Uncertainties in dosimetric data and beam calibration

Comparison of AAPM Addendum to TG-51, IAEA TRS-398, and JSMP 12: Calibration of photon beams in water

The American Association of Physicists in Medicine (AAPM) Working Group on TG-51 published an Addendum to the AAPM's TG-51 protocol (Addendum to TG-51) in 2014, and the Japan Society of Medical Physics (JSMP) published a new dosimetry protocol JSMP 12 in 2012. In this study, we compared the absorbed dose to water determined at the reference depth for high-energy photon beams following the recommendations given in AAPM TG-51 and the Addendum to TG-51, IAEA TRS-398, and JSMP 12. This study was performed using measurements with flattened photon beams with nominal energies of 6 and 10 MV. Three widely used ionization chambers with different compositions, Exradin A12, PTW 30013, and IBA FC65-P, were employed. Fully corrected charge readings obtained for the three chambers according to AAPM TG-51 and the Addendum to TG-51, which included the correction for the radiation beam profile (Prp ), showed variations of 0.2% and 0.3% at 6 and 10 MV, respectively, from the readings corresponding to IAEA TRS-398 and JSMP 12. The values for the beam quality conversion factor kQ obtained according to the three protocols agreed within 0.5%; the only exception was a 0.6% difference between the results obtained at 10 MV for Exradin A12 according to IAEA TRS-398 and AAPM TG-51 and the Addendum to TG-51. Consequently, the values for the absorbed dose to water obtained for the three protocols agreed within 0.4%; the only exception was a 0.6% difference between the values obtained at 10 MV for PTW 30013 according to AAPM TG-51 and the Addendum to TG-51, and JSMP 12. While the difference in the absorbed dose to water determined by the three protocols depends on the kQ and Prp values, the absorbed dose to water obtained according to the three protocols agrees within the relative uncertainties for the three protocols.

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

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.

Determining the incident electron fluence for Monte Carlo-based photon treatment planning using a standard measured data set

An accurate dose calculation in phantom and patient geometries requires an accurate description of the radiation source. Errors in the radiation source description are propagated through the dose calculation. With the emergence of linear accelerators whose dosimetric characteristics are similar to within measurement uncertainty, the same radiation source description can be used as the input to dose calculation for treatment planning at many institutions with the same linear accelerator model. Our goal in the current research was to determine the initial electron fluence above the linear accelerator target for such an accelerator to allow a dose calculation in water to within 1% or 1 mm of the measured data supplied by the manufacturer. The method used for both the radiation source description and the patient transport was Monte Carlo. The linac geometry was input into the Monte Carlo code using the accelerator's manufacturer's specifications. Assumptions about the initial electron source above the target were made based on previous studies. The free parameters derived for the calculations were the mean energy and radial Gaussian width of the initial electron fluence and the target density. A combination of the free parameters yielded an initial electron fluence that, when transported through the linear accelerator and into the phantom, allowed a dose-calculation agreement to the experimental ion chamber data to within the specified criteria at both 6 and 18 MV nominal beam energies, except near the surface, particularly for the 18 MV beam. To save time during Monte Carlo treatment planning, the initial electron fluence was transported through part of the treatment head to a plane between the monitor chambers and the jaws and saved as phase-space files. These files are used for clinical Monte Carlo-based treatment planning and are freely available from the authors.

Radiation dose measurements with alanine/agarose gel and thin alanine films around a 192Ir brachytherapy source, using ESR spectroscopy

Alanine/agarose gel and alanine films in stacks have been used for measurements of absorbed dose around an HDR 192Ir source in a vaginal cylinder-applicator, with and without a 180 degrees tungsten shield. The gel and the films were analysed by means of ESR spectroscopy and calibrated against an ion chamber in a 4 MV photon beam to obtain absolute dose values. The gel serves as both dosimeter and phantom material, and the thin (130 microm) films are used to achieve an improved spatial resolution in the dose estimations. Experimental values were compared with Monte Carlo simulations using two different codes. Results from the measurements generally agree with the simulations to within 5%, for both the alanine/agarose gel and the alanine films.

