Inclusion Criteria:  Willingness and ability to provide informed consent consistent with Good Clinical Practice (i.e., legally authorized representative will not be used / allowed for this study). Stated willingness to comply with all study procedures for the duration of the study Aged 18 years or older. Newly diagnosed (i.e., within 6 weeks), histologically or molecularly confirmed glioblastoma or diffuse midline glioma. Therapy to begin within 6 weeks of last surgery Able to take oral medication ECOG performance status of 0, 1, or 2 (KPS of >50) Recommended to receive temozolomide and radiation therapy Medically fit, as determined by the prescribing oncologists, to undergo temozolomide and radiation therapy. Agree to use of highly effective contraception from screening until at least 90 days after the last study treatment (study participant should not discontinue contraception until discussing with their treating oncologist(s)). Not have significant co-morbid central nervous system disease, such as multiple sclerosis. Agree to Lifestyle Considerations throughout study duration  Exclusion Criteria:  Current use of the following drugs and cannot have a drug substitution or decline the drug substitution: warfarin, flecainide, methadone, amphetamines, quinidine, and chlorpropamide. Pharmacologic ascorbic acid may affect urine acidification and, as a result, may affect clearance rates of these drugs. Current use of antiretroviral drugs (e.g., nelfinavir, abacavir, emtricitabine, lamivudine, stavudine, tenofovir disoproxil fumarate, zidovudine). Pharmacologic ascorbate acid is a known CYP450 3A4 inducer, which results in lower serum levels of antiretroviral drugs. Insulin requirement Requires blood glucose monitoring using finger-stick glucose checks. Inability to undergo MR imaging. Pregnancy or lactation (note: potential participants should not engage in 'pump & dump' strategy; lactation must be discontinued). Known allergic reactions to ferumoxytol. History of hemochromatosis. Prior radiation treatment that would result in field overlap. For potential participants who have undergone nuclear medicine therapy, including PRRT, the study's radiation oncologist must approve study entry. G6PD (glucose-6-phosphate dehydrogenase) deficiency Ferritin > 1,000 ng/mL within 21 days of first treatment Platelet count < 100,000 /mm3 within 21 days of first treatment Creatinine ≥ 1.5x the institutional upper limit of normal within 21 days of the first treatment or if creatinine is elevated a creatinine clearance of < 60 mL/(min 1.73 m2) Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection (requiring inpatient admission or a delay to start of therapy), symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements. Treatment with another investigational drug within 30 days prior to study treatment day 1. Imaging trials (including investigational PET or NM tracers) as well as observational trials are acceptable. Clinical trials with an endpoint of treating the patient's cancer.