Comprehensive comparison of salivary gland and cutaneous adnexal tumors: analogous versus discrepant clinical, terminological, histological, and molecular features

The manyfold salivary gland and cutaneous adnexal tumors exhibit a confusing mixture of striking similarities and analogies, but also major discrepancies. Scientific literature focusing on a comparison of these two tumor groups is largely lacking so far. This 2-part review article presents a comparison of both tumor groups, investigating similarities and discrepancies in their clinical aspects, terminology, (immune)-histology, and molecular pathology. An initial comparison of the basic microscopic and functional anatomy of normal salivary and cutaneous adnexal structures is followed by a topographic assessment of practical differential diagnostic difficulties, anatomically highlighted in the overlapping periparotid and perioral areas. Altogether, 21 of 36 benign and malignant salivary tumor entities display histological similarities to adnexal tumors of eccrine, apocrine, and rarely sebaceous, but not trichofollicular differentiation. In this first part of the review on related tumor entities, major histological and partly terminological and/or molecular similarities are compared for salivary pleomorphic adenoma/myoepithelioma versus cutaneous mixed tumor/myoepithelioma, for basal cell adenoma and carcinoma versus spiradenoma, cylindroma, and spiradenocarcinoma, as well as for the principle of secondary malignant transformation ex different types of benign adenomas.