HMB-45 Immunostaining and Ultrastructure of Melanocytic Hyperplasia in Pigmented Basal Cell Carcinomas

Abstract

The pigmented variant of basal cell carcinoma (PBCC) may be clinically misinterpreted as malignant melanoma. Histologically this variant is characterized by the presence of functional melanocytes distributed among epithelial tumor cells, particularly in superficial areas. Our finding of a case of PBCC with numerous atypical HMB-45-positive melanocytes prompted us to analyze the morphologic and immunohistochemical features of 20 consecutive cases of PBCC. Four additional cases with numerous large HMB-45-positive melanocytes, also present in the most invasive tumor nests, were found. Ultrastructural examination was performed in two selected cases. Large melanocytes with immature (stages I and II) and mature melanosomes were found between epithelial 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 should be considered to avoid a misdiagnosis of malignant melanoma.