Immunohistochemical markers for distinguishing Spitz nevi from malignant melanomas.

In this study, we demonstrate that immunostaining with MIB-1 and anti-bcl-2 is a useful tool to distinguish compound Spitz nevi from malignant melanomas. Forty-six cases of Spitz nevi and 50 cases of vertical growth-phase melanomas (Clark III-V) were compared for the immunoreactivity of MIB-1 and bcl-2 in the intradermal component of the lesions. As many as 76% of the Spitz nevus cases showed a low percentage (0-2%) of MIB-1 immunoreactivity. In the malignant melanomas, such a low MIB-1 index was shown in only 2% of the cases. The average MIB-1 index in malignant melanomas and Spitz nevi was 29.7 and 4.0%, respectively. bcl-2 was negative in only 4% of the melanoma cases, whereas the corresponding value was 72% in Spitz nevi. Statistical analyses using Students t test showed that the differences were highly significant (P < 0.001). By considering the immunoreactivity for MIB-1 and bcl-2 in the individual cases, we found that as many as 96% of the melanomas both expressed a bcl-2 positivity and exhibited a MIB-1 index exceeding 2% in the dermal component. The corresponding value was as low as 6% in the Spitz nevi.