Abstract BACKGROUND The administration of radiotherapy (Rt) plus adjuvant temozolamide (TMZ) with IMR and VMAT techniques has become an increasingly valuable tool in the local control of glioblastoma multiforme with a survival current Rt techniques allow specific conformation with the use of better radiological images that allow us greater precision. For many years large Rt field sizes have been associated with significant adverse effects.Although we continue to give very wide margins that lead us to irradiate non-compromised organs at risk, which translates into a higher rate of radionecrosis and acute and chronic neurological toxicity.Therefore, we consider it valid to ask ourselves if we could reduce the standard margins applied in the planning of Rt. MATERIAL AND METHODS Analysis of 98 GBM patients treatedat the barros luco hospital between 2021 and 2023 with surgery followed by adjuvant therapy, radiotherapy plus concomitant chemotherapy at the Institute. Nacional de Cáncer de Santiago de Chile we reviewed the time of recurrence and the location in relation to the surgical margins vs. radiotherapy margins or other nearby areas. RESULTS After the analysis, we can see that 65% of the recurrences presented a dominant failure pattern, within the field and less than 1cm from the margin between the surgical bed and the radiotherapy margin, evaluated with perfusion brain resonance imaging and spectroscopy CONCLUSION n the local control of GBM, the application of adjuvant therapy is essential in a period of not less than 2 months after surgery, in this institutional analysis we can observe a considerable number of patients under 40 years of age with this pathology, for which reason to consider reducing the diameter of Rt margins can be considered an option, in order to limit deleterious effects in healthy organs in the times that there are advanced Rt techniques, the increasingly frequent application of brain radiosurgery (SRS) thatIt can be used as a rescue method in case of recurrences at the edge of the margin, using radiological images (MRI with perfusion and spectroscopy/C-methionine PET/CT. Figure we can see the CTV and PTV to be treated with modulated intensity technique, evidencing the large field size that receives radiation between 54 to 60 Gy.Radiation oncology must continue to develop and improve the delivery, targeting, and dose of radiation therapy in the GBM to improve its survival and treatment-associated toxicity.