HMB-45 immunostaining and ultrastructure of melanocytic hyperplasia in pigmented basal cell carcinomas

The pigmented variant of basal cell carcinoma (PBCC) may be clinically misi nterpreted as malignant melanoma. Histologically this variant is characteri zed by the presence of functional melanocytes distributed among epithelial tumor cells, particularly in superficial areas. Our finding of a case of PB CC with numerous atypical HMB-45-positive melanocytes prompted us to analyz e the morphologic and immunohistochemical features of 20 consecutive cases of PBCC. Four additional cases with numerous large HMB-45-positive melanocy tes, also present in the most invasive tumor nests, were found. Ultrastruct ural examination was performed in two selected cases. Large melanocytes wit h immature (stages I and II) and mature melanosomes were found between epit helial tumor cells having desmosomes and tonofilaments. Our cases highlight the presence of immature melanosomes and subsequent HMB-45 immunostaining in hyperplastic melanocytes within PBCC, an underrecognized feature that sh ould be considered to avoid a misdiagnosis of malignant melanoma.