THERAPY ASSOCIATED DIFFERENTIATION IN RHABDOMYOSARCOMAS

The morphologic and immunophenotypical features of three embryonal and one spindle cell rhabdomyosarcomas that underwent marked cytologic di fferentiation after polychemotherapy are described. In all the cases t he morphologic changes were associated to disappearance of vimentin im munoreactivity and intense positivity for desmin, muscle specific acti ns, and myoglobin; in addition the differentiated tumor cells acquired a strong and continuous pericellular staining for laminin. The mitoti c activity was significantly decreased. Drug associated differentiatio n seems to recapitulate the normal myogenic development to the point t hat the more mature neoplastic cells may represent terminally differen tiated elements withdrawn from cell cycle. However, due to the persist ence of a small number of poorly differentiated rhabdomyoblasts, the c linical consequences of this phenomenon are unclear.