Comparison of dosimetry calibration factors at the NRCC and the NIST. National Research Council of Canada. National Institute of Standards and Technology

Addendum to the AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon beams

An addendum to the AAPM's TG-51 protocol for the determination of absorbed dose to water in megavoltage photon beams is presented. This addendum continues the procedure laid out in TG-51 but new kQ data for photon beams, based on Monte Carlo simulations, are presented and recommendations are given to improve the accuracy and consistency of the protocol's implementation. The components of the uncertainty budget in determining absorbed dose to water at the reference point are introduced and the magnitude of each component discussed. Finally, the consistency of experimental determination of ND,w coefficients is discussed. It is expected that the implementation of this addendum will be straightforward, assuming that the user is already familiar with TG-51. The changes introduced by this report are generally minor, although new recommendations could result in procedural changes for individual users. It is expected that the effort on the medical physicist's part to implement this addendum will not be significant and could be done as part of the annual linac calibration.

The US radiation dosimetry standards for Co60 therapy level beams, and the transfer to the AAPM accredited dosimetry calibration laboratories

This work reports the transfer of the primary standard for air kerma from the National Institute of Standards and Technology (NIST) to the secondary laboratories accredited by the American Association of Physics in Medicine (AAPM). This transfer, performed in August of 2003, was motivated by the recent revision of the NIST air-kerma standards for 60Co gamma-ray beams implemented on July 1, 2003. The revision involved a complete recharacterization of the two NIST therapy-level 60Co gamma-ray beam facilities, resulting in new values for the air-kerma rates disseminated by the NIST. Some of the experimental aspects of the determination of the new air-kerma rates are briefly summarized here; the theoretical aspects have been described in detail by Seltzer and Bergstrom ["Changes in the U.S. primary standards for the air-kerma from gamma-ray beams," J. Res. Natl. Inst. Stand. Technol. 108, 359-381 (2003)]. The standard was transferred to reference-class chambers submitted by each of the AAPM Accredited Dosimetry Calibration Laboratories (ADCLs). These secondary-standard instruments were then used to characterize the 60Co gamma-ray beams at the ADCLs. The values of the response (calibration coefficient) of the ADCL secondary-standard ionization chambers are reported and compared to values obtained prior to the change in the NIST air-kerma standards announced on July 1, 2003. The relative change is about 1.1% for all of these chambers, and this value agrees well with the expected change in chambers calibrated at the NIST or at any secondary-standard laboratory traceable to the new NIST standard.

Shift in absorbed dose for megavoltage photons when changing to TRS-398 in Australia

Australian primary standards of air kerma and absorbed dose are realized in 60Co gamma rays. To calibrate the megavoltage photon beams from linear accelerators, radiotherapy centres have their ionization chamber calibrated in a 60Co beam and then use a protocol to transfer this calibration to the higher energy. The radiotherapy community is in the process of changing from the ACPSEM Protocol (Second Edition 1998) based on an air kerma calibration to the IAEA's TRS-398 Code of Practice, based on an absorbed dose to water calibration. To evaluate the shift in absorbed dose resulting from the new protocol, the absorbed dose should be determined using both protocols and compared. We present a formula for this shift which can be used to check the result. To use this formula the centre needs to measure a displacement correction and know the ratio of the air kerma to absorbed dose to water calibration factors at 60Co. We calculate the change they should expect by using the average ratio of the air kerma and absorbed dose to water calibration factors for NE2571 and NE2561 chambers, based on Australian standards, and by estimating the displacement correction from published depth dose data. We find the absorbed dose in a megavoltage photon beam to increase by between 0.1 and 0.6% for NE2571 chambers and between 0.7 and 1.1% for NE2561 chambers, for beams up to 35 MV. The dose measured using TRS-398 is always higher.

