S1685
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
been developed [2]. These simulations in combination with experiments indicate that Boag's model fails to accurately predict the response of commercially available air-vented ICs [3] even for the dose-per-pulse values typically found in conventional radiation beams. Material/Methods: The response of 13 ionization chambers—comprising parallel-plate and thimble types from PTW, IBA, and Exradin—was characterized under different bias voltages using the ultra-high pulse dose rate reference electron beam at the German national metrology institute, the PTB. The dose per pulse was varied from 1.6 mGy to 1.4 Gy by adjusting the source-to-surface distance, pulse duration, and additional scattering foils at the beamline exit window. The charge from the ICs was corrected for recombination losses using a polynomial function derived from a numerical model [2] where the correction factor depends on the charge ratios measured at different voltages. This new correction formalism was then compared to the recombination correction predicted by the existing Code of Practice and measurements. Results: The TVM based on Boag's model shows deviations up to 20 %, depending on the IC (Figure below).
assumed in Boag's model. An example of the recombination factor as a function of the charge ratio for the Farmer IC is shown below.
Conclusion: The recommended methodology to evaluate the recombination correction factor overcompensates the charge losses due to volume recombination. The proposed new method overcomes the weaknesses of the Boag's model, providing a more accurate evaluation of ks. References: [1] Boag JW. Ionization measurements at very high intensities. Pulsed radiation beams. Br J Radiol. 1950;23(274):601-611. doi:10.1259/0007-1285-23-274- 601[2] Paz-Martín J, Schüller A, Bourgouin A, et al. Numerical modeling of air-vented parallel plate ionization chambers for ultra-high dose rate applications. Phys Med. 2022;103:147-156. doi:10.1016/j.ejmp.2022.10.006[3] Paz-Martín J, Schüller A, Bourgouin A, et al. Evaluation of the two- voltage method for parallel-plate ionization chambers irradiated with pulsed beams. Med Phys. 2025;52(6):4894-4909. doi:10.1002/mp.17814 Keywords: recombination correction, two-voltage method
On the other hand, deviations from linearity using the new parametrizations are typically below ±1 % for ks ≤ 1.05 at the manufacturer recommended operational bias voltage. The recombination correction is found to be dependent not only on the voltage ratio, but also on the selected pair of voltages, contrary to what is
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