Apocrine cystadenoma, apocrine hidrocystoma, and eccrine hidrocystoma: Three distinct tumors defined by expression of keratins and human milk fat globulin 1

Eccrine hidrocystomas and apocrine cystadenomas are morphologically related cystic sweat gland tumors. To elucidate their cellular differentiation we examined by immunohistochemistry the expression of keratins and of human milk fat globulin 1 in 12 of each of these tumors, diagnosed using established conventional histological criteria. All tumors diagnosed as apocrine cystadenomas by these criteria were characterized by a keratin pattern of secretory type. In addition, they expressed human milk fat globulin 1. Tumors diagnosed as eccrine hidrocystomas expressed a keratin pattern of excretory type. A part of the tumors with an excretory keratin pattern expressed human milk fat globulin, while others did not. Some presumed eccrine hidrocystomas expressed the very same antigens as apocrine cystadenomas. Thus, our study reveals three distinct types of tumors, in contrast to the conventional distinction of only eccrine hidrocystomas and apocrine cystadenomas. Apocrine cystadenomas differentiate towards the secretory coil of apocrine sweat glands. Presumed eccrine hidrocystomas may represent cystic tumors of the eccrine sweat duct, or they may represent cystic tumors of the apocrine duct. Thus, the name hidrocystoma should be used without further specification of an eccrine or apocrine nature, unless certainty is reached by immunohistochemical characterization. Also, hidrocystomas often prove to be histologically misdiagnosed apocrine cystadenomas because of a flattened cyst wall secondary to increased intraluminal pressure.