Advances in the determination of absorbed dose to water in clinical high-energy photon and electron beams using ionization chambers

During the last two decades, absorbed dose to water in clinical photon and electron beams was determined using dosimetry protocols and codes of practice based on radiation metrology standards of air kerma. It is now recommended that clinical reference dosimetry be based on standards of absorbed dose to water. Newer protocols for the dosimetry of radiotherapy beams, based on the use of an ionization chamber calibrated in terms of absorbed dose to water, N(D,w), in a standards laboratory's reference quality beam, have been published by several national or regional scientific societies and international organizations. Since the publication of these protocols multiple theoretical and experimental dosimetry comparisons between the various N(D,w) based recommendations, and between the N(D,w) and the former air kerma (NK) based protocols, have been published. This paper provides a comprehensive review of the dosimetry protocols based on these standards and of the intercomparisons of the different protocols published in the literature, discussing the reasons for the observed discrepancies between them. A summary of the various types of standards of absorbed dose to water, together with an analysis of the uncertainties along the various steps of the dosimetry chain for the two types of formalism, is also included. It is emphasized that the NK-N(D,air) and N(D,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.

A dosimetry study comparing NCS report-5, IAEA TRS-381, AAPM TG-51 and IAEA TRS-398 in three clinical electron beam energies

New codes of practice for reference dosimetry in clinical high-energy photon and electron beams have been published recently, to replace the air kerma based codes of practice that have determined the dosimetry of these beams for the past twenty years. In the present work, we compared dosimetry based on the two most widespread absorbed dose based recommendations (AAPM TG-51 and IAEA TRS-398) with two air kerma based recommendations (NCS report-5 and IAEA TRS-381). Measurements were performed in three clinical electron beam energies using two NE2571-type cylindrical chambers, two Markus-type plane-parallel chambers and two NACP-02-type plane-parallel chambers. Dosimetry based on direct calibrations of all chambers in 60Co was investigated, as well as dosimetry based on cross-calibrations of plane-parallel chambers against a cylindrical chamber in a high-energy electron beam. Furthermore, 60Co perturbation factors for plane-parallel chambers were derived. It is shown that the use of 60Co calibration factors could result in deviations of more than 2% for plane-parallel chambers between the old and new codes of practice, whereas the use of cross-calibration factors, which is the first recommendation in the new codes, reduces the differences to less than 0.8% for all situations investigated here. The results thus show that neither the chamber-to-chamber variations, nor the obtained absolute dose values are significantly altered by changing from air kerma based dosimetry to absorbed dose based dosimetry when using calibration factors obtained from the Laboratory for Standard Dosimetry, Ghent, Belgium. The values of the 60Co perturbation factor for plane-parallel chambers (k(att) x k(m) for the air kerma based and p(wall) for the absorbed based codes of practice) that are obtained from comparing the results based on 60Co calibrations and cross-calibrations are within the experimental uncertainties in agreement with the results from other investigators.

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.

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.

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.

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 Wellhöfer-FC65G (previously Wellhöfer-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 Wellhöfer-FC65G for the different beams. The absorbed dose results for the NE2571 and Wellhöfer-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.

Reference dosimetry in clinical high-energy electron beams: comparison of the AAPM TG-51 and AAPM TG-21 dosimetry protocols

