PATH-26. A RARE CASE OF EXOPHYTIC PILOCYTIC ASTROCYTOMA DIAGNOSED VIA MOLECULAR TESTING

Abstract

Abstract INTRODUCTION Pilocytic astrocytoma (PA) is a WHO grade 1 neoplasm that can share histological and radiological features with more malignant entities, including high grade astrocytoma with piloid features. In cases where the differential remains broad after histopathological examination, DNA methylation analysis and next generation sequencing can contribute significantly to making an accurate diagnosis. Machine learning has also played a role in developing multiple classifying algorithms which group tumors by their methylation status. CASE We report a case of an exophytic left frontal mass with adjacent bone erosion that preoperatively was favored to be a meningioma. Intraoperatively, the mass was found to be cortically based. With histopathologic review at two centers, the diagnosis remained IDH wildtype low-grade glioma not otherwise specified. The clinical treatment team interpreted the post-operative increase in T2 signal as radiographic progression consistent with high grade glioma and planned to proceed with chemoradiation. Patient elected for further evaluation; methylation array profiling and next generation sequencing identified a KIAA1549::BRAF fusion, consistent with a final diagnosis of pilocytic astrocytoma, a WHO Grade 1 Glioma. Given the favorable prognosis following gross total resection, the patient was recommended active surveillance and radiation and chemotherapy treatment planning was stopped. DISCUSSION This is a rare case of an exophytic pilocytic astrocytoma in which the tumor’s atypical histologic and radiographic appearance increased the likelihood of diagnostic errors. This case illustrates the clinical importance of utilizing methylation array profiling and next-generation sequencing to confirm a diagnosis.