Calibration Of Electron Beams Research Articles

Electron-beam calibration of aerogel tiles for the HELIX RICH detector

The HELIX cosmic-ray detector is a balloon-borne instrument designed to measure the flux of light isotopes in the energy range from 0.2 GeV/n to beyond 3 GeV/n. It will rely on a ring-imaging Cherenkov (RICH) detector for particle identification at energies greater than 1 GeV/n and will use aerogel tiles with refractive index near 1.15 as the radiator. To achieve the performance goals of the experiment it is necessary to know the refractive index and its position dependence over the lateral extent of the tiles to a precision of O(10−4). In this paper we describe the apparatus and methods developed to calibrate the HELIX tiles in an electron beam, in order to meet this requirement.

Modified electron beam output calibration based on IAEA Technical Report Series 398.

PurposeThe recently worldwide standard measurement of electron beam reference dosimetry include the International Atomic Energy Agency (IAEA) Technical Report Series (TRS)‐398 and Association of Physicists in Medicine (AAPM) Task Group (TG)‐51 protocols. Muir et al. have modified calibration methods for electron beam calibration based on AAPM TG‐51. They found that the use of cylindrical chambers at low energy gave acceptable results. In this study, we propose and report a modified calibration for electron beam based on IAEA TRS‐398, the standard reference dosimetry protocol worldwide.MethodsThis work was carried out with energies of 6, 8, 10, 12, and 15 MeV. The electron beam is generated from Elektra Synergy Platform and Versa HD linear accelerator. The charge readings were measured with PTW 30013, IBA CC13, Exradin A1Sl, and Exradin A11 chambers connected to the electrometer. The dose calculation uses an equation of modified calibration for electron beam using the updated kQ factor in previous work. The absorbed dose to water for electron beam is expressed in dose per monitor unit (cGy/MU). Thus, we compared dose per monitor unit (D/MU) calculation using a modified calibration to TRS‐398.ResultsIn this work, we have succeeded in implementing the modified calibration of electron beam based on TRS‐398 by applying a cylindrical chamber in all energy beams and using the updated kQ factor. The ratio of the absorbed dose to water between original and modified calibration protocols of TRS‐398 (D w) for the cylindrical chamber was 1.002 on the Elekta Synergy Platform and 1.000 on the Versa HD while for the parallel‐plate chamber it was 1.013 on the Elekta Synergy Platform and 1.014 on the Versa HD. Based on these results, both the cylindrical and parallel‐plate chambers are still within the tolerance limit allowed by the TRS‐398 protocol, which is ±2%. Therefore, modified calibration based on TRS‐398 gives acceptable results and is simpler to use clinically.

Scaling factors measurement for intraoperative electron beam calibration inside PMMA plastic phantom

Complicated irradiation setup inside the water for intraoperative electron dosimetry suggests the plastic phantoms as an alternative medium. Depth scaling (cpl) and fluence scaling (hpl) factors for intraoperative electron dosimetry inside the PMMA phantom were measured in this study. cpl and hpl values were obtained at electron energies of 6, 8, 10 and 12 MeV through ionometric measurement. Results showed that the mean cpl value (averaged over all energies) for intraoperative electron beam would be 0.960 (about 2% higher than reported value of 0.941 for conventional one). The mean hpl value for intraoperative electron beam at Zref was obtained as 0.954, which is about 5% lower than the reported values for conventional electron beam. The reported scaling factors, which are valid for a particular plastic phantom (PMMA) and specific type of dedicated IOERT accelerator (LIAC12), can reproduce the directly measured dose and depth dose distribution inside the water.

Structures, g‐tensors, and hyperfine coupling constants of L‐α‐alanine radicals in radiation dosimetry: An ab initio molecular dynamics simulation study

