Multivariate DNA cytometry discriminates between Spitz nevi and malignant melanomas because large polymorphic nuclei in Spitz nevi are not aneuploid.

To elucidate the reasons for the malignant histologic appearance of melanocytic nuclei within benign Spitz nevi (SN), we evaluated nuclear DNA distribution and nuclear size using a computerized image analysis system. In each case of 28 SN and 34 malignant melanomas (MM), about 100 randomly sampled nuclei were analyzed, prepared as monolayers from paraffin-embedded tissues. Large nuclei in MM (nuclear area > mean nuclear area of normal melanocytes + 4 delta) were significantly more likely to be aneuploid (DNA content > or = 5c) than large nuclei in SN chi2 test, p < 0.0001). Only two of 990 large SN nuclei exhibited DNA values higher than 5c, whereas 236 of 2,024 large MM nuclei were aneuploid or polyploid. Accordingly, in multivariate analysis, five features of DNA distribution proved to be most important for objective discrimination between MM and SN: 2c deviation index, 5c exceeding rate, standard deviation of the nuclear DNA content, and both the 85th and the 95th percentiles of DNA distributions. On the basis of these features, we could define a linear discriminant function that allowed a correct diagnosis in 94% of the cases. Our data demonstrate that diagnostically misleading large nuclei in SN are euploid, in contrast to MM. It is thus possible to discriminate SN and MM with high accuracy using DNA cytometry. Because paraffin-embedded tissue can be used, this technique could be a valuable complement to routine histology in equivocal cases.