Background: Grade II and III Oligodendroglioma associate mutations of isocitrate deshydrogenase 1 or 2 genes and the whole-arm chromosomal loss of 1p and 19q and have a better prognosis than other gliomas. However, even if the preferred treatment consists of a combination of radiotherapy (RT) and chemotherapy, some patients will less respond to this treatment and will relapse faster , in part because of an heterogeneity in the response to RT. In the aim to identify factors of response to RT, we analyzed clinical and molecular data of patients with grade II-III oligodendroglioma exclusively treated with RT in the POLA cohort. Methods: Gene expression profiles on Affymetrix expression arrays of patients from the POLA cohort with co-deleted 1p/19q grade II/II gliomas treated by exclusive RT were used to identify a gene expression set predictive of radiation sensitivity. The primary endpoint was the progression free survival (PFS), defined as the time from treatment start until progression or death. A supervised approach with penalized regression was applied to select most informative predictors, and then a risk score was created based on the linear predictor given by the multivariable model. Results: Forty-five patients corresponded to the study criteria, with a median age at diagnosis of 45 (range 23- 64). The supervised approach allowed identifying a three-gene prognostic set including Semaphorin -3C (SEMA3C), Neuronal Pentraxin 2 (NPTX2 ) and the Metabotropic Glutamate Receptor 5 (GRM5), involved in proliferation, migration and inflammation. The risk score associated to these three genes was statistically associated to PFS (HR = 2.72, p = 0.00005) and remains significant when adjusted on clinical covariates age at diagnosis, necrosis, endothelial proliferation and type of surgery (complete, partial or subtotal surgery) (HRadj = 2.36, p = 0.001). Conclusions: We report an independent three genes SEMA3C-NPTX2-GRM5 risk score signature of response to radiotherapy in patients with oligodendroglioma, which highlights the heterogeneous response in this reputed good prognosis population. This signature could help in determining the adapted treatment as well as potential new targets to address.