Parallel Plate Ionization Chamber Research Articles

On the perturbation effect and LET dependence of beam quality correction factors in carbon ion beams.

In a recent study, we reported beam quality correction factors, fQ , in carbon ion beams using Monte Carlo (MC) methods for a cylindrical and a parallel-plate ionization chamber (IC). A non-negligible perturbation effect was observed; however, the magnitude of the perturbation correction due to the specific IC subcomponents was not included. Furthermore, the stopping power data presented in the International Commission on Radiation Units and Measurements (ICRU) report 73 were used, whereas the latest stopping power data have been reported in the ICRU report 90. The aim of this study was to extend our previous work by computing fQ correction factors using the ICRU 90 stopping power data and by reporting IC-specific perturbation correction factors. Possible energy or linear energy transfer (LET) dependence of the fQ correction factor was investigated by simulating both pristine beams and spread-out Bragg peaks (SOBPs). The TOol for PArticle Simulation (TOPAS)/GEANT4 MC code was used in this study. A 30 × 30 × 50 cm3 water phantom was simulated with a uniform 10 × 10 cm2 parallel beam incident on the surface. A Farmer-type cylindrical IC (Exradin A12) and two parallel-plate ICs (Exradin P11 and A11) were simulated in TOPAS using the manufacturer-provided geometrical drawings. The fQ correction factor was calculated in pristine carbon ion beams in the 150-450 MeV/u energy range at 2 cm depth and in the middle of the flat region of four SOBPs. The kQ correction factor was calculated by simulating the fQo correction factor in a 60 Co beam at 5 cm depth. The perturbation correction factors due to the presence of the individual IC subcomponents, such as the displacement effect in the air cavity, collecting electrode, chamber wall, and chamber stem, were calculated at 2 cm depth for monoenergetic beams only. Additionally, the mean dose-averaged and track-averaged LET was calculated at the depths at which the fQ was calculated. The ICRU 90 fQ correction factors were reported. The pdis correction factor was found to be significant for the cylindrical IC with magnitudes up to 1.70%. The individual perturbation corrections for the parallel-plate ICs were <1.0% except for the A11 pcel correction at the lowest energy. The fQ correction for the P11 IC exhibited an energy dependence of >1.00% and displayed differences up to 0.87% between pristine beams and SOBPs. Conversely, the fQ for A11 and A12 displayed a minimal energy dependence of <0.50%. The energy dependence was found to manifest in the LET dependence for the P11 IC. A statistically significant LET dependence was found only for the P11 IC in pristine beams only with a magnitude of <1.10%. The perturbation and kQ correction factor should be calculated for the specific IC to be used in carbon ion beam reference dosimetry as a function of beam quality.

Uniformity calibration for large area XY strip parallel plate ionization chamber

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 mm2 and 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.

Precise measurement of the Bragg curve for 800 MeV/u 238U ions stopping in polyethylene and its implications for calculation of heavy ion ranges

Stopping power predictions in radiation transport codes are based on the Bethe-Bloch formula and different corrections. For very heavy ions at relativistic energies the available experimental data are scarce and therefore verification of stopping power predictions is only possible to a limited extent.In this work, a full experimental Bragg curve for 800 MeV/u 238U ions stopping in polyethylene is presented. The measurements were conducted at the experimental area Cave A at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. The 800 MeV/u 238U beam was provided by the SIS18 heavy ion synchrotron. The Bragg curve was measured with a setup consisting of a binary range shifter and two large area parallel plate ionization chambers. Complementary Monte Carlo simulations using the FLUKA code were performed and compared with the experimental Bragg curve. The mean ionization potential of polyethylene was fine-tuned to match the measured primary ion range with FLUKA simulations. The impact of the Bloch and Mott corrections to the stopping power calculation were studied by switching them off intentionally in separate simulations. A detailed description of the implementation of the stopping power formulae and the Mott correction in FLUKA is provided.Supplementary data for this article is available online.

