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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.

Image 92 of 95