Characterization of stochastic noise and post-irradiation density growth for reflective-type radiochromic film in therapeutic photon beam dosimetry

The aim of this study is to investigate the dosimetric uncertainty of stochastic noise and the post-irradiation density growth for reflective-type radiochromic film to obtain the appropriate dose from the exactly controlled film density. Film pieces were irradiated with 6-MV photon beams ranging from 0 to 400 cGy. The pixel values (PVs) of these films were obtained using a flatbed scanner at elapsed times of 1 min to 120 h between the end of irradiation and the film scan. The means and standard deviations (SDs) of the PVs were calculated. The SDs of the converted dose scale, u_sd, and the dose increases resulting from the PV increases per ±29 min at each elapsed time, u_time, were computed. The combined dose uncertainties from these two factors, u_c, were then calculated. A sharp increase in the PV occurred within the first 3 h after irradiation, and a slight increase continued from 3 h to 120 h. u_sd was independent of post-irradiation elapsed time. Sharp decreases in u_time were obtained within 1 h after irradiation, and slight decreases in u_time were observed from 1 to 24 h after irradiation. u_c first decreased 1 h after irradiation and remained constant afterward. Assuming that the post-irradiation elapsed times of all of the related measurements are synchronized within ±29 min, the elapsed time should be at least 1 h in our system. It is important to optimize the scanning protocol for each institution with consideration of the required measurement uncertainty and acceptable latency time.