Inclusion Criteria:

• Patients must have a diagnosis of pilocytic astrocytoma, pilomyxoid astrocytoma, pleomorphic xanthoastrocytoma, ganglioglioma, optic pathway glioma, diffuse astrocytoma, low-grade neuroepithelial tumor, low-grade glioneuronal tumor or LGG, NOS, after central pathology review.
• Patients must have had histologic verification of disease at original diagnosis or recurrence EXCEPT for patients with optic pathway gliomas. Patients with optic pathway gliomascan be enrolled without histologic verification but with radiologic verification.
• For recurrent tumors that enhance but were originally non-enhancing or if there is a high index of suspicion regarding high-grade transformation, repeat biopsy is required.
• Tumor must be located in or adjacent to the suprasellar region or midline structures. Midline structures include, but are not limited to: the thalamus, basal ganglia, internal capsule, medial temporal lobe, midbrain, tectum, third ventricle, fourth ventricle, cerebellum, pons, and medulla. Tumors may involve the optic pathway. For questions about tumor locations that are not specified on this list, please contact the Study PI.
• Patients must be aged 6 to < 22 years of enrollment.
• Patients must have a performance status greater or equal to 70 (use Karnofsky scale for patients aged 16 years and greater and Lansky scale for patients aged less than 16 years).
• Patients may not receive concurrent chemotherapy or targeted therapy, including but not limited to BRAF-inhibitors and MEK-inhibitors.
• All patients must be able to undergo contrast-enhanced brain MRI.
• Patients with seizures may be enrolled if well controlled on anticonvulsants.

Exclusion Criteria:

• Patients who have received prior CNS radiation.
• Patients with gross total resection and no measurable disease on MRI.
• Patients with evidence of metastatic disease.
• Patients with WHO grade II midline tumors that harbor the H3K27M mutation, IDH-mutant gliomas, grade II ependymomas and subependymomas, pituicytomas, spindle cell oncocytomas, or granular cell tumors of the sellar region.
• Patients with tumors that directly invade bilateral hippocampi or with gross tumor volumes that extend into bilateral hippocampi.
• Patients with tumors in the spine or cervicomedullary junction.
• Females of child-bearing potential cannot be pregnant or breast feeding. Female participants > 10 years of age or post menarche must have a negative serum or urine pregnancy test before enrollment. Males and females of reproductive potential may not participate unless they have agreed to use an effective contraceptive method.
• Patients with NF-1.
• Patients who are status post resection of bilateral hippocampi. Patients who are status post resection of one hippocampus will be eligible for the study and the hippocampal dose constraints will be applied to the intact hippocampus.
• Patients who are not able to undergo neuro-cognitive testing, including children lacking English comprehension or children with premorbid neurological/neurodevelopmental disorders such as Down’s syndrome or autism.