Atypical and malignant hidradenomas: a histological and immunohistochemical study.

The histological features of atypical hidradenoma are worrisome for increased risk of recurrence and possible malignant potential; however, earlier studies with immunohistochemistry or patient follow-up have not been reported. In addition, immunohistochemical analysis of hidradenocarcinoma exists in the literature mainly as case reports and as a single series of six cases. We compare the histological features and Ki-67, phosphorylated histone H3, epidermal growth factor receptor, and Her2/neu expression profiles of 15 atypical and 15 malignant hidradenomas with those of benign hidradenoma and metastasizing adnexal carcinomas. Infiltrative growth pattern, deep extension, necrosis, nuclear pleomorphism, and > or =4 mitoses per 10 high-power fields are specific features of hidradenocarcinomas. Significant difference in mean Ki-67% was observed between benign and malignant hidradenomas (P<0.001), benign and metastasizing adnexal carcinomas (0.002), atypical and malignant hidradenomas (P<0.001), and between atypical hidradenomas and metastasizing adnexal carcinomas (0.002). Significant difference in mean phosphorylated histone H3% was observed between benign and malignant hidradenomas (P<0.001), benign and metastasizing adnexal carcinomas (0.003), atypical and malignant hidradenomas (P<0.001), and between atypical hidradenomas and metastasizing adnexal carcinomas (P<0.001). Mean epidermal growth factor receptor total score was significantly different in benign and atypical hidradenoma when compared with that in metastasizing adnexal carcinoma (P=0.014 and 0.019, respectively). Equivocal or 2+ Her2/neu positivity was observed in one hidradenocarcinoma and in two metastasizing adnexal carcinomas. Receiver operating characteristic curve analysis for Ki-67 and phosphorylated histone H3% positivity reveals statistically significant criterion values of >11.425 and >0.7, respectively, for distinguishing malignant hidradenomas from atypical hidradenomas. Despite the presence of some worrisome histological features, the significantly different immunoprofile from the malignant counterpart suggests that atypical hidradenomas are likely to recur but are unlikely to metastasize. A tumor with Ki-67>11% and/or phosphorylated histone H3>0.7% would likely be a malignant rather than an atypical hidradenoma. The infrequent Her2/neu overexpression in hidradenocarcinoma suggests its limited therapeutic role.