SPINDLE-CELL RHABDOMYOSARCOMA (SO-CALLED) IN ADULTS - REPORT OF 2 CASES WITH EMPHASIS ON DIFFERENTIAL-DIAGNOSIS

Spindle cell rhabdomyosarcoma (RMS) is a recently described variant of embryonal RMS that carries a relatively favorable prognosis when comp ared with other forms of RMS. To date, spindle cell RMS has been descr ibed only in children. The authors have identified two unusual cases o ccurring in adults using the following criteria: tumors composed mainl y of fascicular, relatively monomorphic spindle-shaped cells that show unequivocal immunohistochemical and ultrastructural evidence of myoge nic differentiation. The tumors were identified in a 38-year-old woman and a 56-year-old man, arising in the cheek and left hemidiaphragm, r espectively. Both were treated with surgical resection and chemotherap y. The first patient died of uncontrolled local recurrence of her tumo r at 27 months after diagnosis, and the second died of metastatic dise ase at 13 months follow-up. The tumors were composed mainly of fascicl es of spindle cells with palely eosinophilic cytoplasm admixed diffuse ly with sparse polygonal, rounded, or strap-shaped rhabdomyoblasts wit h brightly eosinophilic cytoplasm and with cross-striations in the fir st case only. Immunostaining for muscle-related antigens showed staini ng for smooth-muscle actin (focal), pan-actin HHF-35, desmin, fast myo sin, myoglobin, and MyoD1. Both cases were negative for S-100 protein. On electron microscopy, both cases showed neoplastic rhabdomyoblasts with clear-cut sarcomeric differentiation in many of the tumor cells. Spindle cell RMS poses special problems in differential diagnosis when arising in adults and should be distinguished from leiomyosarcoma, ma lignant peripheral nerve sheath tumor with heterologous rhabdomyoblast ic differentiation (malignant Triton tumor), and fibrosarcoma. In view of the good prognosis afforded children with spindle cell RMS and in light of the chemoresponsive behavior of RMS in general, we feel that it is important to identify tumors that meet the criteria for spindle cell RMS occurring in the adult population. However, based on these tw o cases, it is possible that spindle cell RMS occurring in adults may not be associated with such a favorable outcome.