Comparison of the IAEA TRS-398 and AAPM TG-51 absorbed dose to water protocols in the dosimetry of high-energy photon and electron beams.

The International Atomic Energy Agency (IAEA TRS-398) and the American Association of Physicists in Medicine (AAPM TG-51) have published new protocols for the calibration of radiotherapy beams. These protocols are based on the use of an ionization chamber calibrated in terms of absorbed dose to water in a standards laboratory's reference quality beam. This paper compares the recommendations of the two protocols in two ways: (i) by analysing in detail the differences in the basic data included in the two protocols for photon and electron beam dosimetry and (ii) by performing measurements in clinical photon and electron beams and determining the absorbed dose to water following the recommendations of the two protocols. Measurements were made with two Farmer-type ionization chambers and three plane-parallel ionization chamber types in 6, 18 and 25 MV photon beams and 6, 8, 10, 12, 15 and 18 MeV electron beams. The Farmer-type chambers used were NE 2571 and PTW 30001, and the plane-parallel chambers were a Scanditronix-Wellh?fer NACP and Roos, and a PTW Markus chamber. For photon beams, the measured ratios TG-51/TRS-398 of absorbed dose to water Dw ranged between 0.997 and 1.001, with a mean value of 0.999. The ratios for the beam quality correction factors kQ were found to agree to within about +/-0.2% despite significant differences in the method of beam quality specification for photon beams and in the basic data entering into kQ. For electron beams, dose measurements were made using direct N(D,w) calibrations of cylindrical and plane-parallel chambers in a 60Co gamma-ray beam, as well as cross-calibrations of plane-parallel chambers in a high-energy electron beam. For the direct N(D,w) calibrations the ratios TG-51/TRS-398 of absorbed dose to water Dw were found to lie between 0.994 and 1.018 depending upon the chamber and electron beam energy used, with mean values of 0.996, 1.006, and 1.017, respectively, for the cylindrical, well-guarded and not well-guarded plane-parallel chambers. The Dw ratios measured for the cross-calibration procedures varied between 0.993 and 0.997. The largest discrepancies for electron beams between the two protocols arise from the use of different data for the perturbation correction factors p(wall) and p(dis) of cylindrical and plane-parallel chambers, all in 60Co. A detailed analysis of the reasons for the discrepancies is made which includes comparing the formalisms, correction factors and the quantities in the two protocols.     

Protocols for the dosimetry of high-energy photon and electron beams: a comparison of the IAEA TRS-398 and previous international codes of practice. International Atomic Energy Agency

A new international Code of Practice for radiotherapy dosimetry co-sponsored by several international organizations has been published by the IAEA, TRS-398. It is based on standards of absorbed dose to water, whereas previous protocols (TRS-381 and TRS-277) were based on air kerma standards. To estimate the changes in beam calibration caused by the introduction of TRS-398, a detailed experimental comparison of the dose determination in reference conditions in high-energy photon and electron beams has been made using the different IAEA protocols. A summary of the formulation and reference conditions in the various Codes of Practice, as well as of their basic data, is presented first. Accurate measurements have been made in 25 photon and electron beams from 10 clinical accelerators using 12 different cylindrical and plane-parallel chambers, and dose ratios under different conditions of TRS-398 to the other protocols determined. A strict step-by-step checklist was followed by the two participating clinical institutions to ascertain that the resulting calculations agreed within tenths of a per cent. The maximum differences found between TRS-398 and the previous Codes of Practice TRS-277 (2nd edn) and TRS-381 are of the order of 1.5-2.0%. TRS-398 yields absorbed doses larger than the previous protocols, around 1.0% for photons (TRS-277) and for electrons (TRS-381 and TRS-277) when plane-parallel chambers are cross-calibrated. For the Markus chamber, results show a very large variation, although a fortuitous cancellation of the old stopping powers with the ND,w/NK ratios makes the overall discrepancy between TRS-398 and TRS-277 in this case smaller than for well-guarded plane-parallel chambers. Chambers of the Roos-type with a 60Co ND,w calibration yield the maximum discrepancy in absorbed dose, which varies between 1.0% and 1.5% for TRS-381 and between 1.5% and 2.0% for TRS-277. Photon beam calibrations using directly measured or calculated TPR20,10 from a percentage dose data at SSD = 100 cm were found to be indistinguishable. Considering that approximately 0.8% of the differences between TRS-398 and the NK-based protocols are caused by the change to the new type of standards, the remaining difference in absolute dose is due either to a close similarity in basic data or to a fortuitous cancellation of the discrepancies in data and type of chamber calibration. It is emphasized that the NK-ND,air and ND,w formalisms have very similar uncertainty when the same criteria are used for both procedures. Arguments are provided in support of the recommendation for a change in reference dosimetry based on standards of absorbed dose to water.     

