Rhabdomyosarcoma metastasizing as a malignant ectomesenchymoma.

Rhabdomyosarcoma is a common childhood malignancy that may occasionally occur as a component of a mixed mesenchymal tumor, e.g., a triton tumor or malignant ectomesenchymoma. A case is reported of a 13-year-old boy who had resection of a paratesticular embryonal rhabdomyosarcoma with subsequent radiation and chemotherapy. Two years later, a retroperitoneal metastasis was resected. Histology showed a mixture of rhabdomyoblasts, ganglion cells, and a third population of cells with combined features of these two distinct cell types. Electron microscopy confirmed the presence of rhabdomyoblasts with characteristic bundles of myofilaments and Z-band material, and ganglion cells with prominent nuclei and nucleoli, rough endoplasmic reticulum, dense core granules, filaments, and tubules. Notably, the third cell population showed features of both rhabdomyoblasts and ganglion cells. Immunohistochemistry confirmed the mixed population of rhabdomyoblasts (positive for vimentin, desmin, negative for S-100, NSE), ganglion cells (positive for S-100 and NSE, negative for vimentin and desmin), and the third population expressing all test antigens. The features of this metastatic lesion are those of a malignant ectomesenchymoma with combined rhabdomyosarcoma and ganglioneuroma components. It is postulated that this lesion results from biphenotypic expression of tumor cells that previously expressed only rhabdomyoblastic differentiation. The role of prior chemotherapy and radiotherapy in this particular case is unclear.