A form of RHABDOMYOSARCOMA occurring mainly in adolescents and young adults, affecting muscles of the extremities, trunk, orbital region, etc. It is extremely malignant, metastasizing widely at an early stage. Few cures have been achieved and the prognosis is poor. 'Alveolar' refers to its microscopic appearance simulating the cells of the respiratory alveolus. (Holland et al., Cancer Medicine, 3d ed, p2188)