Molecular Approaches to Diagnosis in Ewing Sarcoma: Fluorescence In Situ Hybridization (FISH).

The differential diagnosis of small round cell tumors of bone and soft tissue (SRCT) crucially relies on the synoptic evaluation of morphology, immunohistochemical patterns, and molecular features. Although the implementation of broad RNA sequencing in diagnostic molecular pathology routines has substantially changed the standards for molecular confirmation of diagnoses, fluorescence in situ hybridization (FISH) on formalin-fixed, paraffin-embedded (FFPE) tissue sections remains an essential tool for rapid molecular corroboration of diagnoses, which is essentially required for therapeutic decisions. We discuss the major FISH approaches currently employed in diagnostic molecular pathology, addressing classic Ewing sarcoma and differential diagnoses among SRCT, which often cannot be sufficiently ruled out by immunohistochemistry. This chapter will address technical issues, particularly strategies and pitfalls in interpreting FISH patterns.