EPR dosimetric properties of 2-methylalanine: EPR, ENDOR and FT-EPR investigations

To find an EPR dosimeter material that is sensitive enough for clinical use, the substance 2-methylalanine (2MA) with the chemical structure (CH(3))(2)C(NH(3)(+))COO(-) was tested for its sensitivity to ionizing radiation, dose response, and radical stability over time. At equal and moderate settings of microwave power and modulation amplitude, 2MA was found to be 70% more sensitive than L-alpha-alanine, which is the most common EPR dosimeter material today. The dose response is linear, at least in the dose range of interest (0.5-00 Gy), and the time-dependent variations in signal intensity are very small and may be corrected for easily. The energy dependence of the stopping power and energy absorption was calculated and was found to be similar to that of alanine. The dependence of the signal intensity on microwave power and modulation amplitude was investigated, and the optimal settings were found to be 25 mW (Bruker ER 4102ST) and 12 gauss, respectively. Single crystals of 2MA were analyzed using ENDOR and ENDOR-induced EPR to identify the radiation-induced radicals that formed. Only one radical, in which the amino group is detached from the original molecule, was identified. This radical is obviously dominating and is apparently the only one relevant for dosimetry purposes. The complete set of coupling parameters for three hyperfine couplings is reported. The power saturation properties and spectral line width are ruled by the relaxation times T(1) and T(2). To determine the relaxation times of 2MA, pulsed EPR experiments were performed on single crystals. Two different values of T(1) were obtained, one in the range 1-3 micros, shown to be of importance for the dosimetry properties, and another that is strongly anisotropic with a value between 10 and 35 micros that does not seem to affect the saturation behavior. T(2) was estimated to be of the order of 200-300 ns.

Converting absorbed dose to medium to absorbed dose to water for Monte Carlo based photon beam dose calculations

Current clinical experience in radiation therapy is based upon dose computations that report the absorbed dose to water, even though the patient is not made of water but of many different types of tissue. While Monte Carlo dose calculation algorithms have the potential for higher dose accuracy, they usually transport particles in and compute the absorbed dose to the patient media such as soft tissue, lung or bone. Therefore, for dose calculation algorithm comparisons, or to report dose to water or tissue contained within a bone matrix for example, a method to convert dose to the medium to dose to water is required. This conversion has been developed here by applying Bragg-Gray cavity theory. The dose ratio for 6 and 18 MV photon beams was determined by computing the average stopping power ratio for the primary electron spectrum in the transport media. For soft tissue, the difference between dose to medium and dose to water is approximately 1.0%, while for cortical bone the dose difference exceeds 10%. The variation in the dose ratio as a function of depth and position in the field indicates that for photon beams a single correction factor can be used for each particular material throughout the field for a given photon beam energy. The only exception to this would be for the clinically non-relevant dose to air. Pre-computed energy spectra for 60Co to 24 MV are used to compute the dose ratios for these photon beams and to determine an effective energy for evaluation of the dose ratio.

On the beam quality specification of high-energy photons for radiotherapy dosimetry

An overview of common photon beam quality specifiers used in radiotherapy dosimetry introduces a reasoned discussion on the advantages and disadvantages of TPR20,10 and PDD(10)x. It is shown that some of the potential advantages of PDD(10)x are also present in other well known beam quality specifiers such as d80. However, all PDD-based beam quality indices, including PDD(10)x, are subject to electron contamination and their determination is affected by practical limitations. The proposed filtration of contaminant electrons by Kosunen and Rogers [Med. Phys. 20, 1181-1188 (1993)] and by Li and Rogers [Med. Phys. 21, 791-798 (1994)] is questioned, not only with regard to the adequacy of using lead as an electron filter, but also in relation to its efficiency (if there were no contamination, restrictions for beam calibrations at dmax would be removed) and practical measurement. It is argued that (i) there is no unique beam quality specifier that works satisfactorily in all possible conditions, for the entire energy range of photon energies used in radiotherapy and all possible accelerators used in hospitals and in standards laboratories, and (ii) TPR20,10 remains to be the most appropriate specifier for clinical photon beams as it has less practical drawbacks than PDD-based quality indices. The final impact on clinical photon beam dosimetry resulting from the use of different photon beam quality specifiers, is that they are not expected to yield a significant change (i.e., more than 0.5% and in most cases well within 0.2%) in the absorbed dose to water in reference conditions for most clinical beams.

Dose calculations for external photon beams in radiotherapy

Dose calculation methods for photon beams are reviewed in the context of radiation therapy treatment planning. Following introductory summaries on photon beam characteristics and clinical requirements on dose calculations, calculation methods are described in order of increasing explicitness of particle transport. The simplest are dose ratio factorizations limited to point dose estimates useful for checking other more general, but also more complex, approaches. Some methods incorporate detailed modelling of scatter dose through differentiation of measured data combined with various integration techniques. State-of-the-art methods based on point or pencil kernels, which are derived through Monte Carlo simulations, to characterize secondary particle transport are presented in some detail. Explicit particle transport methods, such as Monte Carlo, are briefly summarized. The extensive literature on beam characterization and handling of treatment head scatter is reviewed in the context of providing phase space data for kernel based and/or direct Monte Carlo dose calculations. Finally, a brief overview of inverse methods for optimization and dose reconstruction is provided.

