Virus-Based Gene Therapy (AdV-HSV1-TK and AdV-Flt3L) in Combination With Valacyclovir for the Treatment of Pediatric and Young Adult Patients With Resectable, Recurrent Primary Malignant Brain Tumors

This study is testing a virus-based gene therapy combination — two engineered viruses that infect and kill cancer cells, given with an antiviral drug — delivered directly into the brain tumor during surgery, for children and young adults with recurring malignant brain tumors. **You may be eligible if...** - You are between 3 and 25 years old with a malignant brain tumor that has come back or progressed after standard treatment — OR — you are 26–39 years old with a specific brain tumor type (diffuse hemispheric glioma, H3 G34-mutant) that has relapsed - You weigh at least 10 kg - Surgery to remove the recurring tumor is planned - You have recovered from side effects of prior treatments **You may NOT be eligible if...** - You have not completed standard-of-care therapy before this trial - You have not waited the required time since your last chemotherapy, radiation, or immunotherapy - Your parent/guardian (or you, if an adult) is not able to consent Talk to your doctor to see if this trial is right for you.