Primary cardiac tumors are rare. Of all primary heart neoplasms, sarcomas account for around 10%, and of these, rhabdomyosarcomas (RMS) constitute a minority. A case of primary left atrial RMS, reminiscent of leiomyosarcoma, is reported herein. Histologically, the tumor was composed of tightly-packed spindle-shaped cells arranged in long fascicles. At immunohistochemistry, the tumor cells stained strongly and diffusely with vimentin, muscle-specific actin, desmin and myogenin, and focally with fast-myosin and sarcomeric actin. WT-1 showed diffuse and intense cytoplasmic staining. Staining for calponin was weak. Staining for alpha-SMA, H-caldesmon, CD34, epithelial membrane antigen (EMA), keratin wide-spectrum (CK w.s.), and S100 protein was negative. Electron microscopy revealed poorly differentiated spindle cells, containing contractile filaments with a "Z-band-like" appearance. The final diagnosis was embryonal RMS, spindle cell variant. Conclusively, spindle cell RMS is a well-recognized variant of embryonal RMS, typically occurring in soft tissue, with only rare cases described in visceral organs. This is the first case of primary cardiac spindle cell RMS ever described.
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