Cutaneous Spindle Cell Neoplasms with Kinase Fusions

This lecture presents a practical approach to cutaneous spindle cell neoplasms harboring kinase alterations. It reviews their morphological and immunohistochemical spectrum, molecular pathogenesis, DNA methylation findings, and diagnostic testing, with particular emphasis on NTRK-rearranged tumors. Important diagnostic pitfalls—including the limitations of pan-TRK immunohistochemistry and the occurrence of non-functional or passenger NTRK fusions—are also discussed, together with the potential implications for classification and targeted therapy.