Fluence Beam Monitor for High-Intensity Particle Beams Based on a Multi-Gap Ionization Chamber and a Method for Ion Recombination Correction

This work presents the tests of a multi-gap detector (MGD), composed of three parallel-plate ionization chambers (ICs) with different gap widths, assembled to prove the capability of correcting for charge volume recombination which is expected to occur when high fluence rates are delivered. Such beam conditions occur with a compact accelerator for charged particle therapy developed to reduce the costs, to accomplish faster treatments and to exploit different beam delivery techniques and dose rates as needed, for example, for range modulation and FLASH irradiations, respectively. The MGD was tested with carbon ions at the Centro Nazionale di Adroterapia Oncologica (CNAO Pavia, Italy), and with protons in two different beam lines: at Bern University Hospital with continuous beams and at the Laboratori Nazionale del Sud (Catania, Italy) of the Italian National Center of Nuclear Physics (INFN) with pulsed beams. For each accelerator, we took measurements with different beam intensities (up to the maximum rate of ionization achievable) and changed the detector bias voltage (V) in order to study the charge collection efficiency. Charge recombination models were used to evaluate the expected collected charge and to measure the linearity of the rate of ionization with the beam fluence rate. A phenomenological approach was used to determine the collection efficiency (f1) of the chamber with thinnest gap from the relative efficiencies, f1/f2 and f1/f3, exploiting the condition that, for each measurement, the three chambers were exposed to the same rate of ionization. Results prove that two calibration curves can be determined and used to correct the online measurements for the charge losses in the ICs for recombination.

PO14: Evaluation of Volume Averaging Effect for Output Measurement of Leipzig Surface Applicators Using a Parallel Plate Ionization Chamber

<h3>Purpose</h3> Leipzig surface applicators are used for skin cancer treatment in high-dose-rate (HDR) brachytherapy. In clinical settings, physicians prescribe dose to a point at a depth between 3 to 5 mm for treatment planning. Therefore, it is important to accurately measure this point dose (related to the applicator's output). A parallel plate ionization chamber is a favorable option because it has a flat detection surface that fits the geometry of the applicator's treatment plane. This facilitates an accurate source-to-surface distance (SSD) setup, which is extremely important for the dose measurement because the applicator has a very short SSD. In this study, we quantified the volume averaging effect for using a Roos parallel plate ionization chamber to measure outputs of the Leipzig applicators. <h3>Materials and Methods</h3> The applicators (H10, H20 and H30) have three sizes (10, 20 and 30 mm). For each size, we measured the applicator's profiles with radiochromic films that were placed at depths of 0, 3 and 5 mm in solid water phantom. An Elekta Flexitron HDR ¹⁹²Ir source was used with the applicators to deliver radiation to the films. We digitized the films with a flatbed scanner and measured the intensities along the crossline profile at two locations: at radiation center and at 7.5 mm away from the center. We then calculated the ratios of the two intensities. The key element behind this design is the Roos chamber has an active, circular detecting area of 7.5 mm radius. Thus, the two points were selected at the center and the edge of the Roos chamber. In this way, the ratio quantifies the volume averaging effect (1 being no volume averaging). <h3>Results</h3> The ratios for H10 were significantly higher than those for H20 and H30. At depths of 0 and 3 mm, H10 had ratios of 1.46 and 1.49, respectively, and H20 and H30 had an averaged ratio of 1.08, indicating a 38.0% to 41.0% difference in heterogeneity. This evidence suggests the Roos chamber has significantly higher volume averaging effect for H10. The heterogeneity difference decreased at 5 mm depth. However, H10 still had approximately 10.0% higher ratio than H20 and H30, which had ratios closer to 1 at all three depths. <h3>Conclusions</h3> We found the Roos parallel plate ionization chamber had overall low volume averaging effect for output measurement of Leipzig surface applicators. However, it should not be used with the smallest applicator.

Numerical modeling of air-vented parallel plate ionization chambers for ultra-high dose rate applications

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.

