Comments on `Ion recombination corrections for plane-parallel and thimble chambers in electron and photon radiation'

High dose-per-pulse electron beam dosimetry - A model to correct for the ion recombination in the Advanced Markus ionization chamber

PURPOSE

The purpose of this work was to establish an empirical model of the ion recombination in the Advanced Markus ionization chamber for measurements in high dose rate/dose-per-pulse electron beams. In addition, we compared the observed ion recombination to calculations using the standard Boag two-voltage-analysis method, the more general theoretical Boag models, and the semiempirical general equation presented by Burns and McEwen.

METHODS

Two independent methods were used to investigate the ion recombination: (a) Varying the grid tension of the linear accelerator (linac) gun (controls the linac output) and measuring the relative effect the grid tension has on the chamber response at different source-to-surface distances (SSD). (b) Performing simultaneous dose measurements and comparing the dose-response, in beams with varying dose rate/dose-per-pulse, with the chamber together with dose rate/dose-per-pulse independent Gafchromic™ EBT3 film. Three individual Advanced Markus chambers were used for the measurements with both methods. All measurements were performed in electron beams with varying mean dose rate, dose rate within pulse, and dose-per-pulse (10^-2  ≤ mean dose rate ≤ 10³ Gy/s, 10²  ≤ mean dose rate within pulse ≤ 10⁷  Gy/s, 10^-4  ≤ dose-per-pulse ≤ 10¹  Gy), which was achieved by independently varying the linac gun grid tension, and the SSD.

RESULTS

The results demonstrate how the ion collection efficiency of the chamber decreased as the dose-per-pulse increased, and that the ion recombination was dependent on the dose-per-pulse rather than the dose rate, a behavior predicted by Boag theory. The general theoretical Boag models agreed well with the data over the entire investigated dose-per-pulse range, but only for a low polarizing chamber voltage (50 V). However, the two-voltage-analysis method and the Burns & McEwen equation only agreed with the data at low dose-per-pulse values (≤ 10^-2 and ≤ 10^-1  Gy, respectively). An empirical model of the ion recombination in the chamber was found by fitting a logistic function to the data.

CONCLUSIONS

The ion collection efficiency of the Advanced Markus ionization chamber decreases for measurements in electron beams with increasingly higher dose-per-pulse. However, this chamber is still functional for dose measurements in beams with dose-per-pulse values up toward and above 10 Gy, if the ion recombination is taken into account. Our results show that existing models give a less-than-accurate description of the observed ion recombination. This motivates the use of the presented empirical model for measurements with the Advanced Markus chamber in high dose-per-pulse electron beams, as it enables accurate absorbed dose measurements (uncertainty estimation: 2.8-4.0%, k = 1). The model depends on the dose-per-pulse in the beam, and it is also influenced by the polarizing chamber voltage, with increasing ion recombination with a lowering of the voltage.

Quantitative characterization of the X-ray beam at the Australian Synchrotron Imaging and Medical Beamline (IMBL)

A critical early phase for any synchrotron beamline involves detailed testing, characterization and commissioning; this is especially true of a beamline as ambitious and complex as the Imaging & Medical Beamline (IMBL) at the Australian Synchrotron. IMBL staff and expert users have been performing precise experiments aimed at quantitative characterization of the primary polychromatic and monochromatic X-ray beams, with particular emphasis placed on the wiggler insertion devices (IDs), the primary-slit system and any in vacuo and ex vacuo filters. The findings from these studies will be described herein. These results will benefit IMBL and other users in the future, especially those for whom detailed knowledge of the X-ray beam spectrum (or `quality') and flux density is important. This information is critical for radiotherapy and radiobiology users, who ultimately need to know (to better than 5%) what X-ray dose or dose rate is being delivered to their samples. Various correction factors associated with ionization-chamber (IC) dosimetry have been accounted for, e.g. ion recombination, electron-loss effects. A new and innovative approach has been developed in this regard, which can provide confirmation of key parameter values such as the magnetic field in the wiggler and the effective thickness of key filters. IMBL commenced operation in December 2008 with an Advanced Photon Source (APS) wiggler as the (interim) ID. A superconducting multi-pole wiggler was installed and operational in January 2013. Results are obtained for both of these IDs and useful comparisons are made. A comprehensive model of the IMBL has been developed, embodied in a new computer program named spec.exe, which has been validated against a variety of experimental measurements. Having demonstrated the reliability and robustness of the model, it is then possible to use it in a practical and predictive manner. It is hoped that spec.exe will prove to be a useful resource for synchrotron science in general, and for hard X-ray beamlines, whether they are based on bending magnets or insertion devices, in particular. In due course, it is planned to make spec.exe freely available to other synchrotron scientists.

