Etoposide Active Not Recruiting Phase 2 Trials for Desmoplastic Small Round Cell Tumor (DSRCT) / Localized Ewing Sarcoma/Peripheral Primitive Neuroectodermal Tumor / Ewing Sarcoma of Bone or Soft Tissue / Metastatic Ewing Sarcoma/Peripheral Primitive Neuroectodermal Tumor Treatment