myosarcoma myosarcoma myosarcoma is a malignant muscle tumor. people with myosarcoma often wake up with the feeling as if they had a cramp during their sleep. myosarcomaspecialtyoncology leiomyosarcoma is sarcoma of smooth muscle, and rhabdomyosarcoma is sarcoma of striated muscle. however, the term myosarcoma itself still appears in the literature.