On temporal evolution of the response of EBT4 radiochromic films

Radiochromic film dosimeters are commonly used in radiation oncology in both clinical and research settings to measure the radiation dose with high spatial resolution over a two-dimensional area. Radiation-induced solid-state polymerization occurring in films leads to a change in their opacity. It is critical to evaluate the temporal stability of the response of new film models to ensure an appropriate post-irradiation time is selected for film scanning.

To investigate the time-dependent growth of optical density (OD) in the recently released EBT4 radiochromic films.

EBT4 film samples were irradiated at 0.125, 0.25, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, and 15 Gy dose levels using a 6 MV X-ray beam produced by a clinical linear accelerator. Two film samples were irradiated at each dose level. The films were repeatedly scanned using a flatbed scanner upon irradiation and at various time intervals for approximately 2 months (1325 h). The net OD was measured, and the films' sensitivity and calibration curves were obtained at several time points.

The net OD grows with time in all color channels, the rate of which reduces with dose. Red channel OD increased by ∼8%-30% over the studied time period for films irradiated at 15-1 Gy. A similar trend was observed in the green channel with slightly higher (∼1%-2%) growth rates compared to the red channel. Films irradiated at > 1 Gy dose reached over ∼90% of their final net OD within the first 12 h post-irradiation, followed by a gradual growth over the following weeks. The OD-per-Gy decreased with dose and increased with elapsed time post-irradiation.

Since the response of the films continues to develop with time, to adhere to good practice of film dosimetry, the calibration and test films must be read at the same time interval post-irradiation, otherwise significant errors in dose measurement can occur. Since the temporal dynamics of EBT4 and EBT3 films were found to be similar, both models can be treated similarly in terms of the optimum post-irradiation wait-time prior to film readout.