e23039 Background: Proton therapy is one of the most expensive treatment modalities offered to cancer patients, and only 44 proton therapy centers are operational in the U.S. Of those, 5 are Medicare Prospective Payment System (PPS)-exempt and are reimbursed based on their reported costs rather than predetermined prices. These PPS-exempt cancer centers set their adjusted payment rates to reflect the expense required for providing specialized care. We hypothesized that PPS-exempt centers would have higher rates for complex proton therapy delivery compared to non-exempt centers. Methods: We obtained online standard charge files for 44 proton therapy centers in the U.S. We used the CPT code 77525, the per fraction technical charge billed for complex proton treatment delivery, to extract the gross price (base charge for service), de-identified minimum and maximum negotiated price (lowest/highest charge negotiated with third-party payers), and cash price (charge for cash payment of service) per fraction of complex proton therapy. Prices were compared between PPS-exempt and non-exempt proton therapy centers using Wilcoxon/Kruskal Wallis test with a p-value < 0.05 considered significant. Results: Of the 44 proton therapy centers, 39 (88.6%) had publicly available standard charge files, and 5 (11.4%) are private practice not required to provide a standard charge file. Five of the 39 (12.8%) centers are PPS-exempt, and the remaining 34 (87.2%) are PPS non-exempt. Four (80%) PPS-exempt centers had their gross price, minimum and maximum negotiated price, and cash price available in their standard charge file for CPT 77525, while 23 (67.6%) PPS non-exempt centers had their gross price and cash price listed and 19 (55.9%) had the minimum and maximum price available. The median gross charge per fraction for complex proton treatment delivery was $8,035.25 (IQR $7,022.00-$9,427.50) at PPS-exempt versus $6,434.00 (IQR $5,213.00-$12,350.00) at PPS non-exempt centers. The median minimum price was $656.47 (IQR $178.04-$1,353.23) at PPS-exempt versus $1,294.70 (IQR $734.20-$2,267.46) at PPS non-exempt centers. The median maximum price was $8,120.10 (IQR $5,576.40-$11,529.86) at PPS-exempt versus $5,814.00 (IQR $4,830.00-$9,524.70) at PPS non-exempt centers. The median cash price was $5,360.65 (IQR $2,979.51-$6,897.08) at PPS-exempt versus $4,170.40 (IQR $2,828.00-$5,640.00) at PPS non-exempt centers. Conclusions: In summary, 80% of PPS-exempt and 67.6% of PPS non-exempt centers had publicly available standard charge files. Median charges for PPS-exempt centers are higher for gross charge, maximum price, and cash price, however did not meet statistical significance. [Table: see text]