e23034 Background: Since 2019, the Centers for Medicare & Medicaid Services (CMS) has required hospitals to publish gross charges for all goods and services provided. In 2021, CMS amended this requirement to include publishing standard charge files, which include cash and payer-negotiated prices for all goods and services provided. This was enacted in an attempt to reduce healthcare prices by reducing information asymmetry and generating competition among both insurers and hospitals. Since complex proton therapy is one of the most expensive modalities of cancer treatment, we aimed to determine proton therapy center compliance with CMS price transparency requirements. Methods: We searched for online standard charge files for the 44 proton therapy centers in the United States. To assess compliance with the nine CMS price transparency criteria, we obtained prices for proton therapy using keyword searches and billing code 77525, the CPT code for the per fraction technical charge billed for complex proton treatment delivery. Results: Out of 44 operating proton therapy centers, 5 (11.4%) are free-standing private practice centers that are not affiliated with a hospital system, and thus not required to publish standard charge files per CMS policy. Of the 39 centers required to publish, 39 (100%) published their standard charge files free of charge with appropriately named files, thus meeting Criteria 1-3. Twenty-seven (69.2%) centers met Criteria 4 by including data on complex proton therapy (CPT code 77525) and Criteria 5a by including the gross charge for the service. One (2.6%) center partially met Criteria 4. Seventeen (43.6%) centers included negotiated prices for commercial insurers, thus meeting Criteria 5b, while 3 (7.7%) partially met this criterion. Twenty-two (56.4%) centers met Criteria 5c and 5d by including the deidentified minimum and maximum negotiated commercial prices, respectively. Twenty-six (66.7%) centers met Criteria 5e by listing the cash price for complex proton therapy, which is typically paid out-of-pocket by patients. One (2.6%) center partially met Criteria 5c-e. In total, 17 (43.6%) out of 39 proton therapy centers were in complete compliance with the 2021 CMS price transparency requirements. Conclusions: In summary, 17 out of 39 eligible proton centers were fully compliant with the 2021 CMS price transparency requirements in regard to complex proton radiation therapy. This suggests that deficiencies exist among hospital compliance and the CMS review process used to identify non-compliance. Future directions are aimed at expanding this analysis to include evaluation of proton center compliance with other billing codes, such as simple and intermediate proton treatment delivery. [Table: see text]