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Appendageal tumors of the skin comprise a wide spectrum of benign and malignant neoplasms that exhibit morphological differentiation toward one or more adnexal structures in normal skin. The classification and diagnosis of appendageal tumors is challenging due to the wide variety of tumor types, complicated nomenclature, and numerous classification systems that categorize these neoplasms.1–8 Traditionally, cutaneous adnexal tumors are classified into four groups according to differentiation toward follicular, sebaceous, apocrine, and eccrine structures. These adnexal tumors can be further classified by a gradient of decreasing differentiation into three groups: hyperplasias and hamartomas, benign neoplasms, and malignant neoplasms (Table 119-1). This classification is similar to the approach of the World Health Organization International Histological Classification of tumor monographs.4 The hyperplasias are characterized by an increased number of normal cells in a normal arrangement. Hamartomas are described as an abnormal arrangement of normal tissue. Benign neoplasms lack the potential to metastasize, whereas malignant tumors have the ability to cause local destruction and to metastasize to lymph nodes and viscera.
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While adnexal tumors are classified based on differentiation, often a tumor is not easily classified into one group because the lesion exhibits histologic features of two or more adnexal cell lines. Since adnexal tumors originate from pluripotent stem cells in the epidermis and its appendages, neoplastic cells may aberrantly express one or more lines of appendageal differentiation. The ultimate histologic characteristics of a tumor are related to the activation of ...