Benchmarking Radiochromic EBT4 Film for Clinical Proton DosimetryRadiochromic Films for Proton Dosimetry

This study presents an evaluation of the dosimetric performance of radiochromic EBT4 films for clinical proton therapy, benchmarked against the widely used EBT3 model. Key parameters assessed include dose-response behavior, energy dependence, sensitivity, reproducibility, temporal stability, and longitudinal and lateral linear energy transfer (LET) effects.

EBT4 films from three independent batches and one batch of EBT3 films were irradiated using monoenergetic therapeutic clinical proton beams ranging from 70 to 225 MeV. Film irradiation was done at various depths in a solid water phantom, while doses from 0.25 to 20 Gy were delivered. Following scanning, the film responses were quantified as net optical density and calibrated with absolute dose measurements from a parallel plate ionization chamber. Temporal kinetics of net optical density were studied at various time points up to 120 hours post irradiation. Film dependency on LET of the proton beams was assessed through both lateral beam profiles and longitudinal depth-dependent analyses beyond pristine Bragg peaks.

EBT4 films exhibited a highly linear and reproducible dose-response (R² > 0.998), with minimal energy dependence (<3%) across 70 to 225 MeV proton energies. Compared to EBT3, EBT4 films showed an under-response of approximately 13%-20% at 10 Gy, depending on the batch, with a batch-to-batch variation of ∼8% observed between EBT4 films. Reproducibility between independent irradiations was within 1%, and sensitivity tests confirmed the ability to resolve dose variations as small as ±5% for doses as low as 0.5 Gy. The film's optical density stabilized within 24 hours post-irradiation. LET-dependent response was observed in high-LET regions for EBT4 films, similar to EBT3 films.

While EBT4 films exhibit relatively lower sensitivity compared to EBT3 and LET corrections remain necessary in high-gradient dose regions, EBT4 films are a promising candidate for routine and high-resolution proton dose measurements, provided that batch-specific calibration is performed.