New optically stimulated luminescence dosimetry film optimized for energy dependence guided by Monte Carlo simulations.

Optically stimulated luminescence (OSL) film dosimeters, based on BaFBr:Eu2+phosphor material, have major dosimetric advantages such as dose linearity, high spatial resolution, film re-usability, and immediate film readout. However, they exhibit an energy-dependent over-response at low photon energies because they are not made of tissue-equivalent materials. In this work, the OSL energy-dependent response was optimized by lowering the phosphor grain size and seeking an optimal choice of phosphor concentration and film thickness to achieve sufficient signal sensitivity. This optimization process combines measurement-based assessments of energy response in narrow x-ray beams with various energy response calculation methods applied to different film metrics. Theoretical approaches and MC dose simulations were used for homogeneous phosphor distributions and for isolated phosphor grains of different dimensions, where the dose in the phosphor grain was calculated. In total 8 OSL films were manufactured with different BaFBr:Eu2+median particle diameters (D50): 3.2µm, 1.5µm and 230 nm and different phosphor concentrations (1.6%, 5.3% and 21.3 %) and thicknesses (from 5.2 to 49µm). Films were irradiated in narrow x-ray spectra (N60, N80, N-150 and N-300) and the signal intensity relative to the nominal dose-to-water value was normalized to Co-60. Finally, we experimentally tested the response of several films in Varian 6MV TrueBeam STx linear accelerator using the following settings: 10 × 10 cm2field, 0deggantry angle, 90 cm SSD, 10 cm depth. The x-ray irradiation experiment reported a reduced energy response for the smallest grain size with an inverse correlation between response and grain size. The N-60 irradiation showed a 43% reduction in the energy over-response when going from 3µm to 230 nm grain size for the 5% phosphor concentration. Energy response calculation using a homogeneous dispersion of the phosphor underestimated the experimental response and was not able to obtain the experimental correlation between grain size and energy response. Isolated grain size modeling combined with MC dose simulations allowed to establish a good agreement with experimental data, and enabled steering the production of optimized OSL-films. The clinical 6 MV beam test confirmed a reduction in energy dependence, which is visible in small-grain films where a decrease in out-of-field over-response was observed.

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 (Zeff , 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 ( D ̇ $\dot{D}$ ) of 600, 300, and 200 MU/min for irradiation times (tir ) of 0.2, 1, 2, 4.5, 12, and 23 min and various readout times (tscan ) between 4 and 1440 min from the start of the exposure (beam-on time). The OSL signal, S ( D ̇ , t i r , t s c a n ) $S(\dot{D},{t}_{ir},{t}_{scan})$ , 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 i r ${t}_{ir}$ (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 i r ${t}_{ir}$ and t s c a n ${t}_{scan}$ . For different beam-on times, 3, 10.5, and 20 min, the optimum t s c a n ${t}_{scan}$ 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(Mi , j , t s c a n $\ {t}_{scan}$ ), 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 s c a n ${t}_{scan}\ $ = 4 min). 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 ( t s c a n ${t}_{scan}$ ). 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 i r ${t}_{ir}$ .