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Ewing Sarcoma (ES) is defined by molecular markers, being t(11,22) the most frequent. Intracranial ES/pPNET usually represent metastases from extracranial sites. Primary Central Nervous System (CNS) lesions are extremely rare. MATERIAL AND METHODS Retrospective review of clinical records from patients with primary CNS ES/pPNET assessed at a neurological center in Argentina between 2007–2019. All confirmed with molecular marker. Clinical characteristics, imaging, histopathology, and treatment response were evaluated. Extensive workup included whole-body CT scan, skeletal-scintigraphy, and positron-emission tomography, excluding extracranial primary lesions in all cases. RESULTS Total 24p. Median age 22yo (2–65); M:F 2:1. Clinical presentation: intracranial hypertension, focal neurological deficit or seizures. In brain MRI 11 supratentorial lesions, 7 infratentorial and 6 diffuse leptomeningeal. Histopathology: diffuse pattern with small round blue cells 13/24p, other patterns were also described. CD99 marked positive in all cases and T(11.22) confirmed as well: 22 PCR and 2 FISH for EWSR1-rearrangement. Misdiagnosis lead to median delay for accuracy of 7,5mo (0–124); including other CNS primary tumors (n= 13), infectious diseases (n= 5) and other diagnoses (n= 4). Most patients (n= 10) were treated with Euro99-protocol (6 cycles plus local radiotherapy (RT)), 4 with Temozolomide, Irinotecan plus RT, 7 with other regimens and 2 received no treatment. Mean overall survival (OS): 58mo (0–132) and progression-free survival 22mo (0–85). Five year OS:41%. CONCLUSION Clinical behavior in primary CNS ES deferred from extraosseous ES, being necessary to apply a specific regimen for this pathology as there is no standard regime for intracranial ES/pPNET. Results showed two different groups of patients with long and very short survival. Presentation, histology, and outcomes are so uneven that we propose if a more specific molecular classification should be necessary. This is, to our knowledge, the largest case series reported describing a heterogeneous group despite the presence of accurate molecular diagnosis.

RARE-43. PRIMARY CENTRAL NERVOUS SYSTEM EWING SARCOMA: A SINGLE INSTITUTION EXPERIENCE