Surface Dose and Skin Toxicity in Low-Magnetic Field MR-Linac

<h3>Purpose/Objective(s)</h3> Radiation induced skin toxicity is a common side effect that results in poor patient satisfaction and unplanned treatment breaks thus prolonging the overall treatment time with poor outcome. It is hypothesized that treatments on low-field MR-Linac will not produce higher surface dose which is investigated in this study. <h3>Materials/Methods</h3> Skin dose has always been an area of active clinical interest that is dependent on many parameters including beam energy, field size, material thickness and atomic number in the beam, material distance from surface, source to surface distance, beam angle (obliquity factor), concomitant chemotherapy etc. Along with these parameters, magnetic field is also a critical parameter that affect surface dose. With the development of the MRL system focus has shifted towards Lawrence force giving rise to electron return effect along with other treatment parameters affecting the skin dose. A Linac equipped with 6MV FFF photons with 90 cm source to axis distance is used. A Markus parallel plate ionization chamber is used in a solid water phantom with proper correction factor to investigate surface and buildup dose with six different gantry angles 15⁰ apart gantry (equivalent of standard Linac 0⁰, 15⁰, 30⁰, 45⁰, 60⁰, and 75⁰) for field sizes of 1.66 × 1.66 cm² (small), 9.96 × 9.9 cm² (reference) and 24.1 × 24.1 cm² (maximum). Measurements were made with increasing buildup thickness in steps of 1 mm to 20 mm of solid water in 0.35 T magnetic field. <h3>Results</h3> In traditional linear accelerators surface dose is 30-40% for a 6MV photon beam, without any modifier. This value increases significantly with beam angle. In MR-Linac, surface dose is relatively low, 12.6%, 14.9% and 16.7% for small, reference and maximum fields at zero-degree gantry angle, respectively. These values are 12% to 2% higher (at 0⁰ and 75⁰ respectively) for maximum field size compared to the reference field size. For lower field sizes minimum variation of 10% was observed at 75° for reference field size. The surface dose increases with increasing radiation field size but magnitude is still lower as the case with traditional lilacs. Table 1 shows the surface dose with field size and gantry angles. <h3>Conclusion</h3> Surface dose in low-field magnetic environment is relatively lower than traditional accelerators. It is concluded that low-field MR-Linac system may not add any additional skin toxicity compared to conventional Linac. However, the effect may be different for high-field MR-Linac system, and appropriate skin evaluation needs to be performed to differentiate between radiation induced toxicity specially for the oblique beams versus RF-thermal effect.

Assessment of Surface and Build-up Doses for a 6 MV Photon Beam using Parallel Plate Chamber, EBT3 Gafchromic Films, and PRIMO Monte Carlo Simulation Code.

Background: Accurate assessment of surface and build-up doses has a key role in radiotherapy, especially for the superficial lesions with uncertainties involved while performing measurements in the build-up region.Objective: This study aimed to assess surface and build-up doses for 6 MV photon beam from linear accelerator using parallel plate ionization chamber, EBT3 Gafchromic films, and PRIMO Monte Carlo (MC) simulation code.Material and Methods: In this experimental study, parallel plate chamber (PPC05) and EBT3 Gafchromic films were used to measure doses in a build-up region for 6 MV beam from the linear accelerator for different field sizes at various depths ranging from 0 to 2 cm from the surface with 100 cm source to surface distance (SSD) in a solid water phantom. Measured results were compared with Monte Carlo simulated results using PENELOPE-based PRIMO simulation code for the same setup conditions. Effect of gantry angle incidence and SSD were also analyzed for depth doses at the surface and build-up regions using PPC05 ion chamber and EBT3 Gafchromic films.Results: Doses measured at the surface were 14.78%, 19.87%, 25.83%, and 31.54% for field sizes of 5×5, 10×10, 15×15, and 20×20 cm2, respectively for a 6 MV photon beam with a parallel plate chamber and 14.20%, 19.14%, 25.149%, and 30.90%, respectively for EBT3 Gafchromic films. Both measurement sets were in good agreement with corresponding simulated results from the PRIMO MC simulation code; doses increase with the increase in field sizes.Conclusion: Good agreement was observed between the measured depth doses using parallel plate ionization chamber, EBT3 Gafchromic films, and the simulated depth doses using PRIMO Monte Carlo simulation code.

