key comparison bipm

Metrologia 2010 47 Tech. Suppl. 06025

Final Report 2010-10-22

Comparison of the standards for absorbed dose to water of the NRC and the BIPM for accelerator photon beams

S. Picard1, D. T. Burns1, P. Roger1, P. J. Allisy-Roberts1

M. R. McEwen2, C. D. Cojocaru2, C. K. Ross2

1Bureau International de Poids et Mesures, Pavillon de Breteuil, F-92312 Sèvres cedex 2National Research Council of Canada, Ionizing Radiation Standards, 1200 Montreal Rd, Ottawa, ON K1A 0R6, Canada

______________________________________________________________________

ABSTRACT

A comparison of the dosimetry for high-energy accelerator photon beams was carried out between the National Research Council of Canada (NRC) and the Bureau International des Poids et Mesures (BIPM) in June 2009. The comparison was based on the determination of absorbed dose to water for three radiation qualities. The comparison result, reported as a ratio of the NRC and the BIPM evaluations, is 0.997 at 6 MV, 1.001 at 10 MV and 0.994 at 25 MV, each with a relative standard uncertainty of 6 × 10−3. This result is the first of the ongoing BIPM.RI(I)-K6 comparison.

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1. INTRODUCTION

With the objective to compare the absorbed dose to water determinations of the National Metrology Institutes (NMIs) for accelerator photon beams, the Bureau International des Poids et Mesures (BIPM) has developed a transportable standard for absorbed dose to water based on a graphite calorimeter [1]. A comparison programme was adopted in 2008, currently proposed for ten NMIs registered in the BIPM key comparison database (KCDB) under BIPM.RI(I)-K6 [2]. Within this framework, a first comparison of accelerator photon-beam dosimetry has been made between the National Research Council of Canada (NRC) and the BIPM. The measurements were carried out in the accelerator laboratory of the Ionizing Radiation Section of the Institute for National Measurement Standards in Ottawa during the period 3 to 16 June 2009. The BIPM calorimeter, water phantom and electronics were shipped in advance for the purpose, while the transfer ionization chambers were hand-carried.

The comparison between an NMI and the BIPM is established by the reciprocal determination of absorbed dose to water at several accelerator radiation qualities. The BIPM absorbed-dose measurement, Dw,BIPM, is made directly at the institute. The NMI has an established standard Dw,NMI for these qualities, and it is realized during the comparison using one or more NRC calibrated transfer ionization chambers. The comparison result for each quality is the ratio

BIPMw,

NMIw,

DD

R = (1)

and its associated uncertainty uc(R). A comparison protocol was developed and adopted for guidance before, during and after the comparison [3].

2. METHOD AND MEASUREMENT ARRANGEMENTS

2.1. NRC Determination of Absorbed Dose to Water

2.1.1. Description of calorimeter system

The Canadian primary standard for absorbed dose to water in both 60Co and high-energy photon beams operated at the NRC is a sealed water calorimeter. The calorimeter has been described in detail by Ross et al. [4]. The radiation-induced temperature rise is measured by thermistors mounted in a glass vessel, filled with high purity water. The glass vessel is positioned at the reference depth in a cubic water phantom of side length 300 mm. The dose to water, Dw is then given by:

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HDddpvctwww 1

1k

kkkkkkcT = D−

⋅⋅⋅⋅⋅⋅⋅⋅Δ ρ (2)

where ΔTw is the radiation-induced temperature rise and cw is the specific heat capacity of water. The various k-factors correct for the following: kt – transient effects on the thermistor response, due to dose deposition in the

thermistor itself;

kc – conductive heat transfer. The 3-D heat transport equation is solved using finite element methods to determine the effect due to excess heat in the glass components and temperature gradients (radial and axial);

kv – convective heat transfer. This correction is assumed to be unity since the calorimeter is operated at 4 ºC;

kp – perturbation of the radiation field by the vessel and probes;

kdd – non-uniformity of the dose profile;

kρ – correction for the change in density (equivalent to a change in depth) when comparing the dose measured by the water calorimeter at 4 ºC and ion chambers at room temperature;

kHD – heat defect. This correction accounts for the difference between the energy absorbed and the energy appearing as heat, which is due primarily to radiation-induced chemical reactions.