AbstractAlanine is used as a transfer standard dosimeter for gamma ray and electron beam calibration. An important factor affecting its dosimetric response is humidity which can lead to errors in absorbed dose calculations. Ab initio molecular dynamics calculations were performed to determine the environmental effects on the electron paramagnetic resonance (EPR) parameters of L‐α‐alanine radicals in acidic and alkaline solutions. A new result, not dissimilar to the closed‐shell amino acid molecule alanine, is that the non‐zwitterionic form of the alanine radical is the stable form in the gas phase while the zwitterionic neutral alanine radical is not a stable structure in the gas phase. Geometric and EPR parameters of radicals in both gas and solution phases are found to be dependent on hydrogen bonding of water molecules with the polar groups and on dynamic solvation. Calculations on the optimized free radicals in the gas phase revealed that for the neutral radical, hydrogen bonding to water molecules drives a decrease in the magnitudes of g‐tensor components g xx and g yy without affecting neither g zz component nor the hyperfine coupling constants (HFCCs). The transfer from the gas to solution phase of the alanine radical anion is accompanied with an increase in the spin density on the carboxylic group's oxygen atoms. However, for the neutral radical, this transfer from gas to solution phase is accompanied with the decrease in the spin density on oxygen atoms. Calculated isotropic HFCCs and g‐tensor of all radicals are in good agreement with experiment in both acidic and alkaline solutions.

A Fuzzy Inference System for Unsupervised Deblurring of Motion Blur in Electron Beam Calibration

This paper presents a novel method of restoring the electron beam (EB) measurements that are degraded by linear motion blur. This is based on a fuzzy inference system (FIS) and Wiener inverse filter, together providing autonomy, reliability, flexibility, and real-time execution. This system is capable of restoring highly degraded signals without requiring the exact knowledge of EB probe size. The FIS is formed of three inputs, eight fuzzy rules, and one output. The FIS is responsible for monitoring the restoration results, grading their validity, and choosing the one that yields to a better grade. These grades are produced autonomously by analyzing results of a Wiener inverse filter. To benchmark the performance of the system, ground truth signals obtained using an 18 μm wire probe were compared with the restorations. Main aims are therefore: (a) Provide unsupervised deblurring for device independent EB measurement; (b) improve the reliability of the process; and (c) apply deblurring without knowing the probe size. These further facilitate the deployment and manufacturing of EB probes as well as facilitate accurate and probe-independent EB characterization. This paper’s findings also makes restoration of previously collected EB measurements easier where the probe sizes are not known nor recorded.

Design and development of high pressure high temperature water circulation system for HHFTF

High Heat Flux Test Facility (HHFTF) at Institute for Plasma Research in Bhat in India is established for testing thermal performance of plasma facing materials and components under intense heat loads expected in plasma fusion devices. It mainly consists of the following sub-systems like vacuum system, high power electron beam system, diagnostic and calibration system, data acquisition & control system and high pressure high temperature water circulation system (HPHT-WCS). In order to test the water cooled test mock-ups or components at ITER like relevant parameters in HHFTF, HPHT-WCS is established that can supply water at desired pressures, temperatures and flow rates. HPHT-WCS, is capable of providing demineralised water up to 160°C temperature, 60bar pressure and 4.85kg/s mass flow rate. The loop is designed on a skid base frame which has piping connected to various process components and the system is operated remotely through a data acquisition and control system. The main requirements and characteristics, design and development of the HPHT-WCS are presented.

Design and performance of vacuum system for high heat flux test facility

High heat flux test facility (HHFTF) at IPR is used for testing thermal performance of plasma facing materials or components. It consists of various subsystems like vacuum system, high power electron beam system, diagnostic and calibration system, data acquisition and control system and high pressure high temperature water circulation system. Vacuum system consists of large D-shaped chamber, target handling system, pumping systems and support structure. The net volume of vacuum chamber is 5 m3 was maintained at the base pressure of the order of 10-6 mbar for operation of electron gun with minimum beam diameter which is achieved with turbo-molecular pump (TMP) and cryo pump. A variable conductance gate valve is used for maintaining required vacuum in the chamber. Initial pumping of the chamber was carried out by using suitable rotary and root pumps. PXI and PLC based faster real time data acquisition and control system is implemented for performing the various operations like remote operation, online vacuum data measurements, display and status indication of all vacuum equipments. This paper describes in detail the design and implementation of various vacuum system for HHFTF.

Use of parallel-plate ionization chambers in reference dosimetry of NOVAC and LIAC® mobile electron linear accelerators for intraoperative radiotherapy: a multi-center survey.

