Exhibit Hall | Forum 4
Purpose: Optically Stimulated Luminescence (OSL) dosimeters produce a linear-to-dose signal, which fades with time due to spontaneous recombination of energetically unstable electron/hole traps. When used for radiotherapy applications, fading affects the signal-to-dose conversion determining an error in the evaluation of the delivered dose. Moreover, the degree of signal fading depends on treatment-specific irradiation conditions such as dose rates, irradiation times. In this work an OSL calibration function is derived accounting for fading to allow for a suitable and accurate dosimetry for different irradiation schemes (e.g.,IMRT/VMAT,SBRT).
Methods: A novel BaFBr:Eu²⁺-based OSL-film was irradiated on a TrueBeamTM STx using a 6MV beam quality, setup: 0° gantry angle, 90cm SSD, 10cm depth, 10x10cm2 field. 86 measurements were acquired for dose-rates (D_rate) of 600, 300, 200 MU/min for irradiation times (t_irr) of 0.2, 1, 2, 4.5, 12, 23min, and various readout times (t_scan) between 4 and 1440min. The OSL signal, S(D_rate,t_irr,t_scan), was modelled via robust nonlinear regression. Two different power-law fading models were tested, respectively independent and dependent on the specific t_irr: eq.1, A∙D_rate∙t_irr∙(t_scan+τ)⁻ⁿ, eq.2, (A∙D_rate)(1-n)⁻¹∙[(t_scan+τ)⁽¹⁻ⁿ⁾-(t_scan+t_irr+τ)⁽¹⁻ⁿ⁾]
Results: Both models are accurate with adjusted-R² of 0.98 (eq.1) and 0.99 (eq.2). The signal-to-dose function, D(S,t_scan), for eq.1 is a calibration surface with a residual mean error <0.05mGy, while for eq.2 the conversion is unpractical since the fading power-law terms is t_irr-dependent. The difference between the models is <3.01% and increases for longer t_irr and shorter t_scan. If t_scan is 30min, the difference between models is <0.7%, for treatments >10min (e.g.,some SBRT).
Conclusion: The calibration of a novel OSL-film usable for accurate dosimetry in different RT treatments was corrected for fading with two different models. If t_scan is tens of minutes higher than t_irr, the fading can be considered approximately independent on irradiation conditions as the signal intensity decreases by a fading factor dependent on t_scan.
Absolute Dosimetry, Calibration, Optical Dosimetry
TH- External Beam- Photons: Calibration protocol and primary standards