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BACKGROUND/AIM: This study compared two types of parallel-plate ionization chamber to clarify the pitfalls of dosimetry in electron radiation therapy. MATERIALS AND METHODS: The ion recombination correction factor and polarity effect correction factor, sensitivity, and percentage depth doses (PDDs) of PPC05 and PPC40 parallel-plate ionization chambers were compared in a small-field electron beam. The output ratios were measured for 4-20 MeV electron beams with field sizes of 10 cm × 10 cm, 6 cm × 6 cm, and 4 cm × 4 cm. Furthermore, the films were placed in water and positioned in the beam with their surface perpendicular to the beam axis, and lateral profiles were obtained for each beam energy and each field. RESULTS: Regarding PDDs, at depths greater than the peak dose, the percentage depth dose for PPC40 was smaller than that for PPC05 in small fields and at beam energies greater than 12 MeV, which could be attributed to the lack of lateral electron equilibrium at small depths and multiple scattering events at large depths. The output ratio of PPC40 was approximately 0.025-0.038, which was lower than that of PPC05 in a 4 cm × 4 cm field. For large fields, the lateral profiles were similar, regardless of the beam energy, however, for small fields, the flatness of the lateral profile was beam energy dependent. CONCLUSION: The PPC05 chamber, which has a smaller ionization volume, is therefore more suitable than the PPC40 chamber for small-field electron dosimetry, in particular at high beam energies.
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Electrones , Radiometría , Humanos , AguaRESUMEN
Demand for large area parallel plate ionization chamber (PPIC) or large area ionization chamber (LAIC) has risen in recent years due to several advantages of the large effective area in monitoring therapeutic radiation beams. PPICs are designed for the measurements of beam profile and dosimetry in radiation therapy quality assurance (QA) procedures.Objective. Heterogeneous responses over the large sensitive area pose an undeniable concern for the straightforward applications of PPICs in clinical dosimetry. Uniformity calibration for the detector response is thus essential for the accurate performance of each PPIC unit.Approach.A large area XY strip PPIC, characterized by a large effective area of 345.44 × 345.44 mm2and 256 readout channels, was investigated in this study. A new systematic uniformity calibration is developed to improve the lateral response of the PPIC over the measurements for both narrow beams and large square field beams. A 2D response map of the PPIC was obtained by a spot-scanning method using a compact x-ray tube (mini x-ray). The mini x-ray, providing stable radiation (uncertainty <0.1%), was moved with a step size of 20 mm in 2 dimensions across the entire PPIC surface to collect a complete spot scan. Different uniformity calibration methods were introduced for the measurement of the PPIC by adopting the information from the detector 2D response map.Main results.Deviation of the detector response, before calibration, was observed to reach about 7% for the testing PPIC unit which is much higher than the recommended uniformity response of 1% (IAEA TRS-398). The uniformity response of the PPIC improved significantly to less than 1% across the detector surface after calibration.Significance.The proposed methods enable the practical application of PPIC in routine clinical dosimetry and can be reliably adopted by any radiation facility to perform daily and monthly QA.
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Radiometría , Calibración , Rayos X , Radiometría/métodos , IncertidumbreRESUMEN
PURPOSE: Air-vented ionization chambers have been the secondary standard for radiation dosimetry since the origins of radiation metrology. However, the feasibility of their use in ultra-high dose rate pulsed beams has been a matter of discussion, as large losses are caused by ion recombinations and no suitable theoretical model is available for their correction. The theories developed by Boag and his contemporaries since the 1950s, which have provided the standard ion recombination correction factor in clinical dosimetry, do not provide an accurate description when used under the limit conditions of ultra-high dose rates (UHDRs). Moreover, the high-ion recombination effects of ionization chambers under extreme dose-rate applications are an obstacle to the development of adequate dosimetry standards. METHODS: In this article, the charge carrier transport equations within a parallel plate ionization chamber (PPIC) have been solved numerically with a double aim. First, this numerical model provides a more accurate tool that can be used to evaluate ion recombination correction for established PPICs in pulsed ultra-high dose rate regimes. Second, studying the chamber behavior in detail allow as to explore the limits of new chamber designs in order to improve their performance under UHDRs. The model presented here has been tested by measuring the instantaneous current of one unit of a Roos chamber (i.e., the time-resolved current during and after the irradiation pulse under UHDR conditions) and comparing these results with the absolute value of the simulated current. RESULTS: The experimental data show consistent agreement with the results obtained using the numerical model. The experimental instantaneous current reveals effects such as the variation of the free electron fraction with the dose per pulse that are supported by the numerical model but cannot be explained in the framework of Boag's theory. CONCLUSIONS: Numerical solutions of the charge carrier released and transport in ionization chambers are able to estimate the effects observed when PPICs are irradiated with ultra-high dose rate beams and to provide new insight into processes related to recombination losses at UHDRs. These models can be reliably extended to include regions where current analytical solutions are not valid. An agreement of better than 5 % between the experimental and simulated effective free electron fraction is found. We were able to reproduce the instantaneous current from a Roos chamber. The discrepancies observed between the experimental data and the numerical simulations can be attributed to the uncertainty about the transport parameters involved in the calculation.