LIAC® and NOVAC are two mobile linear accelerators dedicated to intraoperative radiation therapy (IORT), generating electron beams in the energy range of 3-12MeV. Due to high dose-per-pulse (up to 70mGy per pulse), in 2003 the Italian National Institute of Health (ISS) stated that "for the measure of dose to water in reference conditions, ionization chambers cannot be employed and no published dosimetry protocol can be used". As a consequence, ferrous sulphate (or, alternatively alanine) dosimetry was recommended. Based on a retrospective multi-center survey, a comparison with ferrous sulphate dosimetry is now used to validate the parallel-plate ionization chambers for reference dosimetry of NOVAC and LIAC. The IAEA TRS-398 dosimetry protocol was applied except for the reference irradiation setup and the determination of the ion-recombination correction factor ks . Moreover the depth of maximum dose (R100 ) instead of zref as measurement depth was chosen by the majority of centers, thus implying a renormalization of the beam-quality correction factor kQ,Qo , based on water-air stopping power ratios. Regarding the ks determination, a previously published method, independent of ferrous sulphate dosimetry, was adopted. All the centers participating in this study had used both ferrous sulphate dosimeters and ionization chambers in water phantoms for dosimetry under reference conditions. The mean percentage difference between ionization chambers and ferrous sulphate dosimetry was -0.5% with a dispersion of 3.9% (2σ). Moreover, the uncertainty analysis allowed the agreement between ionization chambers and ferrous sulphate dosimetry to be verified. These results did not show any significant dependence on electron energy, thus indirectly confirming kQ,Qo renormalization. The results from the centers using zref as the measurement depth were similar to the other data, but further focused studies could aim at investigating possible dependences of the dose differences on the chosen reference depth. The present study confirms that parallel-plate ionization chambers can properly and accurately substitute ferrous sulphate detectors in reference dosimetry of LIAC and NOVAC mobile linear accelerators. Therefore, we hope that the most commonly used protocols for reference dosimetry in external-beam radiotherapy will be updated in order to provide guidance in the calibration of electron beams from linear accelerators dedicated to IORT, so that users may benefit from specific, authoritative and up-to-date recommendations.

Output Factor Measurement of a Dedicated Intra-Operative Radiotherapy (IORT) Accelerator Using Ethanol Chlorobenzene (ECB) Dosimeter and Comparing the Results with Ionometric Dosimetry and Monte Carlo Simulation

Background: The accurate dose delivery in intraoperative radiotherapy (IORT) tightly depends on the precision of measured dose by the ion chamber. Output factor (OF) measurement of dedicated intraoperative electron radiotherapy (IOERT) accelerators using ion chamber faces some technical challenges including determination of Ksat. Objectives: The goal was to evaluate the performance of ethanol chlorobenzene (ECB) dosimeter in measuring the OF of intraoperative electron beam and to compare the results to those of ionometric dosimetry and Monte Carlo simulation. Consequemtly we determined the Ksat of employed ion chamber through comparison of the ECB response to ion chamber. Materials and Methods: LIAC dedicated accelerator (LIAC Sordina SpA, Italy) was used for irradiation. To calculate the Ksat, ECB and Advanced Markus chamber were placed at the depth of maximum dose for different energies of LIAC accelerator. Then, Ksat was calculated through comparison of the obtained results. To measure the OF of electron beam, ECB was placed at the depth of maximum dose for each combination of energy/applicator size, and the absorbed dose was determined. Obtained results were compared to those of Advanced Markus chamber and Monte Carlo simulation. Results: The results of Ksat measurement showed that there is a very good agreement between the practically obtained Ksat and theoretical values determined by Laitano formalism at different energies (the maximum difference between the results was lower than 1%). The results of ECB OF measurement were in accordance to the results of ionometric dosimetry and Monte Carlo simulation (the maximum difference between the results was about 1.5% and 1.7%, respectively). Conclusion: Based on the results, it may be concluded that the ECB dosimeter could be considered as an accurate tool for both OF measurement of intraoperative electron beam and cross calibration of employed ion chambers for absolute dosimetry (determination of the Ksat).

