CLASSIFICATION OF RHABDOMYOSARCOMAS AND RELATED SARCOMAS - PATHOLOGICAL ASPECTS AND PROPOSAL FOR A NEW CLASSIFICATION - INTERGROUP RHABDOMYOSARCOMA STUDY

Background. There is a need to develop a single prognostically signifi cant classification of rhabdomyosarcomas (RMS) and other related tumor s of children, adolescents, and young adults which would be a current guide for their diagnosis, allow valid comparison of outcomes between protocols carried out anywhere in the world, and should enhance recogn ition of prognostic subsets. Method. Sixteen pathologists from eight p athology groups, representing six countries and several cooperative gr oups, classified by four histopathologic classification schemes 800 re presentative tumors of the 999 eligible cases treated on Intergroup Rh abdomyosarcoma Study II. Each tumor was classified according to each o f the four systems by each of the pathologists. In addition, two indep endent subsamples of 200 of the 800 patients were reviewed according t o the new system, so that 343 distinct patients were reviewed once, an d 57 of these twice. Results. A study of the survival rates of all sub types in the sample of 800 patients led to the formation of a new syst em. This was tested on two independent subsets of 200 of the original cases and found to be reproducible and predictive of outcome by univar iate analysis. A multivarlate analysis of the 343 patients classified according to the new system indicated that a survival model including pathologic classification and known prognostic factors of primary site , clinical group, and tumor size was significantly better at predictin g survival than a model with only the known prognostic factors. Conclu sion. This new classification, termed International Classification of Rhabdomyosarcoma (ICR) by the authors, was reproducible and predictive of outcome among patients with differing histologies treated uniforml y on the Intergroup Rhabdomyosarcoma II protocols. We believe it shoul d be utilized by all pathologists and cooperative groups to classify r habdomyosarcomas in order to provide comparability among and within mu lti-institutional studies.