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Electrones , Radiometría , Radiometría/métodos , Modelos TeóricosRESUMEN
The free electron fraction is the fraction of electrons, produced inside the cavity of an ionization chamber after irradiation, which does not bind to gas molecules and thereby reaches the electrode as free electrons. It is a fundamental quantity to describe the recombination processes of an ionization chamber, as it generates a gap of positive charges compared to negative ones, which certainly will not undergo recombination. The free electron fraction depends on the specific chamber geometry, the polarizing applied voltage and the gas thermodynamic properties. Therefore, it is necessary to evaluate such fraction in an accurate and easy way for any measurement condition. In this paper, a simple and direct method for evaluating the free electron fraction of ionization chambers is proposed. We first model the capture process of the electrons produced inside an ionization chamber after the beam pulse; then we present a method to evaluate the free electron fraction based on simple measurements of collected charge, by varying the applied voltage. Finally, the results obtained using an Advanced Markus chamber irradiated with a Flash Radiotherapy dedicated research Linac (ElectronFlash) to estimate the free electron fraction are presented. The proposed method allows the use of a conventional ionization chamber for measurements in ultra-high-dose-per-pulse (UHDP) conditions, up to values of dose-per-pulse at which the perturbation of the electric field due to the generated charge can be considered negligible.
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Electrones , Radiometría , Radiometría/métodos , Aceleradores de PartículasRESUMEN
The performance characteristics of some widely employed parallel-plate ionization chambers in dosimetry of conventional high energy electron beams were evaluated and compared in the present study following the recommendations of the IAEA TRS-398 reference dosimetry protocol. Three different types of PTW-made parallel-plate ionization chambers including Roos (TM34001), Markus (TM23343), and Advanced Markus (TM34045) were employed, and correction factors for polarity (kpol), recombination (ks), and quality conversion factor ( [Formula: see text] ) were determined at different nominal electron energies of 4, 6, 9, 12, 16, and 20 MeV produced by a Varian Trilogy clinical Linac. All measurements were performed inside a MP3-M automatic water phantom in the reference condition of 100 cm SSD (source to surface distance), reference measurement depth (zref), and 10 × 10 cm2 field size at the phantom surface. The maximum and minimum range of kpol deviations from unity were respectively found for Markus and Roos ionization chambers. The maximum ks values also belonged to the Markus ionization chamber, while the minimum ks values were observed for the Advanced Markus chamber. The measured ks values through recommendations of the TRS-398 dosimetry protocol were in good accordance with those obtained by Jaffe-plot analysis for all considered ionization chambers. The type of employed ionization chamber can minimally affect the measured electron beam quality index (R50), while it can have a more considerable impact on [Formula: see text] value, especially in the case of the Markus chamber. From the results, it can be concluded that the Roos and Advanced Markus ionization chambers have a superior performance in the case of electron beam dosimetry, although all considered ionization chambers fulfilled the criteria requested by relevant reference dosimetry protocols.
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Dosímetros de Radiación , Radiometría/métodos , Electrones , Aceleradores de Partículas , Fantasmas de ImagenRESUMEN
The aim of this study is to investigate the perturbation effect of parallel-plate ionization chambers on the buildup dose measurement in transverse magnetic fields, using Monte Carlo (MC) simulation. The NACP-02 and ROOS parallel-plate chambers and a PTW31010 cylindrical chamber were modeled for buildup dose measurement in magnetic fields, using the EGSnrc/cavity code. The irradiation condition was set to a 10â¯×â¯10â¯cm2 field in a water phantom at a source-to-surface distance (SSD) of 100â¯cm, using 6-MV photon spectrum. Magnetic fields of 0 0.35, 1.0, 1.5, and 3.0â¯T were applied perpendicularly to the direction of the photon beam. The overall perturbation factor PQ,B for the ionization chambers in the magnetic fields was also calculated. The dose to water was enhanced with increasing the magnetic field strength at a depth of less than 1â¯cm. Over a depth of 1.5â¯cm, there was no significant difference in the depth doses with and without magnetic field in water. The maximum depth dose (%) for the NACP-02 and ROOS chambers at 1.5â¯T was higher up to 12% and 14% than the maximum depth dose at 0â¯T, respectively. The depth dose curves of a PTW31010 chamber have a similar tendency to those of water. The PQ,B values for each chamber were the largest at the phantom surface. The transverse magnetic field has a greater effect on the dose response of the NACP and ROOS chambers than that of the PTW31010 chamber in the buildup region.