Analysis of Temporal Dependencies of Optical Density and Signal-to-Noise Ratio of the New Gafchromic Film EBT4

Purpose: To evaluate the temporal dependencies of optical density and signal-to-noise ratio (SNR) of the new Gafchromic EBT4 film by comparing its characteristics with the EBT3 film for dosimetry and quality assurance (QA) of stereotactic radiotherapy (SRT/SBRT) treatment plans. Material and methods: EBT3 and EBT4 films were irradiated using the CyberKnife robotic system (6 MV, FFF) in a waterequivalent phantom. Optical density measurements were conducted at 30-minute intervals over 63.5 hours post-irradiation using an Epson Perfection V750 Pro scanner and analyzed with ImageJ software. The NIH Rodbard function was used to construct temporal calibration curves. The SNR was calculated for each time interval. Patient-specific SRT/SBRT treatment plans were verified using temporal calibration curves and 2D gamma analysis (2 %/2 mm criterion) implemented in inhouse Python software. Results: The temporal dependencies of normalized optical density (netOD) for both EBT3 and EBT4 films exhibited a logarithmic pattern, with partial stabilization at 30 hours and full stabilization at 45 hours post-irradiation. The EBT4 film demonstrated an improved SNR across the entire dose range, making it a preferred choice for precision dosimetry. Gamma analysis of SRT/SBRT plans using the temporal calibration curves of both films showed high agreement, with no less than 90 % of points meeting the 2 %/2 mm criterion. Conclusion: The findings confirm the feasibility of using Gafchromic EBT4 film for dosimetry and quality assurance (QA) of radiotherapy treatment plans. Its enhanced characteristics, including a higher SNR and stable temporal dependencies, make it a promising tool for routine clinical practice in SRT/SBRT. The use of temporal calibration curves enables accelerated verification of patient-specific plans while maintaining high measurement accuracy.