PATH-40. <i>YAP-1</i> fusion-positive spinal ependymoma in an adult patient: A case report

Abstract BACKGROUND Ependymomas (EPNs) are a heterogenous group of glial neoplasms arising from ependymal cells lining the ventricular system of the central nervous system (CNS).1 Tumors can develop throughout the neuroaxis, with varying behavior and prognosis.2 Historically, classification relied on histopathologic characteristics; however, this approach demonstrated limited prognostic utility.3 Advances in molecular analysis led to ependymoma sub-classification based on anatomical location and molecular characteristics, which confer clinical and prognostic implications.3,4 The YAP-1 fusion-positive ependymoma is a rare molecular subtype typically of the supratentorial (ST) compartment in pediatric patients associated with a favorable prognosis.5 We report the case of a YAP-1 fusion-positive spinal ependymoma in an adult patient. CASE PRESENTATION A 38-year-old female patient presented with 6 months of progressive bilateral upper extremity weakness. Imaging showed a lobulated contrast-enhancing intramedullary mass at the C1-C2 spinal level with associated peritumoral edema. The patient underwent a subtotal resection with pathology revealing mildly cellular, ovoid to spindle shaped cells with a coarsely fibrillary background without necrosis or microvascular hyperplasia. Molecular sequencing revealed a fusion containing exons 1-7 of YAP-1 on chromosome 11q.22.1- 11q21.2 and the entire coding region of FAM118B containing exons 2-9. The patient has done well clinically over 2 years since resection with regular imaging surveillance and without further intervention such as radiation therapy or systemic chemotherapy. DISCUSSION This case highlights an atypical presentation of a YAP-1 fusion-positive ependymoma not only based on non-ST location but occurrence outside of the traditional age group; moreover, this is only the second reported YAP-1 fusion-positive spinal ependymoma in an adult since the WHO reclassification of CNS tumors in 2021.6 This case challenges current assumptions about the anatomical restriction of this molecular subgroup and calls to question if this subtype differs from traditional YAP1-fused ST-EPNs. This case further underscores the evolving importance of comprehensive molecular profiling in CNS tumor diagnostics.