Basal cell tumor or cutaneous basilar epithelial neoplasm? Rethinking the cytologic diagnosis of basal cell tumors.

A 1-cm-diameter, red, raised, cutaneous mass over the dorsal surface of the left third metacarpal of a 6-year-old neutered male yellow Labrador Retriever was aspirated. The aspirate contained cohesive clusters of cells, some containing cells with increased pleomorphism. Cellular debris (some keratinized), melanin, large numbers of macrophages, a few neutrophils, and fibroblasts were also observed. The cytologic interpretation was malignant neoplasia with histiocytic inflammation. Differentials included a carcinoma or, given the melanin pigment and variable morphology of the cells, possibly malignant melanoma. Histologically, the tumor was diagnosed as a basal cell epithelioma. Neoplasms that once were lumped into the broad histologic diagnosis of basal cell tumors have since been split into distinct entities, dependent on evidence of differentiation into epidermis, trichofollicular epithelium, or sweat or sebaceous glands. Although histologic reclassification has resulted in removal of most of these entities from the original basal cell tumor category, a cytologic diagnosis of basal cell tumor continues to be used to represent the large, heterogeneous group of epidermal, trichofollicular, and adnexal skin tumors with basal cell characteristics. The case in this report demonstrates the heterogeneity of neoplasms that may be diagnosed cytologically as basal cell tumors and supports the need for cytologic criteria and nomenclature that better reflect potential variation in tissue differentiation.