Rhabdomyosarcoma. A new classification scheme related to prognosis.

We classified 159 cases of rhabdomyosarcoma (RMS) according to the conventional scheme adopted by the World Health Organization and a modified conventional scheme established at the National Cancer Institute (NCI), Bethesda, Md. The major modification in the NCI scheme was the inclusion of compact round-cell RMS with scant myogenesis in the group of alveolar RMS despite lack of an alveolar architecture. These tumors were previously considered to be embryonal RMS, but their cytologic features are quite different from those seen in embryonal RMS and are indistinguishable from those encountered in alveolar RMS. These tumors are referred to as "solid alveolar RMS." Survival curves were constructed with the method of Kaplan-Meier and compared with the unstratified and stratified methods of Mantel-Haenszel (with stratification factors being stage, site, and age) and with the Cox regression analysis. Both histologic schemes showed a statistically significant prognostic value in unstratified analyses, but the NCI scheme demonstrated prognostic value even in stratified analyses and in the Cox regression analysis in our series of cases. The data indicate that the NCI scheme can serve as a highly predictive, independent prognostic factor in RMS and that the alveolar category should be expanded to include the solid round-cell RMS, even in the absence of a classic alveolar architecture.