Non-myogenic tumors involving skeletal muscle. A survey with special reference to alveolar soft part sarcoma.

Skeletal muscle is remarkably free of neoplasms despite its great body mass. Metastatic tumors to skeletal muscle are distinctly uncommon. Primary myogenous tumors (defined as those whose cytoplasm contains myofibrils) are infrequent, compared to primary lesions of supporting stroma and vascular origin. Almost all non-myogenic tumors of striated muscle occur also in other areas of the body and many represent extension from contiguous structures. The distinctive non-myogenic tumor which occurs exclusively in skeletal muscle, alveolar soft part sarcoma, has ultrastrucutral features of a secretory lesion with distinctive membrane bound crystalloids which may be lipid in nature. The ultrastructural features of alveolar soft part sarcoma do not support a myogenous derivation.