Core Concepts
The traditional two-voltage method for determining ionization chamber saturation correction factors, while widely recommended, overestimates the correction and shows discrepancies with experimental data, particularly at high dose rates; numerical modeling offers a more accurate alternative.
Abstract
Bibliographic Information: Paz-Mart´ın, J., Sch¨uller, A., Bourgouin, A., Gago-Arias, A., Gonzalez-Casta˜no, D.M., G´omez-Fern´andez, N., Pardo-Montero, J., G´omez, F. (2024). Evaluation of the two-voltage method for parallel-plate ionization chambers irradiated with pulsed beams. [Preprint]. arXiv:2410.02696v1 [physics.med-ph]
Research Objective: This study investigates the accuracy of the two-voltage method (TVM) for determining the saturation correction factor (ksat) in parallel-plate ionization chambers (PPICs) exposed to high dose-per-pulse radiation beams, comparing it to a more realistic numerical model.
Methodology: The researchers experimentally determined the charge collection efficiency (CCE) of four PPICs (two Advanced Markus and two PPC05) at the German National Metrology Institute using a high dose-rate electron beam with varying pulse durations and voltages. They compared the experimental CCE with those predicted by several analytical models, including the TVM based on Boag's formalism, and a recently developed numerical model accounting for free electron fractions and electric field perturbations.
Key Findings: The study found significant discrepancies between the experimentally determined CCE and those predicted by the analytical models, including the TVM, even at moderate dose rates. The TVM consistently overestimated the saturation correction factor. The numerical model, however, showed good agreement with the experimental data, accurately predicting the CCE with an average discrepancy of less than 1%.
Main Conclusions: The authors conclude that the widely recommended TVM for determining ksat in PPICs may not be accurate, especially at high dose rates. They suggest that numerical modeling provides a more accurate approach to determining ksat and recommend its use in dosimetry protocols, particularly for high dose-rate applications like intraoperative radiotherapy.
Significance: This research highlights the limitations of the traditional TVM for ionization chamber dosimetry in high dose-rate applications. The findings have significant implications for the accurate determination of absorbed dose in clinical settings utilizing high dose-rate radiation therapy.
Limitations and Future Research: The study was limited to a specific set of PPIC models and a limited range of beam parameters. Further research is needed to validate these findings across a wider range of ionization chamber types and beam characteristics. Additionally, further investigation into the discrepancies between the analytical models and experimental data could lead to improvements in analytical modeling approaches.
Stats
The relative uncertainty of the fD calibration coefficient was determined to be 3.929 Gy nC^-1.
The reference depth of measurement for the electron beam quality used was 4.10 g cm^-2.
The short-term stability of the linear accelerator, based on the standard deviation of residuals, was 0.22%.
The average combined uncertainty for the CCE measurement was 1.12%, with a range of 1.07% to 1.40%.
The average uncertainty for the polarity correction factors was estimated to be 0.32%.
The electron-ion recombination contribution was estimated to be always below 0.23% and on average 0.1% for the investigated chambers and dose-per-pulse range.
The influence of the beam waveform on the CCE, while showing local deviations up to 0.36%, was on average lower than 0.1%.
Differences in absolute homologous voltages between positive and negative polarity were observed up to 7 V, but their effect on CCE was estimated to be always below 0.1%.
Quotes
"The two-voltage method is the recommended methodology by the TRS-398 and TG-51 code of practice...to determine the saturation factor for ionization chambers."
"The classical two-voltage method, systematically overestimates the saturation factor, with differences increasing with dose per pulse also present at low dose per pulse."
"The numerical simulation shows a better agreement with the experimental data than the current analytical theories in terms of charge collection efficiency."