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EUS allows the visualization of the 5 layers of the gastric wall. The superficial gastric mucosa is represented by an echogenic first layer, and the deeper mucosa by a hypoechogenic second layer; the submucosa is represented by an echogenic third layer, the muscularis propria as a hypoechogenic fourth layer, and the serosa as an echogenic fifth layer. EUS also has the ability to identify and biopsy submucosal lesions, such as gastric lymphomas and stromal tumors. These lesions typically involve thickening of the submucosa and muscularis propria and may appear as gastric fold thickening on barium studies or endoscopy.
Based on results of a meta-analysis of EUS for gastric cancer staging, EUS has sensitivity of 86% and specificity of 91% to distinguish T1-2 versus T3-4 tumors.336 Intramucosal lesions (T1a) are identified with 83% sensitivity and 79% specificity. EUS may be particularly useful for identifying early gastric cancer lesions amenable to endoscopic mucosal resection or submucosal dissection337(Fig. 54-8). In terms of N staging, the rate of detection of perigastric nodes with EUS is comparable to staging with CT.338,339 EUS is slightly less accurate in the assessment of nodal status as compared to depth of tumor invasion, with 69% sensitivity and 84% specificity to distinguish positive from negative lymph node status.336 A particular difficulty with N staging lies in the fact that many small lymph nodes can also harbor metastases, and thus understaging can occur.