Context: The World Health Organization Classification Since 1995, the International Classification of Rhabdomyosarcoma has provided prognostically relevant classification for rhabdomyosarcoma (RMS) and allowed risk stratification for children with RMS. The International Classification of Rhabdomyosarcoma includes botryoid and spindle cell RMS as superior-risk groups, embryonal RMS as an intermediate-risk group, and alveolar RMS as an unfavorable-risk group. The 2013 World Health Organization (WHO) classification of skeletal muscle tumors modified the histologic classification of RMS to include sclerosing RMS as a type of spindle cell RMS separate from embryonal RMS. The current WHO classification includes embryonal, alveolar, spindle cell/sclerosing, and pleomorphic subtypes of RMS and does not separate the botryoid subtype.

Objective: To determine if the WHO classification applies to pediatric RMS.

Design: To accomplish this goal, we reviewed 9 consecutive Children's Oncology Group clinical trials to compare the WHO and International Classification of Rhabdomyosarcoma classifications with outcome and site of disease.

Results: Except for a subset of low-risk RMS, the outcome for botryoid was not significantly different from typical embryonal RMS when analyzed by primary site. Similarly, pediatric spindle cell and sclerosing patterns of RMS did not appear significantly different from typical embryonal RMS, with one exception: spindle cell RMS in the parameningeal region had an inferior outcome with 28% event-free survival.

Conclusion: Our data support use of the WHO RMS classification in the pediatric population, with the caveat that histologic diagnosis does not necessarily confer the same prognostic information in children as in adults.