Ependymosarcomas are rare, biphasic tumors composed of ependymal and sarcomatous components. Due to their rarity, their biologic basis is not well understood. We report the case of a 38-year-old male with anaplastic ependymoma (WHO grade III) that progressed to ependymosarcoma in less than 2 years after multiple resections, chemoradiotherapy, and anti-PD1 immunotherapy. Next-generation sequencing was performed on both high-grade anaplastic ependymoma and ependymosarcoma samples to detect small base changes, insertions, and deletions in exons and splice junctions from a panel of over 400 genes. We identify genetic variants in the tumor suppressors RB1, TP53, and TSC2 in these samples and discuss the potential significance of an additional TSC2 genetic variant in the progression to ependymosarcoma.