Evaluation of the polarity effect of Roos parallel plate ionization chamber in build-up region

Abstract Purpose: Despite widespread studying of the polarity effect of Roos parallel plate ion chamber in electron beams as mentioned in several protocols, no investigations have up till now studied this effect in photon beams in the build-up region. It is important to examine its polarity effect in the build-up region for photon beams, so this is the first work that focuses in to evaluate the polarity effect of the Roos chamber in the surface and build-up region and comparing its effect with other chambers. Methods: In this study, the Roos chamber was irradiated by a Theratron 780E 60Co beam to a known polarity effect. The Polarity effects of 5×5 up to 35×35 cm2 field sizes at positive and negative polarizing voltages were measured in the build-up region from surface to 0.7 cm in a solid water phantom. Results: The polarity ratios (PRs) were obtained at 1.020 ± 0.00 and 1.015 ± 0.00 for field sizes 5 × 5 up to 35 × 35 cm2, respectively. For the same fields, the percentage of polarity effects (%PEs) was obtained at 1.99% ± 0.00% and 1.47% ± 0.02%, respectively. The results found that the %PEs decrease with increased field sizes and depths. Moreover, the %PEs exhibited a decrease with an increased percentage surface dose (%SD). The uncertainty of %PE was estimated as 0.01% for all measurements in this study. Conclusions: As a result, the average %PE of the Roos chamber described here is equal to 0.756% ± 0.013% for all depths and field sizes for the 60Co γ-ray beam. It has introduced a less percentage of polarity effect than other chambers.

A 2D strip ionization chamber array with high spatiotemporal resolution for proton pencil beam scanning FLASH radiotherapy.

Experimental measurements of two-dimensional (2D) dose rate distributions in proton pencil beam scanning (PBS) FLASH radiation therapy (RT) are currently lacking. In this study, we characterize a newly designed 2D strip-segmented ionization chamber array (SICA) with high spatial and temporal resolution and demonstrate its applications in a modern proton PBS delivery system at both conventional and ultrahigh dose rates. A dedicated research beamline of the Varian ProBeam system was employed to deliver a 250-MeV proton PBS beam with nozzle currents up to 215nA. In the research and clinical beamlines, the spatial, temporal, and dosimetric performances of the SICA were characterized and compared with measurements using parallel-plate ion chambers (IBA PPC05 and PTW Advanced Markus chamber), a 2D scintillator camera (IBA Lynx), Gafchromic films (EBT-XD), and a Faraday cup. A novel reconstruction approach was proposed to enable the measurement of 2D dose and dose rate distributions using such a strip-type detector. The SICA demonstrated a position accuracy of 0.12±0.02mm at a 20-kHz sampling rate (50μs per event) and a linearity of R2 >0.99 for both dose and dose rate with nozzle beam currents ranging from 1 to 215nA. The 2D dose comparison to the film measurement resulted in a gamma passing rate of 99.8% (2mm/2%). A measurement-based proton PBS 2D FLASH dose rate distribution was compared to simulation results and showed a gamma passing rate of 97.3% (2mm/2%). The newly designed SICA demonstrated excellent spatial, temporal, and dosimetric performances and is well suited for commissioning, quality assurance, and a wide range of clinical applications in proton PBS clinical and FLASH-RT.

Effect of electrode separation and electrode backscatter thickness of a parallel plate ionization chamber on the measurement of dose to tissue in beta radiation fields

Plane parallel plate ionization chamber (PPC) is used in dosimetry especially for beta particles and low energy electron beam. The response of the PPC is affected by the electrode separation, thickness and material of backscatterer. The effect of electrode separation arises due to the well-known inscattering effect of electron which causes fluence perturbation inside the chamber cavity. The perturbation is caused due to reduced electron scattering in the chamber cavity compared to an ideal phantom material and it depends on the thickness of the cavity. Furthermore, the response of PPC also gets affected by the material behind the air cavity. Variation in the response depends on the thickness and the atomic number of the material behind collecting electrode of the PPC. The theoretical studies on the effect of collecting electrode backscatter thickness and electrode separation on the response of a PPC used as a transfer standard in ISO 6980 reference beta radiation field from 85Kr, 90Sr–90Y and 106Ru–106Rh radionuclides are presented. Multi-particle transport code FLUKA is used for the studies. The Polymethyl Methacrylate (PMMA) plate having 3.5 mm thickness is found to provide full backscatter for the above beta radionuclides. It is also observed that even for a well-guarded PPC, as the chamber electrode separation increases, the measured depth dose curve deviates from the ideal depth dose curve and the effective point of measurement (EPOM) of the PPC shifts towards downward direction from chamber reference point. It is also observed that the deviation between ideal and measured depth dose curve (related to EPOM shift) depends on the cavity thickness of the PPC. In the present work, optimization of design parameters of a PPC is carried out to establish it as a transfer standard in compliance with ISO 6980 for the standardization of reference beta radiation fields from 85Kr, 90Sr–90Y and 106Ru–106Rh radionuclide.