A comparison of the determination of absorbed dose to water in reference conditions with high-energy electron beams (Enominal of 6, 8, 10, 12, 15, and 18 MeV) following the recommendations given in the AAPM TG-51 and in the original TG-21 dosimetry protocols has been made. Six different ionization chamber types have been used, two Farmer-type cylindrical (PTW 30001, PMMA wall; NE 2571, graphite wall) and four plane parallel (PTW Markus, and Scanditronix-Wellhöfer NACP, PPC-05 and Roos PPC-40). Depending upon the cylindrical chamber type used and the beam energy, the doses at dmax determined with TG-51 were higher than with TG-21 by about 1%-3%. Approximately 1% of this difference is due to the differences in the data given in the two protocols; another 1.1%-1.2% difference is due to the change of standards, from air-kerma to absorbed dose to water. For plane-parallel chambers, absorbed doses were determined by using two chamber calibration methods: (i) direct use of the ADCL calibration factors N(60Co)D,w and Nx for each chamber type in the appropriate equations for dose determination recommended by each protocol, and (ii) cross-calibration techniques in a high-energy electron beam, as recommended by TG-21, TG-39, and TG-51. Depending upon the plane-parallel chamber type used and the beam energy, the doses at dmax determined with TG-51 were higher than with TG-21 by about 0.7%-2.9% for the direct calibration procedures and by 0.8%-3.2% for the cross-calibration techniques. Measured values of photon-electron conversion kecal, for the NACP and Markus chambers were found to be 0.3% higher and 1.7% lower than the corresponding values given in TG-51. For the PPC-05 and PPC-40 (Roos) chamber types, the values of kecal were measured to be 0.889 and 0.893, respectively. The uncertainty for the entire calibration chain, starting from the calibration of the ionization chamber in the standards laboratory to the determination of absorbed dose to water in the user beam, has been analyzed for the two formalisms. For cylindrical chambers, the observed differences between the two protocols are within the estimated combined uncertainty of the ratios of absorbed doses for 6 and 8 MeV; however, at higher energies (10< or =E< or =18 MeV), the differences are larger than the estimated combined uncertainties by about 1%. For plane-parallel chambers, the observed differences are within the estimated combined uncertainties for the direct calibration technique; for the cross-calibration technique the differences are within the uncertainty estimates at low energies whereas they are comparable to the uncertainty estimates at higher energies. A detailed analysis of the reasons for the discrepancies is made which includes comparing the formalisms, correction factors, and quantities in the two protocols, as well as the influence of the implementation of the different standards for chamber calibration.

Comparison of dosimetric standards of Canada and France for photons at 60Co and higher energies

We report the results of a comparison of the dosimetric standards of Canada and France for photon beams at 60Co and a few higher energies. The present primary standard of absorbed dose to water for NRC, Canada is based on measurements made with a sealed water calorimeter. The corresponding standard of the LNHB, France is based on measurements made with a graphite calorimeter at 60Co energy and transferred to absorbed dose to water for 60Co and higher-energy photon beams using both ion chambers and Fricke dosemeters as transfer instruments. To make this comparison, we used three graphite-walled NE2571 Farmer chambers. The absorbed dose to water determined by the LNHB was greater than that determined by NRC by 0.20% at 60Co energy. This difference is not significant given the uncertainties on the standards. In order to do the comparison for higher-energy photons, we interpolated the NRC data set at the beam qualities used at the LNHB. When %dd(10)x is used as the method of specifying beam quality, the determination of absorbed dose to water by the LNHB is about 0.2% greater than that determined by NRC and consistent with the results at 60Co. However, when using TPR20,10 as the beam quality specifier, the LNHB determination is greater than the NRC's determination by 0.8% and 1.2% at 12 and 20 MV respectively. This discrepancy, which systematically increases with increasing energy, eventually exceeds the uncertainties in the ratio of the standards, estimated to be 0.7%. This underscores the importance of selecting the method of specifying beam quality, either %dd(10)x or TPR20,10, at least for the 'soft' beams used by NRC in this comparison. In the case of the air kerma standards, which were also compared at 60Co energy, the LNHB determination was greater than NRC's by 0.14%, which is not significant given the uncertainties on the standards.