The NRC water calorimeter has been in use for over a decade and the primary

validation route is through the key comparison programme for Co-60 absorbed dose to water organized by the BIPM. Also, a number of high-energy bilateral comparisons have been carried out with other primary standards laboratories [5, 6]. The water calorimeter has been used to determine absorbed-dose calibration coefficients for Farmer-type ionization chambers and results covering a period of nearly ten years are summarized by McEwen [7]. The NE2571 is the default NRC reference chamber and was used as the transfer standard in this comparison.

2.1.2. Uncertainties

The relative standard uncertainty in measuring absorbed dose to water using the NRC water calorimeter is estimated to be 3.0 × 10–3. The relative standard uncertainty in the determination of ion chamber calibration coefficients is estimated to be 3.5 × 10–3, while kQ factors measured using the calorimeter have a reduced relative standard uncertainty of 2.7 × 10–3 due to correlation. These uncertainty values, smaller than reported by Seuntjens et al. [8], result from the re-evaluated heat defect of the water calorimeter made by Ross et al. [9] which formerly was the

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dominant source of uncertainty in the measurements. The revised uncertainty of the water heat defect 1.5 × 10–3 is consistent with the analysis previously made by Krauss [10].

For the routine dissemination of absorbed dose to water in megavoltage beams the NRC uses a set of reference thimble chambers, type NE2571, calibrated against the NRC water calorimeter. The stability of this chamber set is monitored using the NRC 60Co facility and by internal comparison in accelerator beams. Since water calorimetry is very time consuming the same approach was used in this comparison.

The uncertainties for the determination of Dw,NRC for photon beam energies of 6 MV and above using a NE2571 transfer standard ionization chamber are given in Table 1. In effect, this is the uncertainty of the calibration of the NRC NE2581 thimble-type monitor chamber (see Section 2.4).

Table 1. Standard uncertainty components for the determination of Dw,NRC for photon beam energies of 6 MV and above using a NE2571 reference ionization chamber. It should be noted that this is for the reference conditions (SDD1 = 100 cm) used for the present comparison, which are different from those normally used at NRC (SSD2 = 100 cm).

Type A relative standard uncertainty component uA(y)/y / 10–3

typical standard uncertainty of the mean 0.2 transfer to NE2581 monitor chamber (within day) 1.7(i)

Type B relative standard uncertainty component uB(y)/y / 10B

–3

ND,w 3.5 SDD correction (110 cm → 100 cm) 0.5 Positioning 0.5

krn for NE2571 (radial non-uniformity correction) 0.4

PTP (temperature and pressure correction) 0.6 Pion (ion recombination correction) 0.8 Ppol (polarity correction) 0.3

Pelec (electrometer calibration) 0.2

combined relative standard uncertainty [uc(y)/y] / 10–3: 4.1

(i) This value is larger than normal (and larger than in Tables 2 and 3) because of a site-wide power failure during this part of the comparison. Due to time constraints and other issues with the accelerator operation it was not possible to repeat the measurements with the NRC reference chamber (see 4.1).

1 Source to Detector Distance 2 Source to Surface Distance

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2.2. BIPM Determination of Absorbed Dose to Water

The BIPM absorbed-dose graphite calorimeter is described in detail in [1]; a short description is given below.

2.2.1. The BIPM graphite calorimeter and transfer ionization chambers

The calorimeter consists of a graphite core 45 mm in diameter and 6.7 mm thick placed in a cylindrical graphite jacket with outer diameter 60 mm. The core is equipped with three thermistor pairs connected to three independent d.c. bridges. This core and jacket are placed in an evacuated cubic PMMA3 vacuum phantom with side length 300 mm. The specific heat capacity cp of the graphite core has been determined previously in a separate experiment [11, 12]. The absorbed dose Dc in the graphite core is hence determined by measuring the temperature rise, ΔT, at the temperature T when the calorimeter is exposed to a photon beam:

TTcD p Δ⋅= )(c . (3)

Two nominally-identical parallel-plate ionization chambers with graphite walls and collector, similar in design to the existing BIPM standards for air kerma and absorbed dose to water, were fabricated for the determination of the absorbed dose to water from the measured absorbed dose to the graphite core. The first chamber is housed in a graphite jacket, nominally identical to the calorimeter jacket, and is positioned in the same PMMA vacuum phantom but at ambient air pressure, replacing the calorimeter core and its jacket. The second chamber is housed in a waterproof polyethylene sleeve and mounted in a PMMA water phantom with the same outer dimensions and PMMA window thickness as the vacuum phantom.