Development of an ultra-thin parallel plate ionization chamber for dosimetry in FLASH radiotherapy.

BackgroundConventional air ionization chambers (ICs) exhibit ion recombination correction factors that deviate substantially from unity when irradiated with dose per pulse magnitudes higher than those used in conventional radiotherapy. This fact makes these devices unsuitable for the dosimetric characterization of beams in ultra‐high dose per pulse as used for FLASH radiotherapy.PurposeWe present the design, development, and characterization of an ultra‐thin parallel plate IC that can be used in ultra‐high dose rate (UHDR) deliveries with minimal recombination.MethodsThe charge collection efficiency (CCE) of parallel plate ICs was modeled through a numerical solution of the coupled differential equations governing the transport of charged carriers produced by ionizing radiation. It was used to find out the optimal parameters for the purpose of designing an IC capable of exhibiting a linear response with dose (deviation less than 1%) up to 10 Gy per pulse at 4 μs pulse duration. As a proof of concept, two vented parallel plate IC prototypes have been built and tested in different ultra‐high pulse dose rate electron beams.ResultsIt has been found that by reducing the distance between electrodes to a value of 0.25 mm it is possible to extend the dose rate operating range of parallel plate ICs to ultra‐high dose per pulse range, at standard voltage of clinical grade electrometers, well into several Gy per pulse. The two IC prototypes exhibit behavior as predicted by the numerical simulation. One of the so‐called ultra‐thin parallel plate ionization chamber (UTIC) prototypes was able to measure up to 10 Gy per pulse, 4 μs pulse duration, operated at 300 V with no significant deviation from linearity within the uncertainties (ElectronFlash Linac, SIT). The other prototype was tested up to 5.4 Gy per pulse, 2.5 μs pulse duration, operated at 250 V with CCE higher than 98.6% (Metrological Electron Accelerator Facility, MELAF at Physikalisch‐Technische Bundesanstalt, PTB).ConclusionsThis work demonstrates the ability to extend the dose rate operating range of ICs to ultra‐high dose per pulse range by reducing the spacing between electrodes. The results show that UTICs are suitable for measurement in UHDR electron beams.

Dosimetry of absolute dose to water from elekta synergy platform linear accelerator using clinical electron beam

The use of electron beams in external beam radiation therapy started in 1930 but was limited to very few medical centers due to manual calculations of dose. The development of Van de Graaff generators, Betatrons, and various generations of commercial linear accelerators led to Elekta Synergy platform linear accelerators which were capable of performing dose calculations using computer software. In the present paper, we present a study on dose calculations for electron energies in the range of 4-15 MeV. This study was pursued following the recommendations of the International Commission on Radiation Units and Measurements (ICRU) and International Atomic Energy Agency (IAEA) reports and protocols of the international code of practice TRS-381. It is based on an experimental setup using a radiation field analyzer (RFA). This comprises of PTW-Freiburg Parallel plate ionization chamber (waterproof with cap, Model TM 23343) with a sensitive volume of 0.055 cm3 and a semi-flex chamber (waterproof with cap, Model TM 30010) with a sensitive volume of 0.125 cm3 both coupled with a digital electrometer (PTW UNIDOSE). The measurements were performed on a linear accelerator (Make &amp; Model: ELEKTA, Synergy Platform).

