2041 Background: IDH-mutant astrocytomas represent the most prevalent primary tumors in younger adults (<45yo). A substantial minority of these tumors exhibit the deletion of CDKN2A (Cyclin-Dependent Kinase Inhibitor Gene 2A). Homozygous deletion of this gene– which encodes the tumor suppressor protein p16 – is associated with a malignant phenotype and poorer prognosis in IDH-mutant astrocytoma. CDKN2A deletions are primarily observed in tumors that have received radiation therapy (RT), suggesting a potential mechanistic relationship between RT and deletion. This observation gives rise to two alternative hypotheses: either RT is causing DNA damage in irradiated cells, leading to the induction of subclonal CDKN2A deletion, or a subset of cells already possesses the CDKN2A deletion and is resistant to RT, resulting in the treatment selecting for the subsequent emergence of these cells. Here, we sought to ascertain the influence of CDKN2A deletion on IDH-mutant astrocytoma cellular response to RT. Methods: The antitumor effect of RT was evaluated in vitro using patient-derived IDH-mutant astrocytoma cells, MGG152. A homozygous CDKN2A deletion was engineered in this parental line, creating a paired line to permit an isogenic comparison between CDKN2A intact versus CDKN2A deleted tumor cells. A photon irradiator was used to deliver a range of clinically-relevant RT doses spanning from 0 to 20 gray (Gy). Differences in cell viability after a predetermined incubation period were examined and relevant statistical comparisons performed. Results: Dose-response curves in response to radiation therapy, comparing CDKN2A intact and deleted cell lines, were generated (Table). Both CDKN2A intact and deleted cell lines exhibited significant reductions in viability following radiation exposure at and above 2Gy (p < 0.001). CDKN2A-deleted cells displayed a small but significantly greater sensitivity than CDKN2A-intact at 1, 2, and 5Gy (Table). Conclusions: Our findings offer evidence indicating that p16 loss does not bestow radioresistance in IDH-mutant astrocytomas. These findings align with a hypothesis that radiotherapy (RT) might promote DNA damage, and consequently, subclonal CDKN2A deletion, in irradiated IDH-mutant tumors. [Table: see text]