Absorbed dose to water based dosimetry versus air kerma based dosimetry for high-energy photon beams: an experimental study

In recent years, a change has been proposed from air kerma based reference dosimetry to absorbed dose based reference dosimetry for all radiotherapy beams of ionizing radiation. In this paper, a dosimetry study is presented in which absorbed dose based dosimetry using recently developed formalisms was compared with air kerma based dosimetry using older formalisms. Three ionization chambers of each of three different types were calibrated in terms of absorbed dose to water and air kerma and sent to five hospitals. There, reference dosimetry with all the chambers was performed in a total of eight high-energy clinical photon beams. The selected chamber types were the NE2571, the PTW-30004 and the Wellh?fer-FC65G (previously Wellh?fer-IC70). Having a graphite wall, they exhibit a stable volume and the presence of an aluminium electrode ensures the robustness of these chambers. The data were analysed with the most important recommendations for clinical dosimetry: IAEA TRS-398, AAPM TG-51, IAEA TRS-277, NCS report-2 (presently recommended in Belgium) and AAPM TG-21. The necessary conversion factors were taken from those protocols, or calculated using the data in the different protocols if data for a chamber type are lacking. Polarity corrections were within 0.1% for all chambers in all beams. Recombination corrections were consistent with theoretical predictions, did not vary within a chamber type and only slightly between different chamber types. The maximum chamber-to-chamber variations of the dose obtained with the different formalisms within the same chamber type were between 0.2% and 0.6% for the NE2571, between 0.2% and 0.6% for the PTW-30004 and 0.1% and 0.3% for the Wellh?fer-FC65G for the different beams. The absorbed dose results for the NE2571 and Wellh?fer-FC65G chambers were in good agreement for all beams and all formalisms. The PTW-30004 chambers gave a small but systematically higher result compared to the result for the NE2571 chambers (on the average 0.1% for IAEA TRS-277, 0.3% for NCS report-2 and AAPM TG-21 and 0.4% for IAEA TRS-398 and AAPM TG-51). Within the air kerma based protocols, the results obtained with the TG-21 protocol were 0.4-0.8% higher mainly due to the differences in the data used. Both absorbed dose to water based formalisms resulted in consistent values within 0.3%. The change from old to new formalisms is discussed together with the traceability of calibration factors obtained at the primary absorbed dose and air kerma standards in the reference beams (60Co). For the particular situation in Belgium (calibrations at the Laboratory for Standard Dosimetry of Ghent) the change amounts to 0.1-0.6%. This is similar to the magnitude of the change determined in other countries.     

Comparison of dosimetry recommendations for clinical proton beams

The formalism and data in the two most recent dosimetry recommendations for clinical proton beams, ICRU Report 59 and the forthcoming IAEA Code of Practice, are compared. Chamber calibrations in terms of air kerma and absorbed dose to water are considered, including five different cylindrical ionization chamber types commonly used in proton beam dosimetry. The methodology for both types of calibration for ionization chambers is described in ICRU Report 59. The procedure based on air kerma calibrations is compared with an alternative formalism based on IAEA Codes of Practice (TRS-277, TRS-381), modified for proton beams. The new IAEA Code of Practice is exclusively based on calibrations in terms of absorbed dose to water and a direct comparison with ICRU Report 59 recommendations is made. Common to the two formalisms are the fundamental quantities Wair and w(air) and their atmospheric conditions of applicability. The difference in the recommended values of the ratio w(air)/Wair (protons to 60Co) is as large as 2.3%. The use of Wair and w(air) values for dry air (IAEA) and for ambient air (ICRU) is a contribution to the discrepancy, and the ICRU usage is questioned. For air kerma based chamber calibrations, ICRU Report 59 does not take into account the effect of different compositions of the build-up cap and chamber wall on the calibration beam quality. For the chamber types included in the study, this introduces discrepancies of up to 1.1%. Combined with differences in the recommended basic data, discrepancies in absorbed dose determination in proton beams of up to 2.1% are found. For the absorbed dose to water based formalism, differences in the formalism, notably the omission of perturbation factors for 60Co in ICRU 59, and data yield discrepancies in calculated kQ factors, and in absorbed dose determinations, between -1.5% and +2.6%, depending on the chamber type and the proton beam quality.     