Impact of detector selection on commissioning of Leipzig surface applicators with improving immobilization in high-dose-rate brachytherapy

PURPOSECommission and treatment setup of Leipzig surface applicators, because of the steep dose gradient and lack of robust immobilization, is challenging. We aim to improve commissioning reliability by investigating the impact of detector choice on percentage depth dose (PDD) verifications, and to enhance accuracy and reproducibility in calibration/treatment setup through a simple and novel immobilization device. METHODS AND MATERIALSPDD distributions were measured with radiochromic films, optically stimulated luminescent dosimeters (OSLDs), a diode detector, and both cylindrical and parallel plate ionization chambers. The films were aligned to the applicators in parallel and transverse orientations. PDD data from a benchmarking Monte Carlo (MC) study were compared with the measured results, where surface doses were acquired from extrapolation. To improve setup accuracy and reproducibility, a custom-designed immobilization prototype device was made with cost-effective materials using a 3D printer. RESULTSThe measured PDD data with different detectors had an overall good agreement (<±10%). The parallel plate ionization chamber reported unreliable doses for the smallest applicator. There was no remarkable dose difference between the two film setups. The two-in-one prototype device provided a rigid immobilization and a flexible positioning of the applicator. It enhanced accuracy and reproducibility in calibration and treatment setup. CONCLUSIONWe recommend using radiochromic films in the transverse orientation for a reliable and efficient PDD verification. The applicator's clinical applicability has been limited by a lack of robust immobilization. We expect this economical, easy-to-use prototype device can promote the use of Leipzig applicators in surface brachytherapy.

Technical note: Bremsstrahlung dose in the electron beam at extended distances in total skin electron therapy.

Electron beam from a linear accelerator is commonly used in total skin electron therapy (TSET) at extended distances. Since Das et al (Med Phys 21, p.1733, 1994) reported 5% bremsstrahlung dose for a 6 MeV electron beam at extended distance of 500cm it has been accepted as common knowledge. However, measurements by Chen et al (Int J. Rad Onc Biol Phys 59 p.872, 2004) and Monte Carlo simulations by Ding et al (Phys. Med. Biol. 66, 075010, 2021) were unable to reproduce such high bremsstrahlung dose. As bremsstrahlung dose contributes to whole-body dose, which could produce bone marrow toxicity with serious complications for the outcome of the TSET, it is important to re-evaluate the magnitude of bremsstrahlung dose accurately. The EGSnrc Monte Carlo system is used to investigate bremsstrahlung doses from 6 MeV high dose rate total skin electron (HDTSe) beams from Varian TrueBeam and Clinac Accelerators. The measurements were carried out at a depth of dmax and 5cm in Solid Water and Acrylic phantoms at extended distances using a parallel-plate chamber and a cylindrical ion chamber. We were able to reproduce previously reported high bremsstrahlung dose at extended distances by using a parallel plate ionization chamber. However, both the measurements made by using a cylindrical chamber and Monte Carlo simulations showed an insignificant bremsstrahlung dose (∼1%) even at SSD=500cm. The bremsstrahlung doses of a 6 MeV electron beam are 0.5% to 1% for SSD from 100 to 700cm, although it increases with the increasing extended distance. The common belief of up to 5% bremsstrahlung dose at large extended distances is incorrect. Previously reported high bremsstrahlung doses might be due to poor signal-to-noise ratio of using a parallel plate chamber for measuring very low dose or particular setup.

Experimental investigation of ion recombination in a locally made low-voltage ionization chamber

In this study, the weak alpha radiation sources and the beta radiation by the effect of them on the electrical conductivity of air inside the ionization chamber are studied by measuring the produced current. The aim of this study is to make an ionization chamber to measure the ionization caused by weak radioactive sources which produce the electrical current at the order of femto- to pico-amperes and try to estimate the ion recombination coefficient at low ion densities. A parallel-plate ionization chamber with proper shielding and noise reduction circuits are made which measure data and sent them to a personal computer by USB port. The current-voltage characteristics of the ion chamber are measured when different radioactive sources are placed inside the chamber at the different separation of electrodes. The recombination constant obtained is estimated equal to 1.18 × 10−6 cm3/s. It is shown that current-voltage characteristic for ion chamber with different electrode separation lies on a unique curve by a proper normalization in the pico- and femto-ampere current range.

Design of the front-end electronics for parallel plate ion chamber

Design of the front-end electronics for parallel plate ion chamber

Comparing the dosimeter-specific corrections for influence quantities of some parallel-plate ionization chambers in conventional electron beam dosimetry

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 (kQ,Q0) 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 kQ,Q0 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.