Absorbed-dose calibrations in high-energy photon beams at the National Physical Laboratory: conversion procedure

The absorbed-dose calibration service from NPL is based on a primary-standard calorimeter that measures absorbed dose to graphite. Secondary-standard dosemeters are calibrated in absorbed dose to water in a 60Co gamma-ray beam and in x-ray beams over a range of generating potentials from 4 MV to 19 MV. Two methods were used to convert the calibrations of working-standard ionization chambers from absorbed dose to graphite into absorbed dose to water. One method involved the use of published interaction data for photons and secondary electrons, and required a knowledge of the chamber construction. The second method involved the calculation of the ratio of absorbed dose in graphite and water phantoms irradiated consecutively in the same photon beam using the photon-fluence scaling theorem. The two methods were in agreement to 0.1%.

Uncertainty of calibrations at the accredited dosimetry calibration laboratories

The American Association of Physicists in Medicine, through a subcommittee (formerly Task Group 3) of the Radiation Therapy Committee, has accredited five laboratories to perform calibrations of instruments used to calibrate therapeutic radiation beams. The role of the accredited dosimetry calibration laboratories (ADCLs) is to transfer a calibration factor from an instrument calibrated by the National Institute of Standards and Technology (NIST) to a customer's instrument. It is of importance to the subcommittee, to physicists using the services of the ADCLs, and to the ADCLs themselves, to know the uncertainty of instrument calibrations. The calibration uncertainty has been analyzed by asking the laboratories to provide information about their calibration procedures. Estimates of uncertainty by two procedures were requested: Type A are uncertainties derived as the standard deviations of repeated measurements, while type B are estimates of uncertainties obtained by other methods, again expressed as standard deviations. Data have been received describing the uncertainty of each parameter involved in calibrations, including those associated with measurements of charge, exposure time, and air density, among others. These figures were combined with the uncertainty of NIST calibrations, to arrive at an overall uncertainty which is expressed at the two-standard deviation level. For cable-connected instruments in gamma-ray and x-ray beams of HVL > 1 mm Al, the figure has an upper bound of approximately 1.2%.

Proton dosimetry intercomparison

BACKGROUND AND PURPOSE

Methods for determining absorbed dose in clinical proton beams are based on dosimetry protocols provided by the AAPM and the ECHED. Both groups recommend the use of air-filled ionization chambers calibrated in terms of exposure or air kerma in a 60Co beam when a calorimeter or Faraday cup dosimeter is not available. The set of input data used in the AAPM and the ECHED protocols, especially proton stopping powers and w-value is different. In order to verify inter-institutional uniformity of proton beam calibration, the AAPM and the ECHED recommend periodic dosimetry intercomparisons. In this paper we report the results of an international proton dosimetry intercomparison which was held at Loma Linda University Medical Center. The goal of the intercomparison was two-fold: first, to estimate the consistency of absorbed dose delivered to patients among the participating facilities, and second, to evaluate the differences in absorbed dose determination due to differences in 60Co-based ionization chamber calibration protocols.

MATERIALS AND METHODS

Thirteen institutions participated in an international proton dosimetry intercomparison. The measurements were performed in a 15-cm square field at a depth of 10 cm in both an unmodulated beam (nominal accelerator energy of 250 MeV) and a 6-cm modulated beam (nominal accelerator energy of 155 MeV), and also in a circular field of diameter 2.6 cm at a depth of 1.14 cm in a beam with 2.4 cm modulation (nominal accelerator energy of 100 MeV).

RESULTS

The results of the intercomparison have shown that using ionization chambers with 60Co calibration factors traceable to standard laboratories, and institution-specific conversion factors and dose protocols, the absorbed dose specified to the patient would fall within 3% of the mean value. A single measurement using an ionization chamber with a proton chamber factor determined with a Faraday cup calibration differed from the mean by 8%.

CONCLUSION

The adoption of a single ionization chamber dosimetry protocol and uniform conversion factors will establish agreement on proton absorbed dose to approximately 1.5%, consistent with that which has been observed in high-energy photon and electron dosimetry.

