Pseudocarcinomatous hyperplasia with follicular differentiation overlying basal cell carcinoma.

We present a series of 25 cases of basal cell carcinoma (BCC) with overlying cytologically bland epidermal hyperplasia and cyst formation. Eight of the BCCs were nodular and 11 were infiltrative. Immunohistochemical staining for Ki-67 and cytokeratin 17 (CK17) was performed to evaluate the proliferative and differentiation characteristics of the hyperplastic epithelium compared with the adjacent BCC and normal epidermis. Fourteen of the 25 cases had sufficient tissue to evaluate the staining patterns. In the majority of cases, Ki-67 expression was prominent throughout the BCCs, but only expressed in the basal and suprabasal layers of the adjacent hyperplastic epithelium, which was equivalent to normal epidermis. CK17 expression was prominent throughout the BCC and surrounding incident hair follicles. CK17 was expressed to a lesser extent in the central more keratinized portions of the hyperplastic epithelium, but rarely in normal epidermis. The morphologic characteristics and immunohistochemical staining patterns in these tumors suggest that the epidermal hyperplasia represents pseudocarcinomatous hyperplasia with follicular differentiation. Recognizing this pattern of epidermal change should alert the pathologist to the possibility of an associated BCC in cases in which the biopsy specimen or sectioning is initially too superficial to reveal the underlying malignancy.