On the calibration of plane-parallel ionization chambers for electron beam dosimetry.

The procedure recommended by different dosimetry protocols for the determination of the absorbed dose to air chamber factor, ND,pp, of plane-parallel chambers, comparing absorbed dose determinations in a high-energy electron beam with a reference cylindrical chamber having a known ND,cyl factor, has been investigated. Attention has been focused on the case that the chamber serving as reference has a solid aluminium central electrode. It has been found that using a wide spread Farmer-type chamber (NE 2571), together with recommendations which specifically take into account central electrode corrections for electron beam dosimetry, kcelpcel = pcel-global(IAEA) = 1.008, yields inconsistent results compared with those obtained from a fully homogeneous ionization chamber; for the NE 2571 chamber, a value kcelpcel = pcel-global(IAEA) congruent to 1.0 has been obtained. Analytical calculations of kmkatt for Farmer-type cylindrical chambers and experimental determinations of the product kmkattkcelpcel in electron beams agree within experimental uncertainties, with no evidence of statistical significance for the commonly used assumption pcel = 1, which yields a 0.8% correction (due to kcel only) for the effect of the NE 2571 aluminium electrode in electron beam dosimetry. The use of a 'NACP-chamber' specific factor (kpp or kmkatt) to obtain ND,pp from NK,pp in NACP plane-parallel chambers has been found unsatisfactory, and direct experimental determinations of ND,pp are recommended instead. It is suggested that Standard Dosimetry Laboratories provide ND,pp calibration factors in 60Co beams.     

The use of plane-parallel chambers in electron dosimetry without any cross-calibration

Current dosimetry protocols from AAPM, DIN and IAEA recommend a cross-calibration for plane-parallel chambers against a calibrated thimble chamber for electron dosimetry. The rationale for this is the assumed chamber-to-chamber variation of plane-parallel chambers and the large uncertainty in the wall perturbation factor (p(wall)60Co)pp at 60Co for plane-parallel chambers. We have confirmed the results of other authors that chamber-to-chamber variation of the investigated chambers of types Roos, Markus, Advanced Markus and Farmer is less than 0.3%. Starting with a calibration factor for absorbed dose to water and on the basis of the three dosimetry protocols AAPM TG-51, DIN 6800-2 (slightly modified) and IAEA TRS-398, values for (p(wall)60Co)Roos of 1.024 +/- 0.005, (p(wall)60Co)Markus of 1.016 +/- 0.005 and (p(wall)60Co)Advanced Markus of 1.014 +/- 0.005 have been determined. In future this will permit electron dosimetry with the above-listed plane-parallel chambers having a calibration factor N(D, w)60Co without the necessity for cross-calibration against a thimble chamber.     

Calibration of plane-parallel chambers and determination of p(wall) for the NACP and Roos chambers for 60Co gamma-ray beams

Procedures for the calibration and use of plane-parallel ionization chambers in high-energy electron and photon beams have been given in the international code of practice IAEA TRS-381. In the present work, plane-parallel ionization chambers of the type PTW-34001 Roos and Scanditronix NACP02 have been calibrated using two N(K)-based procedures. For the NACP chamber the difference between the N(D,air) chamber factors determined in an electron beam and in a 60Co gamma-ray beam, respectively, is of the same magnitude as the experimental uncertainty. Results for the PTW Roos chambers, however, do not agree, in accordance with recent findings of other authors. The value determined in a 60Co gamma-ray beam is questioned and the reason for the discrepancy assigned to the correction factor for the perturbation due to the chamber wall, p(wall). New values of p(wall) have been experimentally determined by comparing absorbed dose measurements based on air-kerma and absorbed dose to water calibration procedures. A new p(wall) factor for the Roos chamber in 60Co gamma-ray beams in water (1.009+/-0.6%) was derived as the weighted average of the different determinations. The value is not significantly higher than the p(wall) factor given in TRS-381 (1.003+/-1.5%), but the combined standard uncertainty is reduced. The chamber to chamber variation for six commercial PTW Roos chambers and a Roos prototype was found to be very small.     

