Previous work has demonstrated that socioeconomic factors affect the use of PT in children1. Medicaid expansion came into effect in 2014. In 2014, there were 15 proton centers in the US, which increased to 31 by 20202. The effects of increased PT center number and expansion of insurance coverage on PT access has yet to be investigated. This was an observational cohort study using the National Cancer Database (NCDB) to assess for changes in utilization of PT for pediatric cancers before and after Medicaid expansion. Demographics and characteristics were summarized with one-way analysis of variance for continuous variables and χ2 for categorical variables. Data regarding geography of treated patients was suppressed. Logistic regression was performed to investigate the effect of SES and insurance on treatment type. All p values are two-sided, with a significance level of 0.05 used. Statistical analyses were performed in R (version 4.1.2). There were 17,096 patients diagnosed at ages ≤21 years from 2004-2020 treated with either photon or PT with known insurance status. 14,491 patients were treated with photons; 2,605 patients received PT. The number of uninsured patients dropped after 2014 from 3.5% to 2.0%, p<0.001. The number of patients with Medicaid who received PT increased after 2014 (21.9% to 28.5%, p<0.001). From 2004-2020, PT increased from 0.4% to 12.4%, peaking in 2018 at 13.9%. Patients with private insurance had higher odds (OR1.5, CI 1.28-1.65) of PT compared to those with Medicaid when controlling for the increased likelihood of PT over time. Patients of higher SES had a higher likelihood of getting treated with PT both before and after Medicaid expansion, while those of the lowest quartile SES did not witness a significant change in the proportion of patients being treated with PT, (11.5% to 12.1% after 2014). Utilization based on race did not significantly change after 2014 among White, Black, and Asian/Pacific Islander populations, but did decrease for patients who identified as Spanish/Hispanic origin (16.9% to 15.0%, p<0.001), and American Indians from 0.7 to 1.3% (p<0.001). Despite the benefits of Medicaid expansion from the ACA since 2014, pediatric patients of Black race and Hispanic ethnicity failed to experience improved access to PT. Pediatric patients with private insurance continued to be more likely to receive PT than patients on Medicaid. Investigation by geography, as well as tumor type, is necessary to provide essential data for creating effective policies to increase equitable access to PT.