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

Inclusion Criteria: * Age 3 to 25 years with: * Diagnosis of malignant primary brain tumor after tumor recurrence, relapse, or progression who have completed up-front, standard-of-care therapy * Age 26 to 39 years with: * Diagnosis of diffuse hemispheric glioma, H3 G34-mutant, per 2021 World Health Organization (WHO) classification, after tumor recurrence, relapse, or progression who have completed up-front, standard-of-care therapy * At least 10 kg (and body surface area \[BSA\] \> 0.5 m\^2) * Participants who are receiving corticosteroids must be on a stable or decreasing dose for at least 3 days prior to baseline MRI * Surgical resection of the tumor recurrence/relapse/progression is clinically indicated at the time of enrollment * A legal parent/guardian or patient must be able to understand, and willing to sign, a written informed consent and assent document, as appropriate * Participant must be willing to provide archival formalin-fixed embedded (FFPE) and/or frozen tissue specimens, if available * Participant must have recovered from all acute side effects of prior therapy. * From the projected start of scheduled study treatment, the following time periods must have elapsed: At least 7 days after last dose of a biologic agent or beyond time during which adverse events are known to occur for a biologic agent, 5 half-lives from any investigational agent, 4 weeks from cytotoxic therapy (except 23 days for temozolomide and 6 weeks from nitrosoureas), 6 weeks from…