Immunohistochemical study of Spitz nevi and malignant melanoma with use of antibody to proliferating cell nuclear antigen.

Spitz nevi and malignant melanoma may be difficult to differentiate histologically. We applied an antibody to proliferating cell nuclear antigen (PCNA) to cases of Spitz nevi and malignant melanoma to determine if there were differences in reactivity. Cases were selected from the material of the Department of Pathology, University of Iowa Hospital. Formalin-fixed, paraffin-embedded cases were immunohistochemically stained with anti-PCNA (PC 10) with use of avidin-biotin complex technique. The cases were evaluated for the presence or absence of reactivity, staining intensity, and semiquantitative percentage of positive cells. Positive staining was observed in six of the 10 cases of Spitz nevi. The intensity of staining was weak and the number of positive cells varied from 10% to 40%. Positive staining was observed in all 10 cases of malignant melanoma. In eight of these 10 cases the intensity of staining was strong and the number of positive cells was greater than 40%. In the remaining two cases of melanoma, the intensity of staining was weak and the percentage of positive cells was 20-30%. These results indicate that PCNA staining may be observed in both Spitz nevi and malignant melanoma. Presence or absence of reactivity cannot be used in distinguishing these processes. However, a pattern of strong, diffuse reactivity may be used as an adjunct for supporting a diagnosis of melanoma.