Comparison of high-energy photon and electron dosimetry for various dosimetry protocols

The American Association of Physicists in Medicine Task Group 51 (TG-51) and the International Atomic Energy Agency (IAEA) published a new high-energy photon and electron dosimetry protocol, in 1999 and 2000, respectively. These protocols are based on the use of an ion chamber having an absorbed-dose to water calibration factor with a 60Co beam. These are different from the predecessors, the TG-21 and IAEA TRS-277 protocols, which require a 60Co exposure or air-kerma calibration factor. The purpose of this work is to present the dose comparison between various dosimetry protocols and the AAPM TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams. The absorbed-dose to water calculated according to the Japanese Association of Radiological Physics (JARP), International Atomic Energy Agency Technical Report Series No. 277 (IAEA TRS-277) and No. 398 (IAEA TRS-398) protocols is compared to that calculated using the TG-51 protocol. For various Farmer-type chambers in photon beams, TG-51 is found to predict 0.6-2.1% higher dose than JARP. Similarly, TG-51 is found to be higher by 0.7-1.7% than TRS-277. For electron beams TG-51 is higher than JARP by 1.5-3.8% and TRS-277 by 0.2-1.9%. The reasons for these differences are presented in terms of the cavity-gas calibration factor, Ngas, and a dose conversion factor, Fw, which converts the absorbed-dose to air in the chamber to the absorbed-dose to water. The ratio of cavity-gas calibration factors based on absorbed-dose to water calibration factors, N60Co(D,w), in TG-51 and cavity-gas calibration factors which are equivalent to absorbed-dose to air chamber factors, N(D,air), based on the IAEA TRS-381 protocol is 1.008 on average. However, the estimated uncertainty of the ratio between the two cavity-gas calibration factors is 0.9% (1 s.d.) and consequently, the observed difference of 0.8% is not significant. The absorbed-dose to water and exposure or air-kerma calibration factors are based on standards traceable to the National Institute of Standards and Technology (NIST). In contrast, the absorbed-dose to water determined with TRS-398 is in good agreement with TG-51 within about 0.5% for photon and electron beams.     

Comparing calibration methods of electron beams using plane-parallel chambers with absorbed-dose to water based protocols

Recent absorbed-dose-based protocols allow for two methods of calibrating electron beams using plane-parallel chambers, one using the N(Co)D,w for a plane-parallel chamber, and the other relying on cross-calibration of the plane-parallel chamber in a high-energy electron beam against a cylindrical chamber which has an N(Co)D,w factor. The second method is recommended as it avoids problems associated with the Pwall correction factors at 60Co for plane-parallel chambers which are used in the determination of the beam quality conversion factors. In this article we investigate the consistency of these two methods for the PTW Roos, Scanditronics NACP02, and PTW Markus chambers. We processed our data using both the AAPM TG-51 and the IAEA TRS-398 protocols. Wall correction factors in 60Co beams and absorbed-dose beam quality conversion factors for 20 MeV electrons were derived for these chambers by cross-calibration against a cylindrical ionization chamber. Systematic differences of up to 1.6% were found between our values of Pwall and those from the Monte Carlo calculations underlying AAPM TG-51, and up to 0.6% when comparing with the IAEA TRS-398 protocol. The differences in Pwall translate directly into differences in the beam quality conversion factors in the respective protocols. The relatively large spread in the experimental data of Pwall, and consequently the absorbed-dose beam quality conversion factor, confirms the importance of the cross-calibration technique when using plane-parallel chambers for calibrating clinical electron beams. We confirmed that for well-guarded plane-parallel chambers, the fluence perturbation correction factor at d(max) is not significantly different from the value at d(ref). For the PTW Markus chamber the variation in the latter factor is consistent with published fits relating it to average energy at depth.     

Wall correction factors for calibration of plane-parallel ionization chambers with high-energy photon beams

Most dosimetry protocols recommend that calibration of plane-parallel ionization chambers be performed in an electron beam of sufficiently high energy by comparison with cylindrical chambers. For various plane-parallel chambers, the 1997 IAEA TRS-381 protocol includes an overall perturbation factor pQ for electron beams, a wall correction factor p(wall) for a 60Co beam and the product of two wall corrections k(att)k(m) for 60Co in-air calibration. The recommended values of p(wall) for plane-parallel chambers, however, are limited to certain phantom materials and a 60Co beam, and are not given for other phantom materials and x-ray beams. In this work, the p(wall) values of the commercially available NACP, PTW/Markus and PTW/Roos plane-parallel chambers in a solid water phantom have been determined with 60Co and 4 and 10 MV photon beams. The k(att)k(m) values for the NACP and PTW/Markus chambers have also been obtained. The wall correction factors p(wall) and k(att)k(m) have been determined by intercomparison with a calibrated Farmer chamber. The average value of p(wall) for these plane-parallel chambers was 1.005 +/- 0.1% (1 SD) for 60Co beams and 1.007 +/- 0.2% (1 SD) for both 4 MV and 10 MV photons. The k(att)k(m) values for the NACP and PTW/Markus chambers were about 1.5% lower than other published data.     