Conception and realization of a parallel-plate free-air ionization chamber for the absolute dosimetry of an ultrasoft X-ray beam

We report the design of a millimeter-sized parallel plate free-air ionization chamber (IC) aimed at determining the absolute air kerma rate of an ultra-soft X-ray beam (E = 1.5 keV). The size of the IC was determined so that the measurement volume satisfies the condition of charged-particle equilibrium. The correction factors necessary to properly measure the absolute kerma using the IC have been established. Particular attention was given to the determination of the effective mean energy for the 1.5 keV photons using the PENELOPE code. Other correction factors were determined by means of computer simulation (COMSOL™ and FLUKA). Measurements of air kerma rates under specific operating parameters of the lab-bench X-ray source have been performed at various distances from that source and compared to Monte Carlo calculations. We show that the developed ionization chamber makes it possible to determine accurate photon fluence rates in routine work and will constitute substantial time-savings for future radiobiological experiments based on the use of ultra-soft X-rays.

Study of the uncertainty in the determination of the absorbed dose to water during external beam radiotherapy calibration

To achieve a good clinical outcome in radiotherapy treatment, a certain accuracy in the dose delivered to the patient is required. Therefore, it is necessary to keep the uncertainty in each of the steps of the process inside some acceptable values, which implies as low a global uncertainty as possible. The work reported here focused on the uncertainty evaluation of absorbed dose to water in the routine calibration for clinical beams in the range of energies used in external‐beam radiotherapy. With this aim, we considered various uncertainty components (corrected electrometer reading, calibration factor, beam quality correction factor, and reference conditions) associated with beam calibration. Results show a typical uncertainty in the determination of absorbed dose to water during beam calibration of approximately 1.3% for photon beams and 1.5% for electron beams (k=1 in both cases) when the ND,w formalism is used and kQ,Q0 is calculated theoretically. These values may vary depending on the uncertainty provided by the standards laboratory for calibration factor, which is shown in the work. For primary standards based on clinical linear accelerator beam energies, the uncertainty in this step of the process could be placed close to 1.0%. We also discuss the possibility of an uncertainty reduction with the adoption of the absorbed dose to water formalism as compared with the air kerma formalism.

PACS numbers: 87.53.Dq, 87.53.Hv

Determination of the recombination correction factorkSfor some specific plane-parallel and cylindrical ionization chambers in pulsed photon and electron beams

It has been shown from an evaluation of the inverse reading of the dosemeter (1/M) against the inverse of the polarizing voltage (1/V), obtained with a number of commercially available ionization chambers, using dose per pulse values between 0.16 and 5 mGy, that a linear relationship between the recombination correction factor kS and dose per pulse (DPP) can be found. At dose per pulse values above 1 mGy the method of a general equation with coefficients dependent on the chamber type gives more accurate results than the Boag method. This method was already proposed by Burns and McEwen (1998, Phys. Med. Biol. 43 2033) and avoids comprehensive and time-consuming measurements of Jaffé plots which are a prerequisite for the application of the multi-voltage analysis (MVA) or the two-voltage analysis (TVA). We evaluated and verified the response of ionization chambers on the recombination effect in pulsed accelerator beams for both photons and electrons. Our main conclusions are: (1) The correction factor k(S) depends only on the DPP and the chamber type. There is no influence of radiation type and energy. (2) For all the chambers investigated there is a linear relationship between kS and DPP up to 5 mGy/pulse, and for two chambers we could show linearity up to 40 mGy/pulse. (3) A general formalism, such as that of Boag, characterizes chambers exclusively by the distance of the electrodes and gives a trend for the correction factor, and therefore (4) a general formalism has to reflect the influence of the chamber construction on the recombination by the introduction of chamber-type dependent coefficients.