WE‐G‐BRA‐04: Common Errors and Deficiencies in Radiation Oncology Practice

Purpose:Dosimetric errors in radiotherapy dose delivery lead to suboptimal treatments and outcomes. This work reviews the frequency and severity of dosimetric and programmatic errors identified by on‐site audits performed by the IROC Houston QA center.Methods:IROC Houston on‐site audits evaluate absolute beam calibration, relative dosimetry data compared to the treatment planning system data, and processes such as machine QA. Audits conducted from 2000‐present were abstracted for recommendations, including type of recommendation and magnitude of error when applicable. Dosimetric recommendations corresponded to absolute dose errors >3% and relative dosimetry errors >2%. On‐site audits of 1020 accelerators at 409 institutions were reviewed.Results:A total of 1280 recommendations were made (average 3.1/institution). The most common recommendation was for inadequate QA procedures per TG‐40 and/or TG‐142 (82% of institutions) with the most commonly noted deficiency being x‐ray and electron off‐axis constancy versus gantry angle. Dosimetrically, the most common errors in relative dosimetry were in small‐field output factors (59% of institutions), wedge factors (33% of institutions), off‐axis factors (21% of institutions), and photon PDD (18% of institutions). Errors in calibration were also problematic: 20% of institutions had an error in electron beam calibration, 8% had an error in photon beam calibration, and 7% had an error in brachytherapy source calibration. Almost all types of data reviewed included errors up to 7% although 20 institutions had errors in excess of 10%, and 5 had errors in excess of 20%. The frequency of electron calibration errors decreased significantly with time, but all other errors show non‐significant changes.Conclusion:There are many common and often serious errors made during the establishment and maintenance of a radiotherapy program that can be identified through independent peer review. Physicists should be cautious, particularly in areas highlighted herein that show a tendency for errors.

SU‐E‐T‐98: An Analysis of TG‐51 Electron Beam Calibration Correction Factor Uncertainty

Purpose:To analyze the uncertainty of the TG‐51 electron beam calibration correction factors for farmer type ion chambers currently used by institutions visited by IROC Houston.Methods:TG‐51 calibration data were collected from 181 institutions visited by IROC Houston physicists for 1174 and 197 distinct electron beams from modern Varian and Elekta accelerators, respectively. Data collected and analyzed included ion chamber make and model, nominal energy, ND,w, I5₀, R5₀, k'R5₀, dref, Pgr and pdd(dref). k'R5₀ data for parallel plate chambers were excluded from the analysis.Results:Unlike photon beams, electron nominal energy is a poor indicator of the actual energy as evidenced by the range of R5₀ values for each electron beam energy (6–22MeV). The large range in R5₀ values resulted k'R5₀ values with a small standard deviation but large range between maximum value used and minimum value (0.001–0.029) used for a specific Varian nominal energy. Varian data showed more variability in k'R5₀ values than the Elekta data (0.001–0.014). Using the observed range of R5₀ values, the maximum spread in k'R5₀ values was determined by IROC Houston and compared to the spread of k'R5₀ values used in the community. For Elekta linacs the spreads were equivalent, but for Varian energies of 6 to 16MeV, the community spread was 2 to 6 times larger. Community Pgr values had a much larger range of values for 6 and 9 MeV values than predicted. The range in Varian pdd(dref ) used by the community for low energies was large, (1.4–4.9 percent), when it should have been very close to unity. Exradin, PTW Roos and PTW farmer chambers ND,w values showed the largest spread, ≥11 percent.Conclusion:While the vast majority of electron beam calibration correction factors used are accurate, there is a surprising spread in some of the values used.

SU-D-19A-01: Can Farmer-Type Ionization Chambers Be Used to Improve the Accuracy of Low-Energy Electron Beam Reference Dosimetry?

Purpose: To investigate the use of cylindrical Farmer-type ionization chambers to improve the accuracy of low-energy electron beam calibration. Historically, these chamber types have not been used in beams with incident energies less than 10 MeV (R5 0 < 4.3 cm) because early investigations suggested large (up to 5 %) fluence perturbation factors in these beams, implying that a significant component of uncertainty would be introduced if used for calibration. More recently, the assumptions used to determine perturbation corrections for cylindrical chambers have been questioned. Methods: Measurements are made with cylindrical chambers in Elekta Precise 4, 8 and 18 MeV electron beams. Several chamber types are investigated that employ graphite walls and aluminum electrodes with very similar specifications (NE2571, NE2505/3, FC65-G). Depth-ionization scans are measured in water in the 8 and 18 MeV beams. To reduce uncertainty from chamber positioning, measurements in the 4 MeV beam are made at the reference depth in Virtual Water™. The variability of perturbation factors is quantified by comparing normalized response of various chambers. Results: Normalized ion chamber response varies by less than 0.7 % for similar chambers at average electron energies corresponding to that at the reference depth from 4 or 6 MeV beams. Similarly, normalized measurements made with similar chambers at the reference depth in the 4 MeV beam vary by less than 0.4 %. Absorbed dose calibration coefficients derived from these results are stable within 0.1 % on average over a period of 6 years. Conclusion: These results indicate that the uncertainty associated with differences in fluence perturbations for cylindrical chambers with similar specifications is only 0.2 %. The excellent long-term stability of these chambers in both photon and electron beams suggests that these chambers might offer the best performance for all reference dosimetry applications.

