Calibration and time fading characterization of a new optically stimulated luminescence film dosimeter

Background: Optically stimulated luminescence (OSL) dosimeters produce a signal linear to the dose, which fades with time due to the spontaneous recombination of energetically unstable electron/hole traps. When used for radiotherapy (RT) applications, fading affects the signal-to-dose conversion and causes an error in the final dose measurement. Moreover, the signal fading depends to some extent on treatment-specific irradiation conditions such as irradiation times.

Purpose: In this work, a dose calibration function for a novel OSL film dosimeter was derived accounting for signal fading. The proposed calibration allows to perform dosimetry evaluation for different RT treatment regimes.

Methods: A novel BaFBr:Eu²⁺ -based OSL film (Z_{eff , 6 MV} = 4.7) was irradiated on a TrueBeam STx using a 6 MV beam with setup: 0° gantry angle, 90 cm SSD, 10 cm depth, 10 × 10 cm² field. A total of 86 measurements were acquired for dose-rates ( $\dot{D}$ ) of 600, 300, and 200 MU/min for irradiation times (t_ir ) of 0.2, 1, 2, 4.5, 12, and 23 min and various readout times (t_scan ) between 4 and 1440 min from the start of the exposure (beam-on time). The OSL signal, $S(\dot{D},t_{ir},t_{scan})$ , was modeled via robust nonlinear regression, and two different power-law fading models were tested, respectively, independent (linear model) and dependent on the specific $t_{ir}$ (delivery-dependent model).

Results: After 1 day from the exposure, the error on the dose measurement can be as high as 48% if a fading correction is not considered. The fading contribution was characterized by two accurate models with adjusted-R² of 0.99. The difference between the two models is <4.75% for all $t_{ir}$ and $t_{scan}$ . For different beam-on times, 3, 10.5, and 20 min, the optimum $t_{scan}$ was calculated in order to achieve a signal-to-dose conversion with a model-related error <1%. In the case of a 3 min irradiation, this condition is already met when the OSL-film is scanned immediately after the end of the irradiation. For an irradiation of 10.5 and 20 min, the minimum scanning time to achieve this model-related error increases, respectively, to 30 and 90 min. Under these conditions, the linear model can be used for the signal-to-dose conversion as an approximation of the delivery-dependent model. The signal-to-dose function, D(M_{i , j} , $t_{scan}$ ), has a residual mean error of 0.016, which gives a residual dose uncertainty of 0.5 mGy in the region of steep signal fading (i.e., $t_{scan}$ = 4 min). The function of two variables is representable as a dose surface depending on the signal (M_{i , j} ) measured for each i,j-pixel and the time of scan ( $t_{scan}$ ).

Conclusions: The calibration of a novel OSL-film usable for dosimetry in different RT treatments was corrected for its signal fading with two different models. A linear calibration model independent from the treatment-specific irradiation condition results in a model-related error <1% if a proper scanning time is used for each irradiation length. This model is more practical than the delivery-dependent model because it does not need a pixel-to-pixel fading correction for different $t_{ir}$ .