Path-07. Spinal Occurrence of Intracranial Mesenchymal Tumor With Fus::crem Fusion

BACKGROUND Intracranial Mesenchymal Tumor (IMT), with FET::CREB fusion is a rare and recently characterized tumor. Previously, they were classified as intracranial myxoid mesenchymal tumor and angiomatoid fibrous histiocytoma-like neoplasms. Presence of FET::CREB gene fusion defines the entity, however the incidence and prognosis are not well understood. CASE PRESENTATION Our 6-year-old female patient presented with 6 weeks of neck pain. MRI revealed a posterior intradural extramedullary cervical spine mass (2.4 x 1.4 x 1 cm). She subsequently underwent complete tumor resection. H&E showed a round cell neoplasm, in a lobulated architecture with thick interstitial collagen. Mitotic activity was rare. Immunohistochemistry for CD99 and desmin were positive, with focal synaptophysin expression. The tumor lacked specific immunohistochemical features to enable further classification. OncoKids next generation sequencing revealed an in-frame FUS::CREM fusion. DNA methylation profiling matched to an IMT with FET::CREB fusion. Post-operatively, she had pain resolution and is without recurrence 3 months later. DISCUSSION IMTs have gene fusions of the FET RNA-binding protein family (EWSR1 or FUS) with a member of the CREB family of transcription factors (CREB1, ATF1, or CREM). This fusion creates a chimeric protein with altered function. EWSR1 is the most common fusion partner. IMTs have a broad morphological spectrum that overlaps with other entities making molecular studies essential for the diagnosis. While most IMT are supratentorial, few spinal cord examples have been reported. Optimal treatment strategy remains undefined, but adjuvant radiation therapy may be considered, especially for incompletely resected tumors. CONCLUSION FET::CREB fusion defines IMT, a tumor type that is typically intracranial. Spinal cord occurrence is possible, although less well-recognized. They have a broad morphological spectrum, immunohistochemical overlap with other entities, and molecular studies are required to make the diagnosis. Further studies to characterize their clinical behavior to guide future treatment are needed.