Reference dosimetry in clinical high-energy photon beams: comparison of the AAPM TG-51 and AAPM TG-21 dosimetry protocols

Task Group 51 (TG-51) of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM) has recently developed a new protocol for the calibration of high-energy photon and electron beams used in radiation therapy. The formalism and the dosimetry procedures recommended in this protocol are based on the use of an ionization chamber calibrated in terms of absorbed dose-to-water in a standards laboratory's 60Co gamma ray beam. This is different from the recommendations given in the AAPM TG-21 protocol, which are based on an exposure calibration factor of an ionization chamber in a 60Co beam. The purpose of this work is to compare the determination of absorbed dose-to-water in reference conditions in high-energy photon beams following the recommendations given in the two dosimetry protocols. This is realized by performing calibrations of photon beams with nominal accelerating potential of 6, 18 and 25 MV, generated by an Elekta MLCi and SL25 series linear accelerator. Two widely used Farmer-type ionization chambers having different composition, PTW 30001 (PMMA wall) and NE 2571 (graphite wall), were used for this study. Ratios of AAPM TG-51 to AAPM TG-21 doses to water are found to be 1.008, 1.007 and 1.009 at 6, 18 and 25 MV, respectively when the PTW chamber is used. The corresponding results for the NE chamber are 1.009, 1.010 and 1.013. The uncertainties for the ratios of the absorbed dose determined by the two protocols are estimated to be about 1.5%. A detailed analysis of the reasons for the discrepancies is made which includes comparing the formalisms, correction factors and quantities in the two protocols, as well as the influence of the implementation of the different standards for chamber calibration. The latter has been found to have a considerable influence on the differences in clinical dosimetry, even larger than the adoption of the new data and recommended procedures, as most intrinsic differences cancel out due to the adoption of the new formalism.

Absorbed-dose beam quality conversion factors for cylindrical chambers in high energy photon beams

Recent working groups of the AAPM [Almond et al., Med. Phys. 26, 1847 (1999)] and the IAEA (Andreo et al., Draft V.7 of "An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water," IAEA, 2000) have described guidelines to base reference dosimetry of high energy photon beams on absorbed dose to water standards. In these protocols use is made of the absorbed-dose beam quality conversion factor, kQ which scales an absorbed-dose calibration factor at the reference quality 60Co to a quality Q, and which is calculated based on state-of-the-art ion chamber theory and data. In this paper we present the measurement and analysis of beam quality conversion factors kQ for cylindrical chambers in high-energy photon beams. At least three chambers of six different types were calibrated against the Canadian primary standard for absorbed dose based on a sealed water calorimeter at 60Co [TPR10(20)=0.572, %dd(10)x=58.4], 10 MV [TPR10(20)=0.682, %dd(10)x=69.6), 20 MV (TPR10(20)=0.758, %dd(10)x= 80.5] and 30 MV [TPR10(20) = 0.794, %dd(10)x= 88.4]. The uncertainty on the calorimetric determination of kQ for a single chamber is typically 0.36% and the overall 1sigma uncertainty on a set of chambers of the same type is typically 0.45%. The maximum deviation between a measured kQ and the TG-51 protocol value is 0.8%. The overall rms deviation between measurement and the TG-51 values, based on 20 chambers at the three energies, is 0.41%. When the effect of a 1 mm PMMA waterproofing sleeve is taken into account in the calculations, the maximum deviation is 1.1% and the overall rms deviation between measurement and calculation 0.48%. When the beam is specified using TPR10(20), and measurements are compared with kQ values calculated using the version of TG-21 with corrected formalism and data, differences are up to 1.6% when no sleeve corrections are taken into account. For the NE2571 and the NE2611A chamber types, for which the most literature data are available, using %dd(10)x, all published data show a spread of 0.4% and 0.6%, respectively, over the entire measurement range, compared to spreads of up to 1.1% for both chambers when the kQ values are expressed as a function of TPR10(20). For the PR06-C chamber no clear preference of beam quality specifier could be identified. When comparing the differences of our kQ measurements and calculations with an analysis in terms of air-kerma protocols with the same underlying calculations but expressed in terms of a compound conversion factor CQ, we observe that a system making use of absorbed-dose calibrations and calculated kQ values, is more accurate than a system based on air-kerma calibrations in combination with calculated CQ (rms deviation of 0.48% versus 0.67%, respectively).