[Differential diagnosis of epidermotropic neoplasia]

Many benign and malignant neoplasms may involve the epidermis, a phenomenon which is commonly referred to as epidermotropism. Intraepidermal localization is an important diagnostic clue. However, differentiation of neoplastic cells can be challenging, especially when the cells are completely confined to the epidermis. Besides the large group of lymphoid and melanocytic neoplasms, several rare entities may show epidermotropism. Typical examples are mammary and extramammary Paget's disease and Langerhans cell histiocytosis. Other rare differential diagnoses include Merkel cell carcinoma, granular cell tumor, adnexal tumors, and epidermotropic metastases. The Borst-Jadassohn phenomenon describes the mimicry of epidermotropic neoplasia in Bowen's disease, Bowenoid actinic keratosis, clonal seborrheic keratosis, and hidroacanthoma simplex. Inflammatory dermatoses with increased intraepidermal localization of immune cells may mimic intraepidermal neoplasia. We review the different entities and summarize the key morphologic and immunohistochemical features for diagnosis.