Evaluation of the Two-Voltage Method for Recombination Correction in Parallel-Plate Ionization Chambers Exposed to Pulsed Radiation Beams

Grunnleggende konsepter

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.

Sammendrag

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.

Statistikk

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%.

Sitater

"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."

Viktige innsikter hentet fra

Evaluation of the two-voltage method for parallel-plate ionization chambers irradiated with pulsed beams

by José... klokken arxiv.org 10-04-2024

https://arxiv.org/pdf/2410.02696.pdf

Dypere Spørsmål

How might the increasing use of hypofractionated radiotherapy, which often involves higher dose rates, impact the need for more accurate dosimetry methods like those presented in this paper?

Hypofractionated radiotherapy, with its use of higher dose rates and fewer fractions, amplifies the significance of accurate dosimetry for several reasons:

Increased Recombination: Higher dose rates lead to a denser concentration of charge carriers (ions and electrons) generated within the ionization chamber. This increases the probability of ion-ion recombination before the charge can be collected, leading to an underestimation of the dose if not properly corrected for. Accurate determination of the saturation factor (ksat), as discussed in the paper, becomes crucial in these scenarios.
Elevated Sensitivity to Errors: With fewer fractions delivering larger doses, the margin for error decreases. Any inaccuracies in dose measurement are magnified and can have a greater impact on treatment outcome. The paper highlights how traditional methods like the two-voltage method (TVM), based on simplified assumptions, can overestimate the saturation factor, potentially leading to an underdosage in hypofractionated treatments.
Limitations of Traditional Methods: As this study demonstrates, analytical models based on the classic Boag's formalism might not adequately account for the complexities of charge transport and recombination at high dose rates. This underscores the limitations of traditional dosimetry methods in the context of hypofractionated radiotherapy.
Therefore, the growing adoption of hypofractionated radiotherapy necessitates more accurate dosimetry methods, such as the numerical modeling approach presented in the paper. These advanced techniques can better account for the physical phenomena occurring at high dose rates, ensuring more precise dose delivery and improved patient outcomes.

Could the discrepancies between the analytical models and experimental data be attributed to factors not considered in this study, such as variations in chamber manufacturing or environmental conditions during measurement?

Yes, discrepancies between analytical models and experimental data in ionization chamber dosimetry can arise from factors beyond those directly investigated in the study. Here are some possibilities:

Chamber Manufacturing Variations:

Electrode Planarity and Parallelism: Deviations from perfectly parallel electrodes can create non-uniform electric fields within the chamber, affecting ion drift and recombination rates.
Gap Irregularities: Slight variations in the gap distance between electrodes, even on a microscopic scale, can influence the electric field strength and thus the charge collection efficiency.
Material Purity: Impurities in the chamber materials or manufacturing residues can alter electron attachment rates and ion mobilities, impacting recombination.

Environmental Conditions:

Humidity Variations: Changes in air humidity during measurements can affect electron attachment rates, as water molecules can act as electron scavengers.
Temperature Fluctuations: Temperature influences ion mobilities and recombination coefficients. Even small temperature differences between calibration and measurement conditions can introduce discrepancies.
Air Density: Air density, affected by both temperature and pressure, directly impacts the number of ion pairs created by the radiation. Accurate pressure and temperature corrections are essential.

Other Factors:

Radiation Beam Quality: Variations in beam energy spectrum or dose rate can influence the spatial distribution of ionization within the chamber, potentially affecting recombination.
Measurement Uncertainties: Uncertainties associated with electrometer readings, voltage measurements, and positioning of the chamber can all contribute to discrepancies.
It's important to note that while the study aimed to control for many variables, achieving perfect experimental conditions is challenging. The authors acknowledge some limitations, such as assuming a constant humidity level and using a simplified rectangular pulse shape in the numerical model. These factors could contribute to the observed discrepancies.

If numerical modeling proves to be a more accurate method for determining ionization chamber correction factors, what steps need to be taken to make this approach more accessible and user-friendly for clinical physicists in their daily practice?

While numerical modeling offers greater accuracy in determining ionization chamber correction factors, several steps are crucial to bridge the gap between research and routine clinical implementation:

Simplified Software Interfaces: User-friendly software packages specifically designed for clinical physicists are essential. These should allow for easy input of chamber parameters, beam characteristics, and measurement conditions, without requiring extensive programming knowledge.
Pre-Configured Chamber Models: Software should include a library of pre-configured models for commonly used ionization chambers, minimizing the need for users to define chamber geometries and material properties.
Streamlined Workflows: Integration with existing treatment planning systems (TPS) or dosimetry software would streamline workflows. Ideally, correction factors could be automatically calculated and incorporated into dose calculations.
Validation and Benchmarking: Thorough validation of numerical models against experimental data for a wide range of chambers, beam qualities, and dose rates is crucial to build confidence in their accuracy. Benchmarking against established standards and protocols is also essential.
Education and Training: Educational resources, workshops, and training materials tailored for clinical physicists are needed to promote understanding of numerical modeling principles and their practical application in dosimetry.
Computational Resources: While numerical simulations can be computationally demanding, cloud-based solutions or access to high-performance computing clusters could alleviate the need for individual institutions to invest in expensive hardware.
Furthermore, collaboration between researchers, software developers, and clinical physicists is vital to ensure that the developed tools meet the specific needs of the clinical environment. Open-source software initiatives could also accelerate the adoption of numerical modeling by fostering transparency and community-driven development.