Recent studies have reported the overexpression of human epidermal growth factor receptor 2 (HER2) in primary extramammary Paget disease (EMPD). These results indicate that therapies that target HER2 may be useful in treating metastatic EMPD, for which the prognosis is poor. However, there is limited information on the expression and gene amplification of HER2 in metastatic EMPD. Twenty-six corresponding lymph node metastatic sites of primary EMPD underwent immunohistochemical evaluation of HER2 protein overexpression. In cases of HER2 protein overexpression, further analysis into the amplification of the HER2 gene was undertaken using dual colored in situ hybridization. In the corresponding lymph node metastasis of EMPD, HER2 protein overexpression and gene amplification were detected in 38 % and 19 % of cases, respectively. In 22 out of 26 cases (85 %), there were no differences in HER2 protein overexpression between the primary tumors and the corresponding lymph node metastasis (kappa coefficient 0.65). Likewise, HER2 gene amplification status was concordant in 92 % of cases (kappa coefficient 0.75). HER2 status is in good overall concordance between primary tumors and the corresponding metastatic sites. As there was a discrepancy in a minority of cases, HER2 status should be evaluated at both the primary and the metastatic sites, whenever possible.
Keywords: Cancer; Extramammary paget disease; HER2; Metastatic; Trastuzumab.