Abstract BACKGROUND Despite treatment intensification, survival for high-risk subgroups of medulloblastoma (MB) such as MYC-amplified Group 3 (Gr3-II) and p53-mutant SHH (SHH-3p53mut) have remained dismal with overall survival around 40% and 20% respectively. Our previous studies have shown that the DNA-damage response inhibitors (DDRis) prexasertib and ceralasertib enhance the effects of CSI using medulloblastoma cell line-derived orthotopic mouse models. However, patient-derived orthotopic xenografts (PDOXs) are considered superior models for preclinical testing compared to immortalised cell lines. Here, we developed a CSI protocol for multiple PDOX models of high-risk medulloblastoma and tested the optimal protocol with concurrent DDRis. METHOD For our CSI optimisation studies, MED211FH (Gr3-II) or MED813FH (SHH-3p53mut) tumour-bearing mice were monitored for clinical symptoms of tumour before commencing treatment. Multiple, fractionated CSI dosing schedules using a 5-days-on, 2-days-off treatment cycle were trialled based on our lab’s previously established CSI protocols before a schedule of 10 x 0.5 Gy fractions (5.0 Gy total) proved optimal. This schedule was then used in both PDOX models plus an additional SHH-3p53mut model (MED314mCLFH), either alone or in combination with concurrent prexasertib or ceralasertib. RESULTS MED211FH and MED813FH mice treated with 5.0 Gy CSI alone resulted in significantly improved median survival compared to control mice (p<0.0001 and p<0.005 respectively) While 5.0 Gy CSI plus concurrent prexasertib or ceralasertib showed significantly improved survival when compared to CSI alone for each model used. MED211FH: p<0.01 and p<0.001 respectively, MED813FH: p<0.05 and p<0.01 respectively and MED314mCLFH: p<0.05 for both DDRi combination groups. CONCLUSION Remarkably, concurrent use of prexasertib or ceralasertib with CSI significantly improved survival in fatal Gr3-II and SHH-3p53mut models. With these results, we provide robust preclinical evidence for radiosensitising use of DDRi in paediatric high-risk MB and recommend swift clinical translation.