FOLLICULAR BASAL-CELL HYPERPLASIA OVERLYING DERMATOFIBROMA

Follicular basal cell hyperplasia (FBCH) overlying dermatofibroma repr esents aborted or impeded pilar differentiation. Historically, this hy perplasia has been misinterpreted as basal cell carcinoma. In a large series of dermatofibroma (258 cases), those that contained primitive o r malformed follicular structures over the lesion (59 cases) were comp ared with those without such elements (199 cases). Statistical analysi s of various clinicopathologic features showed that FBCH was significa ntly associated with younger age, trunk location, hypercellular dermat ofibroma, loss of a Grenz zone, clear cell hyperplasia, and seborrheic keratosis-like change. There was an inverse correlation between epide rmal atrophy, lichen simplex chronicus-like change, and lower extremit y location with FBCH. Histologic features favoring a diagnosis of FBCH over basal cell carcinoma are the focal nature and superficial locati on of the lesion, lack of cytologic atypia and mitoses, recognizable c omponents of hair follicle differentiation, focal condensation of mese nchymal cells around basal cell proliferation, and the association of epidermal hyperplasia. Our findings suggest that FBCH, clear cell hype rplasia, and seborrheic keratosis-like change all represent an express ion of follicular differentiation overlying dermatofibroma.