Trends and Challenges: A 15-Year Review of Imaging and Radiation Oncology Core Anthropomorphic Phantom Audits for Proton Therapy

This study aims to analyze the trends in institutional performance on IROC proton phantoms and to investigate the impact of treatment parameters on proton therapy delivery accuracy. We analyzed 402 IROC anthropomorphic phantom audits performed by 57 institutions worldwide between 2009 and 2024, including brain, head and neck (H&N), spine, prostate, lung, and liver phantoms. Dosimetric assessments included thermoluminescent dosimeters (TLDs) for dose comparison and gamma analysis of radiochromic film. The lung and liver phantoms were irradiated with simulated motion. Univariate analysis was performed to evaluate the relationships between treatment parameters and phantom outcomes (pass/fail, TLD-to-TPS ratio, and gamma passing rate). Linear regression was used to analyze the long-term trends in the institutional delivery accuracy across the six phantoms. The inter-institutional dose variation in the target was approximately 3% across all phantoms. Of the participating institutions, 66.7% failed at least one IROC proton phantom. Pass rates were 97% for brain, 91% for H&N, 89% for spine, 76% for prostate, 63% for lung, and 57% for liver phantoms. Phantoms incorporating motion exhibited the poorest performance, particularly in gamma analysis, with 33(N=91) lung and 33(N=76) liver irradiations showing gamma values below 85%. No linear temporal trends were observed in the overall passing rates of the six phantoms, while TLD results improved for brain, liver, spine, and prostate phantoms (p