Abstract INTRODUCTION Pilocytic astrocytoma (PA) is a circumscribed low-grade astrocytic glioma, generally considered to be associated with a good prognosis. However, some PA patients experience unfavorable outcomes; advanced age was known as an independent poor prognostic factor. This study retrospectively reviewed PA patients and conducted molecular analyses for elderly patients. METHODS We analyzed data from 29 histologically confirmed PA patients treated at a single center from 2002 to 2021 and conducted integrated molecular analyses including DNA methylation profiling for elderly PA patients. RESULTS The median age at diagnosis was 14 years (range 3-82 years), with 4 patients (14%) being elderly (patients≥60years old). Elderly patients had significantly worse progression-free survival and overall survival. DNA methylation analysis was performed on 2 of the 4 elderly patients. Despite histological diagnoses of PA, methylation profiling classified one case as high-grade astrocytoma with piloid features (methylation class scores below 0.3 in both v11b4 and v12.5) and the other as glioblastoma, IDH-wildtype (score over 0.5 in both v11b4 and v12.5), using classifiers from the German Cancer Research Center and t-SNE analysis. CONCLUSION Elderly patients with PA morphology showed unfavorable outcomes in this cohort. In those patients, DNA methylation profiling revealed the possibility of high-grade astrocytic tumors, including newly defined entities.

PATH-14. UTILITY OF DNA METHYLATION ANALYSIS IN HISTOLOGICALLY-CONFIRMED PA PATIENTS WITH POOR PROGNOSIS