The New International Rhabdomyosarcoma Classification, Its Progenitors, and Considerations beyond Morphology

Abstract

A new classification has recently been proposed, the International Classification of Rhabdomyosarcoma (ICR), which separates rhabdomyosarcomas into distinct, reproducible histopathologic categories that are predictive of outcome. Developed by 16 pathologists using 800 representative tumors treated in Intergroup Rhabdomyosarcoma Study-II, the ICR consists of histologically recognizable types assigned to one of three prognostic groups. Superior prognosis is associated with botryoid and spindle cell rhabdomyosarcoma; intermediate prognosis with conventional embryonal rhabdomyosarcoma; and poor prognosis with alveolar rhabdomyosarcoma and undifferentiated sarcoma. The classification is based strictly on morphologic criteria and does not incorporate immunohistochemical, cytogenetic, or molecular genetic findings.