THE DIAGNOSIS AND CLASSIFICATION OF CHILDHOOD RHABDOMYOSARCOMA

Although the typical subtypes of embryonal, botryoid, alveolar, and pl eomorphic rhabdomyosarcoma are easily recognized by simple light micro scopic examination, poorly differentiated forms are difficult to diagn ose and classify. The availability of markers connoting simple myogeno us or skeletal muscle differentiation has contributed tremendously to the diagnosis of even poorly differentiated rhabdomyosarcoma. Definiti on of typical and minimal ultrastructural criteria has also been instr umental. Proper classification of rhabdomyosarcoma into subtypes with prognostic significance has been a difficult task. Although most patho logists have accepted the original classification scheme of embryonal, botryoid, alveolar, and pleomorphic rhabdomyosarcoma, they have used variable histologic criteria to define the various subtypes. Most of t he major discrepancies occurred in the definition of embryonal versus alveolar rhabdomyosarcoma, especially in those cases that required est ablishment of minimal criteria. For example, poorly differentiated rou nd-cell rhabdomyosarcoma has frequently been erroneously classified as embryonal, because of the absence of an alveolar pattern. This variet y has been recognized as a form of alveolar (solid alveolar) rhabdomyo sarcoma by the National Cancer Institute (NCI) scheme. Comparative ana lysis of several proposed classification schemes has led to the establ ishment of well-defined criteria for the major subtypes of rhabdomyosa rcoma and to the recognition of subtypes with prognostic significance. This is a US government work. There are no restrictions on its use.