicates the depth of penetration (Fig. 54-7): T1a denotes a tumor that invades the lamina propria or mucosa, T1b denotes invasion of the submucosa, T2 denotes invasion of the muscularis propria, T3 denotes invasion of the subserosal connective tissue, T4a denotes invasion of the serosa (visceral peritoneum), and T4b denotes invasion into adjacent organs or structures. N (Nodes) indicates the amount of lymph node invasion: N0 denotes no lymph node involvement, N1 denotes involvement of 1 to 2 lymph nodes, N2 denotes involvement of 3 to 6 lymph nodes, and N3 denotes involvement of 7 or more lymph nodes. M (Metastasis) indicates the presence of metastases, with M0 denoting no metastases and M1 denoting distant metastases, including positive peritoneal cytology (Table 54-4). In the most recent AJCC staging manual, cardia cancer (tumors within 5 cm of and crossing the GE junction) is now classified together with esophageal and GE junction tumors.334
Recent studies have investigated reclassification of gastric cancer based on biological characteristics. Using gastric cancer cell lines, distinct gene expression patterns were identified for intestinal-type and diffuse-type gastric cancer. Subsequent analyses showed that patients who had tumors with intestinaltype gene expression profiles had improved survival when treated with 5-fluorouracil-based chemotherapy.335 The prospect of incorporating tumor biology into staging classification systems is intriguing, although future validation studies are required for this to occur.
Accurate staging in gastric cancer is important for treatment decisions. EUS is the best-studied modality for the staging of gastric cancer and remains the test of choice for assessing tumor depth and nodal involvement. However, improvements in image quality for both CT and MRI make these studies potential alternatives and adjuncts to EUS.