Follicular basal cell hyperplasia overlying dermatofibroma.

Follicular basal cell hyperplasia (FBCH) overlying dermatofibroma represents aborted or impeded pilar differentiation. Historically, this hyperplasia has been misinterpreted as basal cell carcinoma. In a large series of dermatofibroma (258 cases), those that contained primitive or malformed follicular structures over the lesion (59 cases) were compared with those without such elements (199 cases). Statistical analysis of various clinicopathologic features showed that FBCH was significantly associated with younger age, trunk location, hypercellular dermatofibroma, loss of a Grenz zone, clear cell hyperplasia, and seborrheic keratosis-like change. There was an inverse correlation between epidermal atrophy, lichen simplex chronicus-like change, and lower extremity location with FBCH. Histologic features favoring a diagnosis of FBCH over basal cell carcinoma are the focal nature and superficial location of the lesion, lack of cytologic atypia and mitoses, recognizable components of hair follicle differentiation, focal condensation of mesenchymal cells around basal cell proliferation, and the association of epidermal hyperplasia. Our findings suggest that FBCH, clear cell hyperplasia, and seborrheic keratosis-like change all represent an expression of follicular differentiation overlying dermatofibroma.