We reviewed 173 cases of paratesticular rhabdomyosarcoma (RMS) of Intergroup Rhabdomyosarcoma Studies (IRS)-I, -II, and -III for evaluation of possible histological factors that might account for the good prognosis of these patients. Almost all cases (161 of 173 cases, 93.1%) occurring in this site were of embryonal histology. A spindle- cell subtype of embryonal RMS was identified that presented a storiform growth pattern with abundant collagen between the tumor cells in most cases. Other tumors of this subtype showed an arrangement of tumor cells in bundles with a low to moderate amount of collagen, resembling a leiomyosarcoma. The other embryonal RMS in this site had the classical embryonal cytology. The spindle-cell subtype was highly differentiated by immunohistochemistry and electron microscopy. Lymph node metastasis was found in seven of 43 patients (16.3%) with a RMS of spindle-cell subtype, compared with 40 of 112 patients (35.7%) with RMS of non-spindle-cell type. Clinical data from patients with spindle-cell subtypes of the paratesticular lesions revealed that they almost always had an association with clinical groups of limited disease (32 patients, 74.4%, with Group I; 10 patients, 23.3%, with Group II disease) and a significantly better prognosis (95.5% survival at 5 years) when compared with patients with the classic embryonal variant of RMS (80% survival at 5 years, p<0.035). The incidence and anatomic distribution of this spindle cell subtype of embryonal RMS was estimated on 800 randomly selected patients from IRS-II. It was found in the head and neck, extremities, orbit, and some other sites, but 30.6% were located in the paratesticular area. Patients with spindle cell RMS of nonparatesticular sites usually had more extensive disease compared with patients having paratesticular lesions; two thirds of the cases had gross residual tumor after surgery or metastatic tumor at diagnosis. We conclude that spindle-cell RMS is a subtype of embryonal RMS with a very favorable prognosis. The site factor of the paratesticular localization may allow earlier diagnosis of the spindle-cell lesions compared with other sites. Other unknown factors may also play a role
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