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

Abstract

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. 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. A novel BaFBr:Eu2+ -based OSL film (Zeff , 6MV =4.7) was irradiated on a TrueBeam STx using a 6MV beam with setup: 0° gantry angle, 90cm SSD, 10cm depth, 10×10cm2 field. A total of 86 measurements were acquired for dose-rates ( ) of 600, 300, and 200MU/min for irradiation times (tir ) of 0.2, 1, 2, 4.5, 12, and 23min and various readout times (tscan ) between 4 and 1440min from the start of the exposure (beam-on time). The OSL signal, , was modeled via robust nonlinear regression, and two different power-law fading models were tested, respectively, independent (linear model) and dependent on the specific (delivery-dependent model). 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 and . For different beam-on times, 3, 10.5, and 20min, the optimum was calculated in order to achieve a signal-to-dose conversion with a model-related error <1%. In the case of a 3min 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 20min, the minimum scanning time to achieve this model-related error increases, respectively, to 30 and 90min. 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(Mi , j , ), has a residual mean error of 0.016, which gives a residual dose uncertainty of 0.5mGy in the region of steep signal fading (i.e., =4min). The function of two variables is representable as a dose surface depending on the signal (Mi , j ) measured for each i,j-pixel and the time of scan ( ). 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 .