Characteristics of the absorbed dose to water standard at ENEA

The primary standard of absorbed dose to water established at ENEA for the Co-60 gamma-ray quality is based on a graphite calorimeter and an ionometric transfer system. This standard was recently improved after a more accurate assessment of some perturbation effects in the calorimeter and a modification of the water phantom shape and size. The conversion procedure requires two corresponding depths, one in graphite and one in water, where the radiation energy spectra must be the same. The energy spectra at the corresponding points were determined by a Monte Carlo simulation in water and graphite scaled phantoms. A thorough study of the calorimeter gap effect corrections was also made with regard to their dependence on depth and field size. A comparison between the ionization chamber calibration procedures based on the standards of absorbed dose to water and of air kerma was also made, confirming the consistency of the two methods.

Ionization chamber dosimetry of proton beams using cylindrical and plane parallel chambers. Nw versus Nk ion chamber calibrations

Determinations of the absorbed dose in a 170 MeV proton beam have been performed using seven ionization chambers of different types: five cylindrical (two FWT IC-18 and three NE-2571, of which one was modified to have the central electrode made of graphite) and two plane parallel (NACP-02 and Roos FK-6). The ionization was converted into absorbed dose in the proton beam according to the generalization of the formalism provided by the IAEA Code of Practice (TRS 277), which enables the use of the same equations for all kinds of beam used in radiotherapy. The absorbed dose obtained with the two IC-18 chambers, a chamber type commonly used as a reference in proton beams, was up to 1.5% lower than that obtained with the Farmer NE-2571 chamber, which was used as the reference in this work when calibration factors in terms of NK were used. To investigate this difference, experimental ND factors for six chambers (the two IC-18 chambers, the NACP-02, the FK-6 and two of the NE-2571 chambers) were determined in a high-energy electron beam. The procedure commonly recommended for plane parallel ion chambers was used for all the chambers, using the same reference chamber, a Farmer NE-2571. In the 170 MeV proton beam all the ND factors yielded consistent absorbed dose determinations within the estimated experimental uncertainties. This finding calls into question the value of the product kattkm for the IC-18 chamber given by the IAEA Code of Practice used in this comparison, and points at possible chamber to chamber variations that theoretical kattkm factors cannot predict. The investigations enabled the determination of the Pwall(60Co) factor of the Roos FK-6 plane parallel chamber, yielding 1.003 +/- 0.5%, and a correction for the effect of the aluminium central electrode of NE-2571 chambers in proton beams, equal to 1.003 +/- 0.4%. Two of the chambers (the plane parallel FK-6 and the modified cylindrical NE-2571) were provided with calibration factors in terms of absorbed dose to water, Nw, at the quality of 60Co by the Primary Standard Dosimetry Laboratory in Germany (PTB). Using the Nw formalism excellent agreement was found with the determination based on the experimental ND, giving support to the implementation of the NW procedure in therapeutic proton beams.

The status of high-energy photon and electron beam dosimetry five years after the implementation of the IAEA Code of Practice in the Nordic countries

The status of the dosimetry of high-energy photon and electron beams is analysed, taking into account the main developments in the field since the implementation of the IAEA Code of Practice in the Nordic countries. In electron beam dosimetry, energy-range relationships are discussed; Monte-Carlo results with different codes are compared with the experimentally derived empirical expression used in most protocols. Updated calculations of water-to-air stopping-power ratios following the changes in the Monte-Carlo code used to compute actual Sw,air values are compared with the data included in most dosimetry protocols. The validity of the commonly used procedure to select stopping-power ratios for a clinical beam from the mean energy at the phantom surface and the depth of measurement, is analysed for 'realistic' electron beams. In photon beam dosimetry, calculated correction factors including the effect of the wall plus waterproofing sleeve and existing data on the shift of the effective point of measurement of an ionization chamber, are discussed. New calculations of medium-to-air stopping-power ratios and their correlation with the quality of the beam obtained from the convolution of Monte-Carlo kernels are presented together with their possible practical implications in dosimetry. Trends in Primary Standard Dosimetry Laboratories towards implementing calibrations in terms of absorbed dose to water are presented, emphasizing controversial proposals for the specification of photon beam qualities. Plane-parallel ionization chambers are discussed regarding aspects that affect determinations of absorbed dose, either through the different methods used for the calibration of these chambers or by means of correction factors. Recent studies on the effect of the central electrode in Farmer-type cylindrical chambers are described.

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.