The Clinical, Morphologic, and Molecular Spectrum of BRAF Fusion Spitz Tumors

BRAF fusion Spitz neoplasms along with MAP3K8 fusions are among the subtypes of Spitz most likely to be diagnosed as Spitz melanoma. However, the current literature has only limited amounts of clinical follow-up on these cases. In this study, we share our experience with 39 BRAF fusion Spitz neoplasms and provide the greatest number of cases with available clinical follow-up. Among 24 patients with clinical follow-up (mean duration of 26.1 mo), none developed metastatic disease. Detailed biomarker assessment with FISH studies, TERT promoter mutational analysis, PRAME and p16 IHC also strongly favored a benign process. Only 2 of 17 cases were positive by FISH, 37 of 38 were negative for TERT promoter mutations, 24 of 24 were negative for PRAME and 16 of 21 had retained staining with p16. In addition, we identify and describe several distinct morphologic patterns, some of which are highly spitzoid in cytomorphology while others lack convincingly spitzoid cytomorphology. We address classification of those cases with less than classic spitzoid cytomorphology with a nonsupervised PCA plot which shows that independent of how spitzoid the cytomorphology is, BRAF fusions strongly cluster with other subtypes of Spitz neoplasms. In conclusion, we show with clinical follow-up, a meta-analysis of the current literature, and our biomarker analysis, that most BRAF fusion Spitz neoplasms have an indolent course and should be considered either benign or of intermediate grade. Further, our morphologic assessment and PCA plot suggest the morphologic spectrum of Spitz neoplasms may need to be expanded.