The charge produced when the chambers are irradiated is accumulated in a calibrated external capacitor and measured using a commercial electrometer operated in charge mode.

2.2.2. Description of the BIPM method to convert to absorbed dose to water

The method adopted by the BIPM combining calorimetric and ionometric measurements with Monte Carlo simulations to determine the absorbed dose to water is reported in [13]. For completeness, a short description is included here.

When the transfer chamber is positioned in the graphite jacket, replacing the core, the calibration coefficient ND,c for graphite absorbed dose can be written as

3 Polymethyl methacrylate (PMMA)

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c

cc, Q

DN D = , (4)

where Dc is the mean absorbed dose to the graphite core and Qc is the ionization charge. The water absorbed dose Dw at the reference point in a homogeneous water phantom can be written as

ww,w QND D= , (5)

where ND,w is the transfer chamber calibration coefficient in terms of absorbed dose to water and Qw is the ionization charge measured in water. Through the use of Monte Carlo simulations of the complete graphite and water geometries, ND,w is obtained using

MC

c,

w,c,w, ⎟

⎟⎠

⎞⎜⎜⎝

⎛=

D

DDD N

NNN , (6)

where the superscript MC indicates Monte Carlo simulations of the parameters in parenthesis. The ionization charge Q is represented in the Monte Carlo calculations by the mean absorbed dose to the air of the cavity, Dcav. It follows that

cCQQD

DD

DD

QQDD ,w

c

wc

MC

wcav,

ccav,MC

c

w

c

wcw =⎟

⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛= , (7)

where Cw,c represents the total Monte Carlo conversion factor.

In principle, Dw could be obtained from a measurement of Dc and a Monte Carlo calculation of Dw/Dc. However, the direct Monte Carlo calculation of the ratio of absorbed doses will be sensitive to the details of the irradiation geometries, the incident spectra, the radiation transport mechanisms and the choice of data for photon and electron cross sections. By calculating and measuring a transfer ionization chamber response for the same geometries this sensitivity is greatly reduced.

Thus, to determine the absorbed dose to water using the BIPM graphite calorimeter, three different measurement arrangements are realized, and the corresponding parameters are determined using Monte Carlo calculations. The three configurations are illustrated schematically in Figure 1. Note that, for practical reasons, two nominally-identical transfer chambers are used and the

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measured charge ratio Qw/Qc is corrected for the known small difference in the chamber responses (measured free in air in a 60Co beam).

Dca) Qcb) Qwc)Dca) Dca) Qcb) Qcb) Qwc) Qwc)

Figure 1. Schematic representation of the three measurement situations. All measurements are made in a cubic PMMA phantom, here represented by the dark blue square. a) The calorimeter is used in vacuum and the jacket containing the core is represented by the sectioned black disc. The absorbed dose to graphite, Dc, is both measured and calculated. b) The graphite core is replaced by the transfer ionization chamber and the assembly is at atmospheric pressure. The ionization charge in graphite, Qc, is measured and the corresponding cavity dose, Dcav,c, calculated. c) The ionization chamber is placed in a waterproof envelope inside an identical phantom filled with water. The ionization charge in water, Qw, is measured and the corresponding cavity dose, Dcav,w, calculated. The mean absorbed dose to water, Dw, in the absence of the chamber and envelope is also calculated (for a water detector with the same dimensions as the cavity).

2.2.3. Uncertainties

In Table 2, the uncertainties for the determination of Dc are listed. In effect, this is the uncertainty of the calibration of the NE2581 thimble monitor chamber. The combined standard uncertainty of Dc is 1.7 parts in 103. The uncertainties associated with the determination of the ratio Qw/Qc are listed in Table 3. The orientation corrections were determined in a 60Co beam.

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Table 2. Standard uncertainty components for the determination of Dc at the NRC. The statistical uncertainty is based on n = 3 determinations for each radiation quality, each one being the result of ten irradiations of thirty seconds.