Chamber-quality factors in 60Co for three plane-parallel chambers for the dosimetry of electrons, protons and heavier charged particles: PENELOPE Monte Carlo simulations

The IBA-Scanditronix NACP-02, IBA-Wellh?fer PPC-40 and PPC-05 plane-parallel ionization chambers have been simulated with the Monte Carlo code PENELOPE to obtain their chamber- and quality-dependent factors f(c,Qo) for a (60)Co gamma beam. These are applicable to the determination of k(Q) beam-quality factors for the dosimetry of electron, protons and heavier charged particles beams based on standards of absorbed dose to water. The factor f(c,Q) is equivalent to the product s(w,air)p, but it is not subject to the assumed independence of perturbation factors and stopping power (Sempau et al 2004 Phys. Med. Biol. 49 4427-44). The calculations have been carried out using three different (60)Co source models: a monoenergetic point source, a point source with a realistic (60)Co spectrum and the simulated phase space from a radiotherapy (60)Co unit. Both the detailed geometries of the ionization chambers and of the (60)Co unit have been obtained from the manufacturers. In the case of the NACP-02 chamber, values of f(c,Qo) have been compared with those in the IAEA TRS-398 Code of Practice and from other authors, results being in excellent agreement. The PPC-05 and PPC-40 chambers are of relatively new design, and their values have not been calculated before. Within the estimated uncertainty, computed at the 2sigma level (95% confidence limit), the results for each of the three chambers appear to be independent of the degree of sophistication of the (60)Co source model used. For the NACP-02 chamber this assumption is justified by the excellent agreement between the various models, which occurs at the level of one standard uncertainty. This suggests the possibility of adopting the mean value of the three source models, weighted with the inverse of their corresponding uncertainties, as a better estimate of f(c,Qo). A consequence of the above conclusions is that the estimated uncertainty of k(Q) beam-quality factors of all charged particles referred to (60)Co can potentially be decreased considerably using our approach. For example, the estimated relative standard uncertainty of the denominator of k(Q), given in TRS-398 as 1.6% for plane-parallel ionization chambers, can be reduced to 0.06% for a NACP chamber using the mean value of f(c,Qo) given in this work. Similar reductions could be obtained for the combined standard uncertainty of the k(Q) beam-quality factors of all charged particles, notably electrons.     

Consistency of absorbed dose to water measurements using 21 ion-chamber models following the AAPM TG51 and TG21 calibration protocols

In 1999, the AAPM introduced a reference dosimetry protocol, known as TG51, based on an absorbed dose standard. This replaced the previous protocol, known as TG21, which was based on an air kerma standard. A significant body of literature has emerged discussing the improved accuracy and robustness of the absorbed dose standard, and quantifying the changes in baseline dosimetry with the introduction of the absorbed dose protocol. A significant component playing a role in the overall accuracy of beam output determination is the variability due to the use of different dosimeters. This issue, not adequately addressed in the past, is the focus of the present study. This work provides a comparison of absorbed dose determinations using 21 different makes and models of ion chambers for low- and high-energy photon and electron beams. The study included 13 models of cylindrical ion chambers and eight models of plane-parallel chambers. A high degree of precision (<0.25%) resulted from measurements with all chambers in a single setting, a sufficient number of repeat readings, and the use of high quality ion chambers as external monitors. Cylindrical chambers in photon beams show an improvement in chamber-to-chamber consistency with TG51. For electron dosimetry with plane-parallel chambers, the parameters Ngas and the product ND,w x k(ecal) were each determined in two ways, based on (i) an ADCL calibration, and (ii) a cross comparison with an ADCL-calibrated cylindrical chamber in a high-energy electron beam. Plane-parallel chamber results, therefore, are presented for both methods of chamber calibration. Our electron results with technique (i) show that plane-parallel chambers, as a group, overestimate the beam output relative to cylindrical chambers by 1%-2% with either protocol. Technique (ii), by definition, normalizes the plane-parallel results to the cylindrical results. In all cases, the maximum spread in output from the various cylindrical chambers is <2% implying a standard deviation of less than 0.5%. For plane-parallel chambers, the maximum spread is somewhat larger, up to 3%. A few chambers have been identified as outliers.     

