Diagnostic Roles of α-Methylacyl-CoA Racemase (AMACR) Immunohistochemistry in Gastric Dysplasia and Adenocarcinoma

<i>Background and Objectives</i>: This study aimed to elucidate the diagnostic role of α-Methylacyl-CoA racemase (AMACR) immunohistochemistry in gastric dysplasia and adenocarcinoma. <i>Materials and Methods</i>: Immunohistochemistry for AMACR was performed on 39 gastric dysplasia and 40 gastric adenocarcinoma cases. The expression patterns of AMACR were investigated and divided into luminal and cytoplasmic expression patterns in the gastric lesions. In addition, correlations between AMACR expression and patient age, sex, and tumor size were evaluated. <i>Results</i>: AMACR was expressed in 26 of 39 cases of gastric dysplasia (66.7%) and 17 of 40 cases of gastric adenocarcinomas (42.5%). The AMACR expression rates in high- and low-grade dysplasia were 80.0% and 52.6%, respectively. A detailed analysis of the expression patterns revealed that the luminal expression pattern was significantly higher in low-grade dysplasia than in high-grade dysplasia and gastric adenocarcinoma (<i>p</i> < 0.001). The cytoplasmic expression pattern, without luminal expression, was predominant in high-grade dysplasia and gastric adenocarcinoma. In addition, the rates of loss of expression in the overall area were 15.1 ± 23.9%, 49.0 ± 29.9%, and 59.0 ± 32.2% in low-grade dysplasia, high-grade dysplasia, and gastric adenocarcinoma, respectively. The negative rate of low-grade dysplasia was significantly lower than that of high-grade dysplasia and gastric adenocarcinoma (<i>p</i> < 0.001 and <i>p</i> < 0.001, respectively). <i>Conclusions</i>: AMACR is a useful diagnostic marker for differentiating low-grade dysplasia from high-grade dysplasia and gastric adenocarcinoma. Luminal or cytoplasmic expression patterns and the extent of loss of expression are important for differentiation.