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Because cutaneous basal cell carcinoma (BCC) is such a common malignancy, its unusual histologic manifestations are important. We identified a variant of BCC in which thickened basement membranes surround aggregations of neoplastic epithelial cells. Thickened basement membranes of similar appearance have previously been observed in benign cutaneous adnexal neoplasms, in basaloid monomorphic adenomas of the salivary gland and in other benign conditions, such as folliculocentric basaloid proliferation. We identified nine BCCs that otherwise met standard criteria, but which also had thick basement membranes surrounding some of the aggregations, and examined them by routine and histochemical staining. The cases included BCC with nodular, micronodular, and infiltrating patterns. Two neoplasms were composed largely of clear cells, suggesting, together with the thickened membranes, outer root sheath differentiation. CD34, which labels keratinocytes of the outer root sheath, marked only the epithelial cells of one of these cases. The thickened membranes were stained by periodic-acid Schiff with and without diastase (PAS-D) and by antibodies to type IV collagen and laminin, with slightly different staining patterns. Intraepithelial droplets within aggregations stained with PAS-D and type IV collagen antibodies. Thickened basement membranes therefore can occur in most of the common growth patterns of BCC. The absence of CD34 staining of epithelial cells in most cases makes it problematic at this time to prove that the thickened membranes indicate trichilemmal differentiation. BCC with thick basement membranes can closely mimic benign neoplasms, such as cylindroma and trichilemmoma, from which they can be distinguished in routinely stained sections. The presence of a continuous thick basement membrane around aggregates of epithelial cells does not in and of itself distinguish between benign and malignant cutaneous epithelial neoplasms.

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