Reference Dosimetry according to the New German Protocol DIN 6800-2 and Comparison with IAEA TRS 398 and AAPM TG 51

The preceding DIN 6800-2 (1997) protocol has been revised by a German task group and its latest version was published in March 2008 as the national standard dosimetry protocol DIN 6800-2 (2008 March). Since then, in Germany the determination of absorbed dose to water for high-energy photon and electron beams has to be performed according to this new German dosimetry protocol. The IAEA Code of Practice TRS 398 (2000) and the AAPM TG-51 are the two main protocols applied internationally. The new German version has widely adapted the methodology and dosimetric data of TRS-398. This paper investigates systematically the DIN 6800-2 protocol and compares it with the procedures and results obtained by using the international protocols. The investigation was performed with 6 MV and 18 MV photon beams as well as with electron beams from 5 MeV to 21 MeV. While only cylindrical chambers were used for photon beams, the measurements of electron beams were performed by using cylindrical and plane-parallel chambers. It was found that the discrepancies in the determination of absorbed dose to water among the three protocols were 0.23% for photon beams and 1.2% for electron beams. The determination of water absorbed dose was also checked by a national audit procedure using TLDs. The comparison between the measurements following the DIN 6800-2 protocol and the TLD audit-procedure confirmed a difference of less than 2%. The advantage of the new German protocol DIN 6800-2 lies in the renouncement on the cross calibration procedure as well as its clear presentation of formulas and parameters. In the past, the different protocols evoluted differently from time to time. Fortunately today, a good convergence has been obtained in concepts and methods.     

Ion recombination correction in the Clatterbridge Centre of Oncology clinical proton beam

Most codes of practice for dosimetry of proton beams do not give a clear recommendation on the determination of recombination correction factors for ionization chambers. In this work, recombination corrections were measured in the low-energy clinical proton beam of the Clatterbridge Centre of Oncology (CCO) using data collected at different dose rates and different polarizing voltages. This approach allows the separation of contributions from initial and volume recombination and was compared with results from extrapolation and two-voltage methods. A modified formulation of the method is presented for a modulated beam in which the ionization current is time dependent. Using a set-up with two identical chambers placed face-to-face yielded highly accurate data for plane-parallel ionization chambers. This method may also be used for high-energy photon and electron beam dosimetry. At typical dose rates of 26 Gy min(-1) used clinically at the CCO, the recombination correction is 0.8% and thus is of importance for reference dosimetry. The proton beam should be treated as purely continuous given the high pulse repetition frequency of the cyclotron beam. The results show that the volume recombination parameter for protons is consistent with values measured for photon beams. Initial recombination was found to be independent of beam quality, except for a tendency to increase at the distal edge of the Bragg peak; this is only relevant for depth dose measurements. Using a general equation for recombination and generic values for the initial and volume recombination parameters (A = 0.25 V and m2 = 3.97 x 10(3) s cm(-1) nC(-1) V2), the experimental results are reproduced within 0.1% for all conditions met in this work. For the CCO beam and similar proton beams used for treating optical targets operating at high dose rates, the recombination correction factor can be overestimated by up to 2%, resulting in an overestimation of dose to water by the same amount, if the recommendation from IAEA TRS-398, which is only valid for pulsed beams, is followed without consideration.     

Calculated absorbed-dose ratios,TG51/TG21, for most widely used cylindrical and parallel-plate ion chambers over a range of photon and electron energies

Task Group 51 (TG51), of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM), has developed a calibration protocol for high-energy photon and electron therapy beams based on absorbed dose standards. This protocol is intended to replace the air-kerma based protocol developed by an earlier AAPM task group (TG21). Conversion to the newer protocol introduces a change in the determined absorbed dose. In this work, the change in dose is expressed as the ratio of the doses (TG51/TG21) based on the two protocols. Dose is compared at the TG-51 reference depths of 10 cm for photons and d(ref) for electrons. Dose ratios are presented for a variety of ion chambers over a range of photon and electron energies. The TG51/TG21 dose ratios presented here are based on the dosimetry factors provided by the two protocols and the chamber-specific absorbed dose and exposure calibration factors (N60Co(D,w) and Nx) provided by the Accredited Dosimetry Calibration Laboratory (ADCL) at The University of Texas, M. D. Anderson Cancer Center (MDACC). As such, the values presented here represent the expected discrepancies between the two protocols due only to changes in the dosimetry parameters and the differences in chamber-specific dose and air-kerma standards. These values are independent of factors such as measurement uncertainties, setup errors, and inconsistencies arising from the mix of different phantoms and ion chambers for the two protocols. Therefore, these ratios may serve as a guide for institutions performing measurements for the switch from TG21-to-TG51 based calibration. Any significant deviation in the ratio obtained from measurements versus those presented here should prompt a review to identify possible errors and inconsistencies. For all cylindrical chambers included here, the TG51/TG21 dose ratios are the same within +/-0.6%, irrespective of the make and model of the chamber, for each photon and electron beam included. Photon beams show the TG51/TG21 dose ratios decreasing with energy, whereas electrons exhibit the opposite trend. The dose ratio for photons is near 1.00 at 18 mV increasing to near 1.01 at 4 mV while the dose ratio for electrons is near 1.02 at 20 MeV decreasing only 0.5% to near 1.015 at 6 MeV. For parallel-plate chambers, the situation is complicated by the two possible methods of obtaining calibration factors: through an ADCL or through a cross-comparison with a cylindrical chamber in a high-energy electron beam. For some chambers, the two methods lead to significantly different calibration factors, which in turn lead to significantly different TG51/TG21 results for the same chamber. Data show that if both N60Co(D,w) and Nx are obtained from the same source, namely an ADCL or a cross comparison, the TG51/TG21 results for parallel-plate chambers are similar to those for cylindrical chambers. However, an inconsistent set of calibration factors, i.e., using N60Co(D,w) x k(ecal) from an ADCL but Ngas from a cross comparison or vice versa, can introduce an additional uncertainty up to 2.5% in the TG51/TG21 dose ratios.     

