Systematic characterization of new EBT4 radiochromic films in clinical x-ray beams

Objective. We aim to characterize kinetics of radiation-induced optical density in newly released EBT4 radiochromic films exposed to clinical x-rays. Several film models and batches were evaluated for the film sensitivity, optical signal increasing with time, relative film noise, and minimum detectable limits (MDL).Approach. Radiochromic film pieces from a single batch of EBT3 and three batches of EBT4 were exposed to doses of 77.38 cGy, 386.92 cGy, and 773.84 cGy using a 6 MV x-ray beam. The films were scanned with a flatbed scanner at specific time intervals up to 120 h. The time-series net optical density of red, green and blue colors was corrected for response of the scanner with time and studied to establish the saturation characteristics of film polymerization process. Dose-response from 3.86 cGy to 1935 cGy was also determined for each color. MDL of the films was quantitatively defined as the dose that would double the net optical density of red color above the standard deviation of the residual signal at zero dose. The relative noise characteristics of EBT3 versus EBT4 were studied as a function of time, dose and scanner resolution.Main Results. For doses ≥ 100 cGy, analysis revealed a stability of optical density beyond 48 h post-exposure for EBT3 and EBT4 films. EBT3 films attained 80%-90% of their net optical density at 48 h within minutes of irradiation, compared to 72%-88% for EBT4 films. The rate of growth was slowest for blue color, fastest for red, while green was in between the two. The MDL for EBT4 averaged 15 cGy for three batches, whereas EBT3 films reliably detected doses as low as 8.5 cGy.Significance. Several batches of the new EBT4 film showed slightly lower response compared to its predecessor over 3.86 cGy to 1935 Gy range. For all practical purposes, the post-irradiation growth of polymers ceases between 48 to 60 h for both EBT films. Overall, the EBT4 film exhibited noise characteristics similar to EBT3, except for lower doses where the noise was observed to be higher than its predecessor.