Abstract BACKGROUND Diffuse Midline Glioma (DMG) is a fatal, unresectable, pediatric brain tumor with limited treatment options and a median survival of 9-15mths. As radiation therapy (RT) can only extend life by 3mths, novel RT techniques are needed, including ultra-high dose rate RT (FLASH). RT can stimulate immune responses in many cancers especially via type 1 interferon (IFN1) but the effect of RT on DMG is understudied as post treatment tissue is not readily available. METHODS We evaluated the effect of RT on immune response in an orthotopic syngeneic model of DMG, comparing FLASH, given at 90Gy/second, to conventional dose rate RT (CONV), given at 2Gy/min. We evaluated single-cell RNA sequencing (scRNA-seq) on CD45+ immune cells isolated from tumors 4 days post 15Gy FLASH or CONV RT and compared to unirradiated tumor (SHAM). We also performed flow cytometry on tumors at both day 4 and day 10 post-RT. RESULTS We find microglia (MG) are the most abundant immune cell type followed by non-resident myeloid cells including macrophages (MAC) and dendritic cells (DC). While there’s no significant difference in tumor growth or proportion of immune subtypes comparing FLASH, CONV and SHAM, we see similar IFN1 enrichment in MG comparing FLASH and CONV at day 4. Interestingly, at day 10 post-RT we observe higher IFN1 receptor (IFNAR1) by flow in MGs after CONV (p=0.016) and SHAM (p=0.040) compared to FLASH. At day 4 CONV shows higher IFN1 gene set enrichment in MACs and DCs compared to FLASH (both p<0.0001) as well as higher IFNAR1 in DCs comparing CONV to FLASH (p=0.0079), while day 10 post-RT shows equivalent IFNAR1 in MACs and DCs between dose rates. CONCLUSION Changes in DMG tumor immune microenvironment post-RT differ over time. Though both RT modalities increase IFN1, the timing of this response is dependent on both cell phenotype and dose.