Determination of absorbed dose to water in reference conditions for radiotherapy kilovoltage x-rays between 10 and 300 kV: a comparison of the data in the IAEA, IPEMB, DIN and NCS dosimetry protocols

A comparison of four of the most commonly used dosimetry protocols for the determination of absorbed dose to water in therapeutic kilovoltage x-rays using an ionization chamber (IAEA TRS-277, IPEMB, DIN and NCS) has been carried out. Owing to the different energy ranges and HVLs recommended by each protocol, backscatter factors, water-to-air mass energy absorption coefficient ratios and perturbation correction factors have been recast to a common quality range that all protocols satisfy individually to make a comparison possible. The results of the comparison show that in the sometimes reduced quality range originally included by the different protocols, determinations of absorbed dose to water at all beam qualities agree to within +/-1.0% with that obtained using the second edition of the IAEA TRS-277 code of practice (1997). The extrapolation of data to a common beam quality range practically preserves the agreement for all the protocols except for that issued by the NCS at the extremes of the range, where differences of up to 1.8% and 1.4% have been found for low and medium energies respectively. In all cases the DIN protocol yields very good agreement with TRS-277.     

Dosimetry using plane-parallel ionization chambers in a 75 MeV clinical proton beam

Reference ionization chamber dosimetry in clinical proton beams is generally performed with cylindrical ionization chambers. However, when the measurement is performed in the presence of a large depth dose gradient or in a narrow spread out Bragg peak (SOBP), it could be advisable to use a plane-parallel chamber. Few recommendations and studies have been devoted to this subject. In this paper, experimental information on perturbation correction factors for four plane-parallel ionization chamber types in proton beams is presented. The experiments were performed in 75 MeV modulated and non-modulated proton beams. Monte Carlo calculations have been performed to support the conclusions of the experimental work. Overall, we were not able to find experimental evidence for significant differences between the secondary electron perturbation correction factors for plane-parallel chambers and those for a cylindrical NE2571. We found experimental ratios of perturbation correction factors that did not differ by more than 0.6% from unity for a Roos and two NACP02 chambers, and by not more than 1.2% for a Calcam-2 and two Markus chambers. Monte Carlo simulations result in corrections that are limited to 0.6% in absolute value, but given the overall uncertainties of the measurements, the deviations of the correction factors from unity could not be resolved from the experimental results. The results of the simulations thus support the experimental conclusion that perturbation correction factors for the set of plane-parallel chambers in both proton beams (relative to NE2571) do not deviate from unity by more than 1.2%. This confirms, within the experimental uncertainties, the assumption that the overall perturbation correction factor for a plane-parallel chamber in a low-energy proton beam is unity, made in IAEA TRS-398 and other dosimetry protocols. Given the large uncertainties of the gradient correction factors to be applied when using a cylindrical ionization chamber in a narrow SOBP or in the presence of a strong depth dose gradient, the level of agreement between plane-parallel and cylindrical ionization chambers observed in this study shows that plane-parallel chambers are a reliable alternative for reference dosimetry in low-energy proton beams.     

