Rhabdomyosarcoma metastasizing as a malignant ectomesenchymoma

Rhabdomyosarcoma is a common childhood malignancy that may occasionally occ ur as a component of a mixed mesenchymal tumor, e.g., a triton tumor or mal ignant ectomesenchymoma. A case is reported of a 13-year-old boy who had re section of a paratesticular embryonal rhabdomyosarcoma with subsequent radi ation and chemotherapy. Two years later, a retroperitoneal metastasis was r esected. Histology showed a mixture of rhabdomyoblasts, ganglion cells, and a third population of cells with combined features of these two distinct c ell types. Electron microscopy confirmed the presence of rhabdomyoblasts wi th characteristic bundles of myofilaments and Z-band material, and ganglion cells with prominent nuclei and nucleoli, rough endoplasmic reticulum, den se core granules, filaments, and tubules. Notably, the third cell populatio n showed features of both rhabdomyoblasts and ganglion cells. Immunohistoch emistry confirmed the mixed population of rhabdomyoblasts (positive for vim entin, desmin, negative for S-100, NSE), ganglion cells (positive for S-100 and NSE, negative for vimentin and desmin), and the third population expre ssing all test antigens. The features of this metastatic lesion are those o f a malignant ectomesenchymoma with combined rhabdomyosarcoma and ganglione uroma components. It is postulated that this lesion results from biphenotyp ic expression of tumor cells that previously expressed only rhabdomyoblasti c differentiation. The role of prior chemotherapy and radiotherapy in this particular case is unclear.