Diagnosing the Newly Defined CNS Myxoid Glioneuronal Tumor: A Pediatric Case Report (1424)

Objective: To describe a pediatric patient with a newly defined rare CNS tumor. Background: Myxoid glioneuronal tumors are a newly defined CNS tumor that has stereotypic location in the septal region, periventricular white matter, or corpus collosum. It is characterized by a dinucleotide mutation of the PDGFRA oncogene replacing lysine with leucine or isoleucine. Histological features are reminiscent of dysembryoplastic glioneuronal tumors (DNT) or rosette-forming glioneuronal tumors (RGNT) and are characterized by oligodendrocyte-like tumor cells embedded in a myxoid/mucin-rich stroma. There have been only 4 cases of myxoid glioneuronal tumors described in pediatric patients to date. Design/Methods: We report a case of a 7-year-old female who presented with two unprovoked generalized tonic-clonic seizures. MRI brain revealed an 11mm subependymal lesion along the posterior horn of the right lateral ventricle suggestive of low grade glioma. Multiple EEGs showed that seizure activity was spatially unrelated to the tumor. Her tumor remained stable for about 3 years until growth was noted on surveillance imaging. Stereotactic biopsy was performed. Results: Gross tumor analysis showed a glial neoplasm composed of oligodendrocyte-like cells embedded in a fibrillary background. Immunohistochemistry was diffusely positive for GFAP and synaptophysin but negative for BRAFV600E and IDH-1. Somatic next generation sequencing (NGS) on tumor specimen showed a pathogenic dinucleotide mutation in codon 385 of the PDGFRA gene replacing lysine with leucine. Germline testing showed a de novo 343kb deletion in 3q29 of unknown significance. Conclusions: This tumor was found incidentally on MRI as was the case with 50% of pediatric myxoid glioneuronal tumors previously described. A diagnosis of pilocytic astrocytoma was deemed most likely based on the histology of the limited biopsy material although typical findings such as Rosenthal Fibers were absent. This case demonstrates the importance of molecular analysis in accurately differentiating and diagnosing rare CNS tumors in pediatric patients, particularly in small tumor specimens. Disclosure: Mr. Silverman has nothing to disclose. Dr. Kaur has nothing to disclose. JOSEPH PIATT has nothing to disclose.