Type A relative standard uncertainty component uA(y)/y / 10–3

typical standard uncertainty of the mean (n = 3), 1.2 including the transfer using the NRC NE2581 monitor chamber Type B relative standard uncertainty component uB(y)/y / 10B

–3

specific heat capacity of graphite [11] 0.9 impurity correction 0.2 krn for BIPM calorimeter in the NRC beam 1.4 temperature calibration 0.5

linear model for temperature extrapolation 0.7

axial position of calorimeter with the NRC setup 0.5

combined relative standard uncertainty [uc(y)/y] / 10–3: 2.3

Table 3. Standard uncertainty components for the determination of Qw/Qc. The statistical uncertainty is based on n = 3 determinations for each radiation quality, each one the result of ten irradiations of thirty seconds.

Type A relative standard uncertainty uA(y)/y / 10–3

typical standard uncertainty of the mean (n = 3), 0.5 including the transfer using the NRC NE2581 monitor chamber Type B relative standard uncertainty uB(y)/y / 10B

–3

difference in graphite jackets 0.1

chamber orientation for Qc 0.1

chamber orientation for Qw 0.4

difference in air volumes for two chambers 0.1 temperature and pressure correction 0.3 axial position of chamber 0.5 combined relative standard uncertainty [uc(y)/y] / 10–3: 0.9

2.3. High-Energy Photon Irradiation Facilities at the NRC

2.3.1. NRC accelerator used for comparison

The NRC operates an Elekta Precise clinical linear accelerator that has not been modified for operation at a standards laboratory. Although full gantry rotation is available the majority of measurements are made with a horizontal beam geometry

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and this was the geometry used in this comparison. Three photon energies (nominally 6 MV, 10 MV and 25 MV) and five electron energies are available. The maximum dose-rate for photon beams is approximately 5 Gy min–1 at the depth of dose maximum and this can be varied in discrete steps by varying the pulse repetition frequency of the accelerator; the dose per pulse remaining constant. The field size is defined by the accelerator jaws and there is no multi-leaf collimator installed. The source-to-surface distance is set using the mechanical pointer supplied with the accelerator. The stability of this device is monitored as part of the quality assessment of the accelerator.

The calorimeter and ion chamber phantoms are mounted on an experimental table which allows precise movement in all three axes. The uncertainty in positioning using this system is estimated to be 0.2 mm.

2.3.2. Experimental set up

Figure 2 shows the BIPM calorimeter being installed on the NRC Elekta accelerator.

Figure 2. Setting up the BIPM graphite calorimeter. To the right is the head of the Elekta accelerator with the mechanical pointer mounted. Also seen on the right is the top of the shadow tray support (black/yellow) on which the thimble monitor chamber is mounted (see section 2.4).

2.3.3. Choice of radiation qualities and reference conditions

For the three qualities used, the nominal accelerating potential and the corresponding tissue-phantom ratio TPR20,10 [14] measured by the NRC are listed in Table 4. The absorbed dose rate was around 4 Gy min–1. The pulse repetition frequency was 400 Hz at 6 MV, and 200 Hz at 10 MV and 25 MV.

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Table 4. Nominal accelerating potentials and corresponding TPR20,10 values for the NRC clinical accelerator.

Nominal accelerating potential / MV TPR20,10

6 0.681 10 0.731 25 0.800

All the BIPM measurements were made with the detector at 1 m (with the

exception noted in Section 2.4), and the reference depth was chosen as 10 g cm–2 (also the usual reference depth at the NRC). The standard temperature and pressure were taken to be 295.15 K and 101.325 kPa. No correction was made for air humidity; the relative humidity in the accelerator laboratory remained within the interval 30 % to 55 % throughout the comparison. An irradiation time of 30 s was used in all measurements, including those for ion chambers. In this way, any accelerator instability during the first few seconds of irradiation is included in the statistical uncertainties.

