High grade gliomas, particularly glioblastoma, are among the most aggressive brain tumors and are associated with poor outcomes despite standard treatment. Many patients, especially older adults or those with poor general health, are not suitable for surgery and have a life expectancy of less than 12 months. Current standard includes a shortened course of radiotherapy (over 3 weeks) combined with chemotherapy using temozolomide (TMZ), which offers limited survival benefits. This study aims to explore whether delivering radiotherapy in a shorter duration (1 or 2 weeks) at a higher dose, guided by advanced imaging with a PET scan, can improve survival in this group of patients. PET scans help identify the most active parts of the tumor, which aids in targeting of these areas more precisely, potentially improving outcomes while reducing harm to healthy brain tissue. This study will randomly assign 116 eligible patients into two groups: * One group will receive the current standard of care (3-week radiotherapy + TMZ). * The other group will receive PET-guided radiotherapy over a shorter duration (either 5 or 10 sessions) at a higher dose, alongside TMZ. The primary goal is to compare overall survival at one year between the two groups. The study will also assess how the disease progresses, side effects of treatment, and the impact on patients' quality of life. The study will be conducted over a total period of 6 years, including 4 years for patient enrolment and 2 years of follow-up. Participation in the study is entirely voluntary, and all patients will undergo an informed consent process. The study has been designed to follow all applicable ethical and regulatory guidelines. The results may help establish a more effective and convenient treatment option for patients with aggressive brain tumors and poor prognosis.