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

BackgroundOptically 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. PurposeIn 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. MethodsA 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 degrees gantry angle, 90 cm SSD, 10 cm depth, 10 x 10 cm(2) field. A total of 86 measurements were acquired for dose-rates (D?$\dot{D}$) 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(D?,tir,tscan)$S(\dot{D},{t}_{ir},{t}_{scan})$, was modeled via robust nonlinear regression, and two different power-law fading models were tested, respectively, independent (linear model) and dependent on the specific tir${t}_{ir}$ (delivery-dependent model). ResultsAfter 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-R-2 of 0.99. The difference between the two models is tir${t}_{ir}$ and tscan${t}_{scan}$. For different beam-on times, 3, 10.5, and 20 min, the optimum tscan${t}_{scan}$ 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,tscan$\ {t}_{scan}$), 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., tscan${t}_{scan}\ $= 4 min). The function of two variables is representable as a dose surface depending on the signal (M-i(,)j) measured for each i,j-pixel and the time of scan (tscan${t}_{scan}$). ConclusionsThe calibration of a novel OSL-film usable for dosimetry in different RT treatments was corrected for its signal fading with two different models. A linear calibration model independent from the treatment-specific irradiation condition results in a model-related error tir${t}_{ir}$.