M Caprioli¹*, M De Saint-hubert², L De Freitas Nascimento³, R De Roover⁴, L Delombaerde⁵, S Galvez Febles⁶, K Himschoot⁷, P Leblans⁸, B Van Der Heyden⁹, D Vandenbroucke¹⁰, W Crijns¹¹, (1) KU Leuven, Brussels, VBR, BE, (2) Belgian Nuclear Research Centre (SCK CEN), Mol, ,BE, (3) Belgian Nuclear Research Centre (SCK CEN), Mol, ,BE, (4) UZ Leuven, Leuven, ,BE, (5) UZ Leuven, Leuven, ,BE, (6) Belgian Nuclear Research Centre (SCK CEN), Mol, ,BE, (7) Agfa N.V., Mortsel, ,BE, (8) Agfa N.V., Mortsel, ,BE, (9) Belgian Nuclear Research Centre (SCK CEN), Mol, ,BE, (10) Agfa N.V., Mortsel, ,BE, (11) UZ Leuven, Leuven, VBR, BE

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  M Caprioli

TU-F115-IePD-F4-3 (Tuesday, 7/12/2022) 1:15 PM - 1:45 PM [Eastern Time (GMT-4)]

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.

Keywords

Absolute Dosimetry, Calibration, Optical Dosimetry

Taxonomy

TH- External Beam- Photons: Calibration protocol and primary standards

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