Spindle Cell/Sclerosing Rhabdomyosarcoma
Spindle Cell/Sclerosing Rhabdomyosarcoma (RMS): Microscopically, more than 80% of the tumor is composed of fusiform, elongated spindle cells that show high degree of skeletal muscle differentiation. They have eosinophilic fibrillar cytoplasm, cigar-shaped nuclei and prominent nucleoli. Cross-striations are commonly found. Scattered round or oval rhabdomyoblasts may also be present. Some spindle cell RMS are collagen-rich and show abundant collagen fibers separating spindle cells (as in this case). The collagen-poor forms are more cellular and resemble fibrosarcomas. Immunohistochemically, spindle cell RMS are positive for myogenin, muscle specific actin, and desmin. Some cases also express CD99 and WT1.