PATH-19. MOLECULAR, HISTOLOGIC AND CLINICAL CHARACTERISTICS OF OLIGODENDROGLIOMAS: A MULTI-INSTITUTIONAL RETROSPECTIVE STUDY

Abstract In the 2016 WHO classification of CNS tumors, oligodendrogliomas are molecularly defined by IDH1 or IDH2 mutations and 1p/19q co-deletion. Some reports suggest that PI3K pathway alterations may confer increased risk of progression and poor prognosis in oligodendroglioma. However, factors that influence prognosis in molecularly defined oligodendroglioma (mOGD) have not been thoroughly studied. Also, the benefits of adjuvant radiation and temozolomide in mOGDs remain to be determined. 107 mOGDs diagnosed between 2008-2018 at the University of Texas Health Science Center at Houston (n= 39) and MD Anderson Cancer Center (n= 68) were included. A retrospective review of the demographic, clinical, histologic, molecular, and outcomes were performed. Median age at diagnosis was 37 years and 61 (57%) patients were male. There were 64 (60%) WHO Grade 2 and 43 (40%) WHO Grade 3 tumors. Ninety-five (88.8%) tumors were IDH1-mutant and 12 (11.2%) were IDH2-mutant. Eighty-two (77%) patients were stratified as high-risk: older than 40-years and/or subtotal resection (RTOG 9802). Gross-total resection was achieved in 47 (45%) patients. Treatment strategies included observation (n= 15), temozolomide (n= 11), radiation (n= 13), radiation with temozolomide (n= 62) and other (n= 6). Our results show a benefit of temozolomide vs. observation in progression-free survival (PFS). However, no benefit in PFS or overall survival (OS) was observed when comparing radiation vs. radiation with temozolomide. PIK3CA mutations were detected in 15 (14%) cases, and patients with PIK3CA-mutant mOGDs showed worse OS (10.7-years vs 15.1-years, p= 0.009). Patients with WHO Grade 3 tumors had shorter PFS but no significant difference in OS was observed compared to grade 2. Our findings suggest that mOGDs harboring PIK3CA mutations have worse OS. Except for an advantage in PFS in temozolomide treated patients, adjuvant treatment with radiation or the combination of both, showed no significant advantage in terms of OS.