ERG expression in intraductal carcinoma of the prostate: comparison with adjacent invasive prostatic adenocarcinoma

Intraductal carcinoma of the prostate is a growth pattern of prostatic adenocarcinoma that has not been well characterized from the molecular standpoint. It remains debatable whether intraductal carcinoma of the prostate represents colonization of benign glands by pre-existing conventional prostatic adenocarcinoma, or progression of high-grade prostatic intraepithelial neoplasia. TMPRSS2-ERG is the most common gene fusion in conventional prostatic adenocarcinoma, identified in about 40-70% of cases. In this study, we compared the expression of ERG in intraductal carcinoma of the prostate and adjacent conventional prostatic adenocarcinoma. Thirty-one confirmed cases of intraductal carcinoma of the prostate, with adjacent conventional prostatic adenocarcinoma and available tissue blocks, were identified at our institution. Immunohistochemical stains were performed for ERG using a rabbit anti-ERG monoclonal antibody. The ERG expression in the intraductal carcinoma of the prostate component was compared with that in the adjacent conventional prostatic adenocarcinoma. Mean patient age was 65 years (range: 48-79 years). Positive ERG expression was identified in 11/31 (35%) cases of intraductal carcinoma of the prostate. In all 11/11 (100%) cases with positive ERG expression in the intraductal carcinoma of the prostate component, ERG expression was also positive in the adjacent conventional prostatic adenocarcinoma. In the 20/31 cases with negative ERG expression in the intraductal carcinoma of the prostate component, ERG was also negative in the adjacent conventional prostatic adenocarcinoma. It is highly conceivable that based on the identical ERG expression (positive or negative) in intraductal carcinoma of the prostate and the adjacent conventional prostatic adenocarcinoma, intraductal carcinoma of the prostate most likely represents colonization of benign glands by adjacent pre-existing conventional prostatic adenocarcinoma.