This clinical trial studies whether continuous glucose monitoring (CGM) can be used to help patients with glioblastoma manage their blood sugar (glucose) levels and improve survival. Glioblastoma is the most common malignant primary brain tumor in adults, with an average survival time of approximately 15-18 months despite therapy. Studies have shown that having a higher-than-normal amount of glucose in the blood (hyperglycemia) during radiation therapy is associated with poorer survival outcomes in glioblastoma patients. Hyperglycemia in glioblastoma patients is often driven by steroids that are commonly used during treatment. CGM uses a device that places a sensor under the skin that monitors glucose levels at regular intervals, providing real-time, or near real-time, glucose information. This can help to identify when a patient has changes in their glucose levels so they may receive necessary interventions or medications sooner. CGM may be an effective way for glioblastoma patients to manage their glucose levels, which may improve survival.

Inclusion Criteria:

• Presumed newly diagnosed GBM based on imaging findings consistent with GBM on brain MRI (e.g., heterogeneously enhancing mass with central necrosis and surrounding edema), as determined by the treating neuro-oncology team
• Age ≥ 18 years at the time of consent
• Karnofsky performance status (KPS) ≥ 70 at baseline
• Absolute neutrophil count (ANC) ≥ 1.5 × 10^9/L
• Platelet count ≥ 100 × 10^9/L
• Hemoglobin ≥ 9 g/dL
• Serum creatinine ≤ 1.5 × upper limit of normal (ULN) or creatinine clearance ≥ 60 mL/min
• Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5 × ULN
• Total bilirubin ≤ 1.5 × ULN
• Willingness and ability to comply with CGM device use and attend dietary counseling sessions as part of the study protocol

Exclusion Criteria:

• Recurrent glioblastoma or prior therapy for glioblastoma beyond surgical resection or biopsy
• History of eating disorders (e.g., anorexia nervosa, bulimia) or substance use disorder within the past 12 months
• Any other uncontrolled or inadequately managed medical illness (e.g., unstable cardiovascular, hepatic, renal, or psychiatric condition) that, in the opinion of the investigator, would interfere with study participation or interpretation of results
• Concurrent diagnosis of another active malignancy requiring treatment
• Pregnancy or breastfeeding at the time of enrollment
• Documented history of type 1 diabetes mellitus

To reduce bias, all imaging and quality of life assessments will be interpreted by evaluators blinded to group assignment.