Performance analysis and determination of the p(wall) correction factor for 60Co gamma-ray beams for Wellhöfer Roos-type plane-parallel chambers

The wall perturbation correction factor p(wall) in 60Co for Wellh?fer Roos-type plane-parallel ionization chambers is determined experimentally and compared with the results of a previous study using PTW-Roos chambers (Palm et al 2000 Phys. Med. Biol. 45 971-81). Five ionization chambers of the type Wellh?fer PPC-35 (or its equivalent PPC-40) are used for the analysis. Wall perturbation correction factors are obtained by assuming N(D,air) chamber factors determined by cross-calibration in a high-energy electron and in a 60Co gamma-ray beam to be equal, and by assigning any differences to the wall perturbation factor. The procedure yields a p(wall) value of 1.018 (u(c) = 0.010), which is slightly higher than the value 1.014 (u(c) = 0.010) formerly obtained for the PTW-Roos chambers using the N(D,air) method. The chamber-to-chamber variation in p(wall) for the Wellh?fer-Roos chambers is found to be very small, with a maximum difference of 0.3%. The effect of using new p(cav) values for graphite-walled Farmer-type chambers used in water in electron beams is to decrease p(wall) by approximately 0.5%. The long- and short-term stability of the Roos-type chambers manufactured by Wellh?fer is investigated by measurements at the IAEA Dosimetry Laboratory in Vienna, Austria, and at the Sahlgrenska University Hospital in G?teborg, Sweden. Calibrations made at the IAEA over several months show variations in the N(D,w) calibration factors larger than expected. based on previous experiences with PTW-Roos chambers. Measurements of the short-term stability of the Wellh?fer-Roos chambers show a marked increase in chamber response for the time the chambers are immersed in water, pointing to a possible problem in the chamber design. As a consequence of these findings, Wellh?fer is currently working on a re-design of the chamber to solve the stability problem.     

Experimental determination of the effective point of measurement for cylindrical ionization chambers in 60Co gamma radiation

The displacement effect of cylindrical ionization chambers is taken into account either by an effective point of measurement (EPOM) or, alternatively, by using a displacement perturbation factor. The dependence of these effects in water was examined as a function of the cavity radius using cylindrical chambers with different radii and a plane-parallel chamber, whose EPOM is well known. Depth-dose curves were measured in terms of absolute absorbed dose in water and evaluated according to the international protocol IAEA TRS-398 as well as the German protocol DIN 6800-2. As expected, evaluation of absorbed dose under reference conditions following both protocols agreed well within a standard uncertainty of 0.1%. However, values of absorbed dose at depths beyond the dose maximum showed deviations up to 0.3% and 0.5% for IAEA TRS-398 and DIN 6800-2, respectively. Values in the build-up and maximum region did not agree very well. Deviations of more than 1% were found for both protocols. It was concluded that the corrections recommended in both protocols are not fully appropriate. A procedure is suggested to measure the absorbed depth-dose distribution including the build-up region with an improved accuracy by means of cylindrical chambers.     

Absorbed dose beam quality correction factors kappaQ for the NE2571 chamber in a 5 MV and a 10 MV photon beam

Dose to water (Dw) determination in clinical high-energy photon beams with ionization chambers calibrated in terms of absorbed dose to water has been proposed as an alternative to ionization chamber dosimetry based on air kerma calibrations. Dw in the clinical beam is derived using a kappaQ factor that scales the absorbed dose calibration factor in the reference beam to the absorbed dose calibration factor in the user beam. In the present study kappaQ values were determined for the NE2571 chamber in a 5 MV and a 10 MV high-energy photon beam generated at the 15 MeV high-intensity electron linac of the University of Gent. A set of three NE2571 chambers was calibrated relative to the Gent sealed water calorimeter both in 60Co and in the linac beam at a depth of 5 cm and a source to detector distance of 100 cm. Two high-purity chemical water systems were used in the detection vessel of the calorimeter, H2-saturated and Ar-saturated pure water, which are both supposed to give a zero heat defect. TPR20(10) and %dd(10) have been evaluated as beam quality specifiers. Simulations using the BEAM/DOSXYZ Monte Carlo system were performed to evaluate potential corrections on the measured beam qualities. The average kappaQ values measured for the three NE2571 chambers in the 5 MV and 10 MV photon beams are 0.995 +/- 0.005 and 0.979 +/- 0.005 respectively. For the three chambers used, the maximum deviation of individual kappaQ values is 0.2%. The measured beam quality specifiers %dd(10) and TPR20(10) are 67.0 and 0.705 for the 5 MV beam and 75.0 and 0.759 for the 10 MV beam. Although our beam design is very different from those used by other investigators for the measurement of kappaQ values, the agreement with their results is satisfactory showing a slightly better agreement when %dd(10) is used as the beam quality specifier.