Current approach in clinical electron beam dosimetry

The absolute dosimetry calibration of clinical electron beams is increasingly based on the AAPM Task Group #51 (TG-51) protocol. In addition, recently published dosimetry data on electrons beams bring up the question of: how would one need to modify the widely used TG-25 that originally was based on the older AAPM Task Group #21 (TG-21) calibration protocol?

Characterization of a Fricke dosimeter at high energy photon and electron beams used in radiotherapy

The dosimetric features of the Fricke dosimeter in clinical linear accelerator beams are considered. Experimental data were obtained using various nominal energies 6 and 18 MV, 12 and 15 MeV, including the (60)Co γ-ray beam. The calibration of the dosimeters was performed using the ionization chamber as a reference dosimeter. Some general characteristics of Fricke dosimeter such as energy dependence, optical density (OD)-dose relationship, reproducibility, accuracy, dose rate dependence were analyzed. The Fricke solution shows linearity in OD-dose relationship, energy independence and a good reproducibility over the energy range investigated. The Fricke dosimeter was found to be suitable for carrying out absorbed dose to water measurements in the calibration of high energy electron and photon beams.

Recommendations for clinical electron beam dosimetry: Supplement to the recommendations of Task Group 25

The goal of Task Group 25 (TG-25) of the Radiation Therapy Committee of the American Association of.Physicists in Medicine (AAPM) was to provide a methodology and set of procedures for a medical physicist performing clinical electron beam dosimetry in the nominal energy range of 5-25 MeV. Specifically, the task group recommended procedures for acquiring basic information required for acceptance testing and treatment planning of new accelerators with therapeutic electron beams. Since the publication of the TG-25 report, significant advances have taken place in the field of electron beam dosimetry, the most significant being that primary standards laboratories around the world have shifted from calibration standards based on exposure or air kerma to standards based on absorbed dose to water. The AAPM has published a new calibration protocol, TG-51, for the calibration of high-energy photon and electron beams. The formalism and dosimetry procedures recommended in this protocol are based on the absorbed dose to water calibration coefficient of an ionization chamber at 60Co energy, N60Co(D,w), together with the theoretical beam quality conversion coefficient k(Q) for the determination of absorbed dose to water in high-energy photon and electron beams. Task Group 70 was charged to reassess and update the recommendations in TG-25 to bring them into alignment with report TG-51 and to recommend new methodologies and procedures that would allow the practicing medical physicist to initiate and continue a high quality program in clinical electron beam dosimetry. This TG-70 report is a supplement to the TG-25 report and enhances the TG-25 report by including new topics and topics that were not covered in depth in the TG-25 report. These topics include procedures for obtaining data to commission a treatment planning computer, determining dose in irregularly shaped electron fields, and commissioning of sophisticated special procedures using high-energy electron beams. The use of radiochromic film for electrons is addressed, and radiographic film that is no longer available has been replaced by film that is available. Realistic stopping-power data are incorporated when appropriate along with enhanced tables of electron fluence data. A larger list of clinical applications of electron beams is included in the full TG-70 report available at http://www.aapm.org/pubs/reports. Descriptions of the techniques in the clinical sections are not exhaustive but do describe key elements of the procedures and how to initiate these programs in the clinic. There have been no major changes since the TG-25 report relating to flatness and symmetry, surface dose, use of thermoluminescent dosimeters or diodes, virtual source position designation, air gap corrections, oblique incidence, or corrections for inhomogeneities. Thus these topics are not addressed in the TG-70 report.