Impact of Dose Calculation Algorithm on Skin Dose for Breast Cancer Treated With Intensity Modulated Proton Therapy

<h3>Purpose/Objective(s)</h3> Historically, pencil beam algorithms (PBA), have been used for intensity-modulated proton therapy (IMPT) dose calculation. Recently, improvements in calculation speeds and a desire to generate more accurate plans have led to wider adoption of Monte Carlo (MC) algorithms for IMPT. Our goal was to ascertain the impact of both PBA and MC calculation algorithms on the skin dose in breast cancer treated with IMPT by means of measurements. <h3>Materials/Methods</h3> Twenty-one comprehensive chest wall patients were included in this study with treatment planning performed using a treatment planning system. All IMPT plans were generated with 1 or 2 en-face fields. Single- and multiple-field-optimization were used in 19 and 2 patients, respectively. The prescription doses were 50.4 Gy for 15 and 45 Gy for 6 patients, all at 1.8 Gy/fraction. The skin OAR was defined as a 3 mm thick rind anterior to the chest wall target volume. For 12 patients, the skin dose was optimized with the objectives of V95% < = 50%, V100% < = 5%, and D0.03cc < = 50.9 Gy. For the remaining 9 patients, the skin dose was optimized to control D0.03cc alone. The physical skin dose was measured with a parallel plate ionization chamber and a 2D ionization chamber array device at various depths in solid water ranging from 0.2 to 1.3 cm. The agreement of calculated to measured dose was evaluated by 2D gamma analysis (3%/3mm) and point dose comparison. <h3>Results</h3> The average gamma passing rates for PBA and MC plans were 94% and 97%, respectively. The point dose comparison showed that the measured skin doses were systematically lower than the calculated dose in PBA plans by an average of 3.1% [range: -6.76% to 0.03%], whereas the measured doses were systematically higher than the calculated dose in MC plans by an average of 1.3% [range: -0.58% to 3.51%]. For the patients who received 50.4 Gy, the skin mean dose and D1% were 48.1 [range: 45.8 to 49.6] and 50.8 [range: 49.0 to 52.6] Gy. For the patients who received 45 Gy, the skin mean dose and D1% were 45.0 [range: 44.2 to 46.2] and 46.8 [range: 46.1 to 48.0] Gy. <h3>Conclusion</h3> This study demonstrated that comparing with the PBA algorithm generated plans, the MC generated plans yield a higher gamma passing rate (97% for MC vs. 94% for PBA), and a narrower 95% of confidence interval of skin dose (4.1 for MC vs. 6.8 for PBA). Measurements showed that MC computed plans systematically under-estimate the skin dose by an average of 1.3%, in contrast to PBA computed plans which systematically over-estimate the skin dose by an average of 3.1%. The clinical implications are that for individual patient, PBA could overestimate skin dose considerably more than MC. Therefore, cautions for skin dose should to be taken when PBA is replaced by MC in clinical practice, and it is recommended that appropriate optimization objectives be used during treatment planning to control skin dose.

Monte Carlo-derived ionization chamber correction factors in therapeutic carbon ion beams

The accuracy of electromagnetic transport in the GEANT4 Monte Carlo (MC) code was investigated for carbon ion beams and ionization chamber (IC)-specific beam quality correction factors were calculated. This work implemented a Fano cavity test for carbon ion beams in the 100–450 MeV/u energy range to assess the accuracy of the default electromagnetic physics parameters. The Urban and the Wentzel-VI multiple Coulomb scattering models were evaluated and the impact of maxStep, dRover, and final range parameters on the accuracy of the transport algorithm was investigated. The optimal production thresholds for an accurate calculation of values, which is the product of the water-to-air stopping power ratio and the IC-specific perturbation correction factor, were also studied. The correction factors were calculated for a cylindrical and a parallel-plate IC using carbon ions in the 150–450 MeV/u energy range. Modifying the default electromagnetic physics parameters resulted in a maximum deviation from theory of 0.3%. Therefore, the default EM parameters were used for the remainder of this work. The factors were found to converge for both ICs with decreasing production threshold distance below 5 μm. The values obtained in this work agreed with the TRS-398 stopping power ratios and other previously reported results within uncertainty. This study highlights an accurate MC-based technique to calculate the combined stopping power ratio and the perturbation correction factor for any IC in carbon ion beams.