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PURPOSE Pilomyxoid astrocytomas are predominantly located in supra-chiasmatic region and are more clinically aggressive than pilocytic astrocytomas, although recent WHO 2016 classification placed them into the grade I/II category. In our study, we describe imaging correlation of PMA to their genetic signature. MATERIALS AND METHODS We identified 12 pediatric patients with pathologically proven PMA, PA, and PA with myxoid features in an IRB approved study. Three of the tumors had whole exome somatic and germline sequencing. Qualitative MRI characteristics of location, size, enhancement, edema, T2 and T1 intensity, and multifocality were assessed. RESULTS Among the PMA, 3 cases were found to have KIAA1549-BRAF fusion, 1 case BRAF V600E mutation, and 2 cases had wildtype BRAF. The BRAF wildtype tumors had atypical imaging features with intraventricular extension of tumor, involvement of frontal lobe parenchyma and one tumor demonstrating increase in size and development of enhancement at 5 years. Whole exome sequencing of BRAF wildtype tumors identified somatic truncation mutations in NF1 R1534X and R1513X with wildtype germline NF1 and missense mutations in KMT2C and GLTSCR1. Among PAM, one was BRAF wildtype with mutations i PTCH1 M956V and PTPN1 (A72V) and demonstrated atypical features of intratumoral hemorrhage on presentation. Among PA, one was positive for KIAA1549-BRAF, one was BRAF wildtype. CONCLUSIONS BRAF wildtype PMA and PA demonstrate atypical tumor localization and are associated with atypical genetic mutations on whole exome sequencing. On the contrary, presence of KIAA1549-BRAF fusion or BRAF V600E mutation within PMA and PA correlates with classic qualitative imaging characteristics.

neuro-oncology

LGG-09. CORRELATING GENETIC SIGNATURE OF PILOMYXOID ASTROCYTOMAS AND PILOCYTIC ASTROCYTOMAS WITH QUALITATIVE AND QUANTITATIVE MR IMAGING CHARACTERISTICS