Alveolar soft part sarcoma: immunological evidence of rhabdomyoblastic differentiation M.P.FOSCHINI*, C.CECCARELLIt, V.EUSEBI*, 0.SKALLQ & G.GABBIANI3 * Istituto di Anatomia e Istologia Patologica, Universita di Bologna, Italy, Kentro Interdipartimentale per la Ricerca sul Cancro and f Departement de Pathologie, Centre Medical Universitaire, Geneve, Switzerland Accepted for publication 17 November 1987 FOSCHINI M.P., CECCARELLI C., EUSEBI V., SKALLI 0. & GABBIANI G. (1988) Histopthology 13, 101-108 Alveolar soft part sarcoma: immunological evidence of rhabdomyoblastic differentiation Two cases of alveolar soft part sarcoma have been studied immunocytochemically using antisera against epithelial membrane antigen, lysozyme, keratins, S-100 protein, desmin, vimentin, fetal myosin, slow myosin, alpha-skeletal muscle actin, alpha-smooth muscle actin and myoglobin. The neoplastic cells were negative with all antisera employed with the exception of the alpha-skeletal muscle actin antiserum which stained the cytoplasm of numerous neoplastic elements, incluaing the crystalloid rods, typical cytoplasmic inclusions of these tumours. It is suggested:that the presence of this protein indicates rhabdomyoblastic differentiation of these tumours. Keywords: alveolar soft part sarcoma, actin, rhabdomyosarcoma Introduction Alveolar soft part sarconias are tumours clearly delineated as an entity, but of uncertain origin (Christopherson, Foote & Stew:!rt 1952, Shipkey et al. 1964). Many theories concerning their histogenesis have been presented (Fisher & Reidbord 1971 Welsh et al. 1972, DeSchryver-Kecskemeti et al. 1982), but none of these was con- sidered conclusive by Enzinger & Weiss (1983) who included alveolar soft part sarcomas in their chapter on ‘Malignant tumours of uncertain histogenesis’. Recently Mukai et al. (1984), using a digital image processing technique, elaborated a three- dimensional configuration of the crystalloids, typical cytoplasmic inclusions of these tumours (Masson 1956) and concluded that they had a filamentous structure similar to actin. Subsequently Mukai et al. (1986) localized actin immunocytochemically as S. Orsola, Via Massarenti 9, 40138 Bologna, Italy. Address for correspondence: Prof. V. Eusebi, Istituto di Anatomia ed Istologia Patologica, Policlinico 101