Spindle cell tumor with CD34 and S100 co-expression and distinctive stromal and perivascular hyalinization showing EML4-ALK fusion.

Recently, a novel group of CD34+ and S100+ spindle cell tumors with distinctive stromal and perivascular hyalinization showing recurrent gene fusions involving RAF1, BRAF, NTRK1/2/3, and RET has been identified. ALK rearrangements have been rarely reported in this group of tumors. We report a 24-year-old woman with a 1.5-cm pink mass of the scalp. The tumor was made of spindle cells organized in fascicles or haphazardly arranged in a patternless architecture, with areas of stromal and perivascular hyalinization. The tumor cells diffusely expressed CD34 and S100, without SOX-10 expression. The tumor showed diffuse immunopositivity for ALK. RNA sequencing using next-generation sequencing (NGS) detected an EML4-ALK fusion. This case extends the spectrum of this newly described group of CD34+/S100+ spindle cell tumors at the molecular-genetic level. Dermatopathologists should be aware of this recent entity, as it may fall in the differential diagnosis of many other spindle cell tumors with CD34 expression. NGS-based techniques should be performed when facing spindle cell tumors with similar morphology and immunophenotype. Identification of kinase fusions is essential for the precise classification and better knowledge of these tumors, and for targeted therapy in rare aggressive cases.