SU‐FF‐T‐297: Total Electron Therapy Calibration with TG‐51

Purpose: To determine if slight modifications and assumptions can be made to allow for a TG‐51 based absolute dose calibration for a Total Skin Electron (TSE) Therapy program. We show the results of a TG‐51 based absolute dose calibration for TSE Therapy which are compared against three other dose calculation methods. Method and Materials: A TG‐51 calibrated Markus parallel‐plate chamber was used for our absolute dose calculation. This chamber was also used to determine the best gantry angles to produce dose uniformity and to determine the percent depth dose for the six dual field electron beams. Full rotation uniformity was analyzed with thermo luminescent dosimeters (TLDs') and clinical rotation of an anthropomorphic phantom to six different positions (Stanford technique.) Results: The absolute dose calibration of this TSE program was completed with slight modifications of a standard TG‐51 calculation which included extrapolation of the K′R50 value to 1.05 +/− 0.01 and extended target to skin distance (TSD) of 361cm. Verification measurements were done with TLDs, One‐Dose dosimeters, and a TG‐21 calibrated parallel‐plate. All four systems showed agreement within a maximum difference of 3.5%. The agreement between the TG‐21 method and the TG‐51 method was 2.3 %. Conclusion: We do not claim that our TSE absolute calibration is as accurate as a conventional 100 TSD therapy electron beam calibration done with all TG‐51 recommended parameters satisfied. We estimate an additional 2% error on the potential absolute dose calculation accuracy of a TG‐51 measurement. However, with the high degree of uncertainties involved in TSE therapy, our TSE calibration method offers a reasonably accurate absolute dose calibration involving only a few modifications and estimates and allows for the use of commonly used TG‐51 calibrated chambers.

Extrapolation chamber mounted on perspex for calibration of high energy photon and electron beams from a clinical linear accelerator.

The objective of the present study is to establish radiation standards for absorbed doses, for clinical high energy linear accelerator beams. In the nonavailability of a cobalt-60 beam for arriving at Nd, water values for thimble chambers, we investigated the efficacy of perspex mounted extrapolation chamber (EC) used earlier for low energy x-rays and beta dosimetry. Extrapolation chamber with facility for achieving variable electrode separations 10.5mm to 0.5mm using micrometer screw was used for calibrations. Photon beams 6 MV and 15 MV and electron beams 6 MeV and 15 MeV from Varian Clinac linacs were calibrated. Absorbed Dose estimates to Perspex were converted into dose to solid water for comparison with FC 65 ionisation chamber measurements in water. Measurements made during the period December 2006 to June 2008 are considered for evaluation. Uncorrected ionization readings of EC for all the radiation beams over the entire period were within 2% showing the consistency of measurements. Absorbed doses estimated by EC were in good agreement with in-water calibrations within 2% for photons and electron beams. The present results suggest that extrapolation chambers can be considered as an independent measuring system for absorbed dose in addition to Farmer type ion chambers. In the absence of standard beam quality (Co-60 radiations as reference Quality for Nd,water) the possibility of keeping EC as Primary Standards for absorbed dose calibrations in high energy radiation beams from linacs should be explored. As there are neither Standard Laboratories nor SSDL available in our country, we look forward to keep EC as Local Standard for hospital chamber calibrations. We are also participating in the IAEA mailed TLD intercomparison programme for quality audit of existing status of radiation dosimetry in high energy linac beams. The performance of EC has to be confirmed with cobalt-60 beams by a separate study, as linacs are susceptible for minor variations in dose output on different days.

Monte Carlo calculations of correction factors for plane‐parallel ionization chambers in clinical electron dosimetry

