S100/CD34-Positive Spindle Cell Mesenchymal Neoplasm Harboring KIAA1549-BRAF Fusion

Mesenchymal neoplasms with oncogenic kinase activity due to genomic alterations in receptor tyrosine kinase genes are a morphologically heterogeneous group with a variable biologic potential. A subset of these neoplasms are characterized by immunophenotypic property of dual S100 protein/CD34 expression, histopathological resemblance to lipofibromatosis or peripheral nerve sheath tumors, and often alterations in neurotrophic tropomyosin-related kinase genes. In this article, we present a case of an S100 protein/CD34-positive spindle cell neoplasm harboring a rare BRAF gene rearrangement (KIAA1549-BRAF fusion) and discuss the clinical, histopathological, and molecular variations associated with such neoplasms.