Malignant sweat gland tumours: an update.

Cutaneous adnexal tumours can be a diagnostic challenge for the pathologist. This is particularly true in the case of tumours with sweat gland differentiation, due to a large number of rare entities, a multiplicity of names to designate the same neoplasms and consequent lack of consensus regarding their classification and nomenclature. In the traditional view, sweat gland tumours were divided into eccrine and apocrine. However, this has been challenged in recent years, and in fact many of these tumours may have both eccrine and apocrine variants. Some display more complex features and defy classification, due to the presence of other lines of differentiation, namely follicular and/or sebaceous (in the case of apocrine tumours, due to the close embryological relationship between apocrine glands, hair follicles and sebaceous glands). The present paper reviews and updates the basic concepts regarding the following malignant sweat gland tumours: apocrine carcinoma, porocarcinoma, hidradenocarcinoma, spiradenocarcinoma, cylindrocarcinoma, microcystic adnexal carcinoma and related entities, squamoid eccrine ductal carcinoma, digital papillary adenocarcinoma, primary cutaneous mucinous carcinoma, endocrine mucin-producing sweat gland carcinoma and primary cutaneous signet ring cell carcinoma. Particular emphasis is put in recent findings that may have implications in the diagnosis and management of these tumours.