Pediatric Mesenchymal Tumors.

Vascular, fibrous/myofibroblastic, and myogenic tumors account for the majority of mesenchymal tumors in children. These tumors often show significant overlap in morphology and immunophenotype posing diagnostic difficulties and, thus, their classification remains challenging. Recent advances in immunohistochemistry have proved helpful in identifying a specific line of differentiation in some tumors, but other tumors remain difficult to classify. Molecular investigations have provided an existing tool to better understand the pathogenesis of some of these tumors and, in some cases like the EWING family of tumors, expanded the classification resulting in the emergence of previously unknown tumors. Some of these tumors are currently diagnosed according to their underlying molecular abnormality, such as CIC-rearranged sarcoma, BCOR-rearranged sarcoma, etc. This review focuses on the common mesenchymal neoplasms of the pediatric population with emphasis on the salient histologic features, immunoprofile, and molecular characteristics. For practical purposes, the latter are summarized in Supplemental Table 1, Supplemental Digital Content 1, http://links.lww.com/PAP/A46.