Recent standard dosimetry protocols recommend that plane-parallel ionization chambers be used in the measurements of depth-dose distributions or the calibration of low-energy electron beams with beam quality R50 <4 g/cm2. In electron dosimetry protocols with the plane-parallel chambers, the wall correction factor, Pwall, in water is assumed to be unity and the replacement correction factor, Prepl, is taken to be unity for well-guarded plane-parallel chambers, at all measurement depths. This study calculated Pwall and Prepl for NACP-02, Markus, and Roos plane-parallel chambers in clinical electron dosimetry using the EGSnrc Monte Carlo code system. The Pwall values for the plane-parallel chambers increased rapidly as a function of depth in water, especially at lower energy. The value around R50 for NACP-02 was about 10% greater than unity at 4 MeV. The effect was smaller for higher electron energies. Similarly, Prepl values with depth increased drastically at the region with the steep dose gradient for lower energy. For Markus Prepl departed more than 10% from unity close to R50 due to the narrow guard ring width. Prepl for NACP-02 and Roos was close to unity in the plateau region of depth-dose curves that includes a reference depth, dref. It was also found that the ratio of the dose to water and the dose to the sensitive volume in the air cavity for the plane-parallel chambers, Dw/[Dair]pp, at d(ref) differs significantly from that assumed by electron dosimetry protocols.

An experimental study of recombination and polarity effect in a set of customized plane parallel ionization chambers

Plane parallel ionization chambers are an important tool for dosimetry and absolute calibration of electron beams used for radiotherapy. Most dosimetric protocols require corrections for recombination and polarity effects, which are to be determined experimentally as they depend on chamber design and radiation quality. Both effects were investigated in electron beams from a linear accelerator (Varian 21CD) for a set of four tissue equivalent plane parallel ionization chambers customized for the present research by Standard Imaging (Madison WI). All four chambers share the same design and air cavity dimensions, differing only in the diameter of their collecting electrode and the corresponding width of the guard ring. The diameters of the collecting electrodes were 2 mm, 4 mm, 10 mm and 20 mm. Measurements were taken using electron beams of nominal energy 6 to 20 MeV in a 10 cm x 10 cm field size with a SSD of 100 cm at various depths in a Solid Water slab phantom. No significant variation of recombination effect was found with radiation quality, depth of measurement or chamber design. However, the polarity effect exceeded 5% for the chambers with small collecting electrode for an effective electron energy below 4 MeV at the point of measurement. The magnitude of the effect increased with decreasing electron energy in the phantom. The polarity correction factor calculated following AAPM protocol TG51 ranged from approximately 1.00 for the 20.0 mm chamber to less than 0.95 for the 2 mm chamber at 4.1 cm depth in a electron beam of nominally 12 MeV. By inverting the chamber it could be shown that the polarity effect did not depend on the polarity of the electrode first traversed by the electron beam. Similarly, the introduction of an air gap between the overlying phantom layer and the chambers demonstrated that the angular distribution of the electrons at the point of measurement had a lesser effect on the polarity correction than the electron energy itself. The magnitude of the absolute difference between charge collected at positive and negative polarity was found to correlate with the area of the collecting electrode which is consistent with the explanation that differences in thickness of the collecting electrodes and the number of electrons stopped in them contribute significantly to the polarity effect. Overall, the polarity effects found in the present study would have a negligible effect on electron beam calibration at a measurement depth recommended by most calibration protocols. However, the present work tested the corrections under extreme conditions thereby aiming at greater understanding of the mechanism underlying the correction factors for these chambers. This may lead to better chamber design for absolute dosimetry and electron beam characterization with less reliance on empirical corrections.

Calibration of low-energy electron beams from a mobile linear accelerator with plane-parallel chambers using both TG-51 and TG-21 protocols

A new approach to intraoperative radiation therapy led to the development of mobile linear electron accelerators that provide lower electron energy beams than the usual conventional accelerators commonly encountered in radiotherapy. Such mobile electron accelerators produce electron beams that have nominal energies of 4, 6, 9 and 12 MeV. This work compares the absorbed dose output calibrations using both the AAPM TG-51 and TG-21 dose calibration protocols for two types of ion chambers: a plane-parallel (PP) ionization chamber and a cylindrical ionization chamber. Our results indicate that the use of a ‘Markus’ PP chamber causes 2–3% overestimation in dose–output determination if accredited dosimetry-calibration laboratory based chamber factors are used. However, if the ionization chamber factors are derived using a cross-comparison at a high-energy electron beam, then a good agreement is obtained (within 1%) with a calibrated cylindrical chamber over the entire energy range down to 4 MeV. Furthermore, even though the TG-51 does not recommend using cylindrical chambers at the low energies, our results show that the cylindrical chamber has a good agreement with the PP chamber not only at 6 MeV but also down to 4 MeV electron beams.