Mammary Paget's disease and extra‐mammary Paget's disease: two morphologically similar but biologically different diseases

Background: The cells of origin of mammary Paget's disease (MPD) and extra‐mammary Paget's disease (EMPD) have been a controversial subject. The purpose of this study is to examine the expression of the human epidermal growth factor receptor 2 (HER2) pathway members in these two diseases. Design: HER2, AKT, pAKT, PTEN, epidermal growth factor receptor (EGFR) and pEGFR were examined in 16 MPD and 14 EMPD cases. HER2 was graded on a scale from 0 to 3. A score of 3 was considered positive. For AKT, pAKT, PTEN, EGFR and pEGFR, a semi‐quantitative scoring system was used. A score >100 was considered positive. Fisher's exact test was used to analyze the data. Results: HER2 was overexpressed in 87.5% MPD and 35.7% EMPD. While AKT was expressed in all cases, pAKT was expressed in 87.5% MPD and 92.9% EMPD. Both EGFR and pEGFR were negative in all cases. PTEN was positive in 62.5% MPD and 71.4% EMPD. For pAKT+ group, HER2–/PTEN+ was recorded in 0% MPD and 38.5% EMPD (P = .01). Conclusions: In a subset of EMPD, AKT is not activated by HER2 overexpression or by loss of PTEN, which is not the case in MPD. These data suggest that these two diseases are biologically different. Liu W, Iqbal J, Khoury T. Mammary Paget's disease and extramammary Paget's disease: two morphologically similar but biologically different diseases.