The IPEM code of practice for electron dosimetry for radiotherapy beams of initial energy from 4 to 25 MeV based on an absorbed dose to water calibration

This report contains the recommendations of the Electron Dosimetry Working Party of the UK Institute of Physics and Engineering in Medicine (IPEM). The recommendations consist of a code of practice for electron dosimetry for radiotherapy beams of initial energy from 4 to 25 MeV. The code is based on the absorbed dose to water calibration service for electron beams provided by the UK standards laboratory, the National Physical Laboratory (NPL). This supplies direct N(D,w) calibration factors, traceable to a calorimetric primary standard, at specified reference depths over a range of electron energies up to approximately 20 MeV. Electron beam quality is specified in terms of R(50,D), the depth in water along the beam central axis at which the dose is 50% of the maximum. The reference depth for any given beam at the NPL for chamber calibration and also for measurements for calibration of clinical beams is 0.6R(50.D) - 0.1 cm in water. Designated chambers are graphite-walled Farmer-type cylindrical chambers and the NACP- and Roos-type parallel-plate chambers. The practical code provides methods to determine the absorbed dose to water under reference conditions and also guidance on methods to transfer this dose to non-reference points and to other irradiation conditions. It also gives procedures and data for extending up to higher energies above the range where direct calibration factors are currently available. The practical procedures are supplemented by comprehensive appendices giving discussion of the background to the formalism and the sources and values of any data required. The electron dosimetry code improves consistency with the similar UK approach to megavoltage photon dosimetry, in use since 1990. It provides reduced uncertainties, approaching 1% standard uncertainty in optimal conditions, and a simpler formalism than previous air kerma calibration based recommendations for electron dosimetry.

Determination of absorbed dose calibration factors for therapy level electron beam ionization chambers

Over several years the National Physical Laboratory (NPL) has been developing an absorbed dose calibration service for electron beam radiotherapy. To test this service, a number of trial calibrations of therapy level electron beam ionization chambers have been carried out during the last 3 years. These trials involved 17 UK radiotherapy centres supplying a total of 46 chambers of the NACP, Markus, Roos and Farmer types. Calibration factors were derived from the primary standard calorimeter at seven energies in the range 4 to 19 MeV with an estimated uncertainty of +/-1.5% at the 95% confidence level. Investigations were also carried out into chamber perturbation, polarity effects, ion recombination and repeatability of the calibration process. The instruments were returned to the radiotherapy centres for measurements to be carried out comparing the NPL direct calibration with the 1996 IPEMB air kerma based Code of Practice. It was found that, in general, all chambers of a particular type showed the same energy response. However, it was found that polarity and recombination corrections were quite variable for Markus chambers-differences in the polarity correction of up to 1% were seen. Perturbation corrections were obtained and were found to agree well with the standard data used in the IPEMB Code. The results of the comparison between the NPL calibration and IPEMB Code show agreement between the two methods at the +/-1% level for the NACP and Farmer chambers, but there is a significant difference for the Markus chambers of around 2%. This difference between chamber types is most likely to be due to the design of the Markus chamber.

The saturation loss for plane parallel ionization chambers at high dose per pulse values

The use of plane parallel ionization chambers with electron beams with high dose per pulse entails dose uncertainties due to the overestimation of the ion recombination factor, k, up to 20% if conventional dosimetric protocols are used. In this work MD-55-2 radiochromic films have been used as reference dosimeters to obtain dose to water per pulse DGAF(w) values for three Novac7 (Hitesys) electron beams of E0 = 5.8 MeV. However, the beam calibration by MD-55-2 films is time consuming and the use of plane parallel chambers is fundamental for a periodic quality control procedure. Three plane parallel chambers have been used and the general formula for the k determination has been tested using the calibration doses, DGAF(w). In particular, consistent ion recombination factors ksat(V0) (with the ion chamber polarized at V0), that follow the Boag theory, have been estimated at different dose per pulse values for the three plane parallel ionization chambers. This means that at present any ion chamber needs a specific ksat (V0) determination by using a reference dosimeter for which the response is independent of the dose rate. An accurate determination of ksat(V0), using a reference quality beam, can be used to determine the dose to water per pulse for electron beams of different quality and geometrical configuration.

Measurement of the collection efficiency of a large volume spherical ionization chamber in megavoltage therapy beams

The collection efficiency of a 5.7 cm diameter spherical ionization chamber has been measured in 4 MV and 10 MV x-ray beams at various distances from the source. This chamber was found to have a substantial inefficiency due to its large volume and the high dose rate and pulsed nature of the therapy beams. It was also found that the efficiency depended on the dose rate of the machine because the inter-pulse separation time of the linac is significantly less than the ion transit-time for this chamber. Thus, ionization from more than one beam pulse is collected by the chamber at the same time. The efficiency was determined using three techniques (i) the two-voltage technique, (ii) the voltage extrapolation technique and (iii) a method originally devised for determining the collection efficiency of large volume ionization chambers in diagnostic radiology. The results show that methods (ii) and (iii) agree well, but that the two-voltage technique predicts a much lower efficiency. At about 4 m from the source, the collection efficiency for this chamber varied between 98% and 97% for dose rates between 50 and 250 cGy/min for 4 MV and between 97% and 90% for dose rates between 100 and 600 cGy/min for 10 MV. At isocenter, the comparable figures were 78% and 56% respectively for 4 MV and 65% and 38% respectively for 10 MV.

AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams

A protocol is prescribed for clinical reference dosimetry of external beam radiation therapy using photon beams with nominal energies between 60Co and 50 MV and electron beams with nominal energies between 4 and 50 MeV. The protocol was written by Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) and has been formally approved by the AAPM for clinical use. The protocol uses ion chambers with absorbed-dose-to-water calibration factors, N(60Co)D,w which are traceable to national primary standards, and the equation D(Q)w = MkQN(60Co)D,w where Q is the beam quality of the clinical beam, D(Q)w is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions, M is the fully corrected ion chamber reading, and kQ is the quality conversion factor which converts the calibration factor for a 60Co beam to that for a beam of quality Q. Values of kQ are presented as a function of Q for many ion chambers. The value of M is given by M = PionP(TP)PelecPpolMraw, where Mraw is the raw, uncorrected ion chamber reading and Pion corrects for ion recombination, P(TP) for temperature and pressure variations, Pelec for inaccuracy of the electrometer if calibrated separately, and Ppol for chamber polarity effects. Beam quality, Q, is specified (i) for photon beams, by %dd(10)x, the photon component of the percentage depth dose at 10 cm depth for a field size of 10x10 cm2 on the surface of a phantom at an SSD of 100 cm and (ii) for electron beams, by R50, the depth at which the absorbed-dose falls to 50% of the maximum dose in a beam with field size > or =10x10 cm2 on the surface of the phantom (> or =20x20 cm2 for R50>8.5 cm) at an SSD of 100 cm. R50 is determined directly from the measured value of I50, the depth at which the ionization falls to 50% of its maximum value. All clinical reference dosimetry is performed in a water phantom. The reference depth for calibration purposes is 10 cm for photon beams and 0.6R50-0.1 cm for electron beams. For photon beams clinical reference dosimetry is performed in either an SSD or SAD setup with a 10x10 cm2 field size defined on the phantom surface for an SSD setup or at the depth of the detector for an SAD setup. For electron beams clinical reference dosimetry is performed with a field size of > or =10x10 cm2 (> or =20x20 cm2 for R50>8.5 cm) at an SSD between 90 and 110 cm. This protocol represents a major simplification compared to the AAPM's TG-21 protocol in the sense that large tables of stopping-power ratios and mass-energy absorption coefficients are not needed and the user does not need to calculate any theoretical dosimetry factors. Worksheets for various situations are presented along with a list of equipment required.

An algorithm to include the bremsstrahlung contamination in the determination of the absorbed dose in electron beams

None of the existing protocols or codes of practice for high-energy electron dosimetry take any account of the accelerator-generated bremsstrahlung always present in electron beams. This results in a systematic error in the derivation of the absorbed dose. The purpose of this study is to draw attention to this omission which affects the absorbed dose calibration. A method based on available experimental data is presented for dealing with this deficiency in electron dosimetry. A re-defined algorithm for absorbed dose derivation accounting for this bremsstrahlung component is proposed. The question of omission of the bremsstrahlung contamination is important in comparing ionization methods with other dosimetric methods such as calorimetry or the use of ferrous sulphate.

Ion recombination corrections for the NACP parallel-plate chamber in a pulsed electron beam

The NACP electron chamber is one of three parallel-plate chambers recommended for use in the UK. Measurements with this chamber type have indicated a problem in determining the recombination correction. This is due to a variation of the ionization current I with polarizing voltage V which deviates from the accepted Boag theory. It is shown that there is a chamber-dependent threshold voltage below which the NACP chamber follows the Boag theory. Above this voltage the chamber should be used with caution, although it is still possible to correct for the dependence of the chamber response on the dose per pulse. The existence of such deviations from theory demonstrates the usefulness of the 1/I against 1/V plot and the limitations of the Boag two-voltage analysis. Values for the initial recombination and the coefficient of general recombination are measured for several NACP chambers. It is shown that from these one can derive a value for the effective plate separation and the collector radius of each chamber. Differences in the behaviour of NACP chambers manufactured by Scanditronix and Dosetek are discussed and the implications of free-electron collection are considered.