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Predicting clinical behavior of atypical Spitz tumors remains problematic. In this study, we assessed interobserver agreement of diagnosis by 13 expert dermatopathologists for atypical Spitz tumors (n=75). We determined which histomorphologic features were most heavily weighted for their diagnostic significance by the experts and also which histomorphologic features had a statistically significant correlation with clinical outcome. There was a low interobserver agreement among the experts in categorizing lesions as malignant versus nonmalignant (κ=0.30). The histomorphologic features that were given the most diagnostic significance by the experts were: consumption of the epidermis, atypical mitoses, high-grade cytologic atypia, and mitotic rate. Conversely, the histomorphologic features that most correlated with disease progression were: frequent mitoses, deep mitoses, asymmetry, high-grade cytologic atypia, and ulceration. The presence and/or pattern of pagetoid spread, consumption of the epidermis, and lymphoid aggregates demonstrated no association with clinical behavior. The results support the assertion that there is a lack of consensus in the assessment of atypical Spitz tumors by expert dermatopathologists. Importantly, many features used to distinguish conventional melanoma from nevi were not useful in predicting the behavior of atypical Spitz tumors. This study may provide some guidance regarding histologic assessment of these enigmatic tumors.

Death due to melanoma in childhood (up to 20 y of age) is a rare event, with an average of 18 cases reported annually in the United States. In this study we evaluated 2 subgroups of high-risk melanocytic neoplasms in childhood, specifically atypical Spitz tumors (ASTs) with chromosomal copy number changes and conventional melanomas. We analyzed the clinical, histologic, and molecular features of all cases and performed the Fisher exact test, logistic regression, and multivariate analysis to evaluate features associated with aggressive clinical behavior in these cases. Among the ASTs, all of which had 1 or more chromosomal copy number aberrations, the presence of homozygous 9p21 deletions and a positive sentinel lymph node were each found to be correlated with tumor extension beyond the sentinel lymph node, with P-values of 0.046 and 0.01, respectively. Two patients with ASTs that had homozygous 9p21 deletions developed brain metastasis, one of whom died of disease. Among the 21 conventional melanomas, 3 patients developed distant metastasis and died of disease. Chromosomal copy number aberrations evaluated by fluorescence in situ hybridization were present in the majority of the cases (16/18). Among conventional melanomas, we did not identify any clinical, histologic, or molecular features associated with aggressive behavior. The presence of 8q24 gains was seen almost exclusively in 6 amelanotic small cell melanomas in children of whom 1 died of disease. Characteristic chromosomal copy number aberrations may occur in specific subtypes of melanocytic neoplasms in children and may help with the classification and prognostication of these rare tumors.