2.4. Beam Monitoring System The beam intensity of a clinical accelerator is not sufficiently stable in time for comparison purposes and therefore all measurements must be normalized to the reading of a suitable monitor. For clinical dose measurements this is normally the monitor chamber internal to the accelerator. For the present comparison the internal monitor was used but in addition an external thin-windowed parallel-plate transmission chamber constructed at the NRC was used during each series of BIPM calorimeter or transfer chamber irradiations (cf. Fig. 3). Both devices measure the entire beam incident on the calorimeter or chamber. The chambers show good short-term stability but can exhibit changes in response, particularly from day-to-day, of up to 1 part in 10–

2. While remaining well within the clinical specification of the accelerator, these changes would have an impact on the overall uncertainty of comparison and a second external monitor chamber is therefore required.

For this purpose a commercial thimble ionization chamber, type NE2581, was used,mounted in a PMMA block on the shadow tray. This is placed directly in the beam before and after each series of BIPM measurements. The internal and external transmission monitors are used during a short set of measurements while the thimble monitor chamber is used to transfer between sets, either on the same day or from day-to-day. This approach had previously been used successfully with the NRC water calorimeter. A component of uncertainty is included to take account of this day-to-day transfer (cf. Table 8). The monitors were pre-irradiated for around 5 minutes. When the thimble chamber is used, a lead block 5 cm in thickness is used as a shield to avoid irradiating the calorimeter unnecessarily. This block is used in all measurement

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involving the thimble chamber, even when the calorimeter is replaced by a water phantom, to conserve the same backscatter component into the transmission monitor.

Mijnheer [15] has shown that chambers with A-150 thimbles (such as the NE2581) can show significant variations in response with humidity as A-150 plastic is hygroscopic. For this comparison the measurement period was short and the humidity was stable and therefore no correction or additional uncertainty is included for this potential effect.

Figure 3. Photograph showing the external transmission monitor chamber and the thimble monitor chamber in a PMMA block. Note that in this image the chamber is a type PTW30001 but that for the comparison a NE2581 Farmer-type chamber was used. The lead block used to shield the BIPM calorimeter is not shown.

This thimble monitor chamber was also used to link the BIPM measurements to the NRC dose measurements. In the days preceding the BIPM measurements, a reference NE2571 chamber (serial number 667), calibrated against the NRC calorimeter, was used to calibrate the thimble monitor in terms of the NRC absorbed dose to water. It should be noted that these measurements were made with the phantom surface at 100 cm from the source, the NRC reference condition. However, all the BIPM measurements were made with the detector at 100 cm. As a result, the NRC dose estimates were corrected for a 10 cm forward movement of the phantom and reference chamber using measured correction factors at each energy, with the uncertainty given in Table 1.

All measurements using the various NRC chambers were made using the NRC measurement systems for charge, air pressure and temperature, and water temperature. The BIPM acquisition system was used for the charge measurement, air pressure and temperature of the BIPM water and graphite phantoms.

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3. MONTE CARLO SIMULATIONS The Monte Carlo calculations are described in detail in [13] and make use of the PENELOPE code [16]. As noted in Section 2, four geometries are simulated. By its nature, the method relies on the symmetry of the geometries. A novel aspect of this is the use of a disc-shaped transfer chamber whose total graphite thickness on-axis is the same as that of the calorimeter core. Very detailed geometrical models were constructed although, as most of the geometrical bodies appear in two geometries, very fine details should not need to be simulated. Similarly, although detailed electron transport should not be essential for the same reasons, sufficient detail was used to permit the cavity dose to be calculated in a way that gives the same results as a full calculation using event-by-event electron transport (as demonstrated in an earlier work [17]).

Reference [13] includes a detailed uncertainty analysis for the calculation of the conversion factor Cw,c. At the higher accelerator energies the dominant component is that arising from the electron stopping power for graphite, Sc (for Co-60 the transfer chamber is essentially thick-walled and the results are insensitive to the choice of stopping powers). The main set of calculations was made using stopping powers derived using the grain density of graphite (2.265 g cm–3), as recommended by MacPherson [18], and an I-value of 82.5 eV. This choice of stopping power was discussed in [19] and is considered here to give the best estimates for Cw,c. To demonstrate the sensitivity to these choices, further calculations were made for an I-value of 78.0 eV, as currently recommended by the ICRU [20], and using the bulk graphite density (1.78 g cm–3). Note that the graphite density is changed only for the evaluation of the stopping power, the physical bodies are simulated with their measured bulk densities.

To test the extent to which the symmetry reduces the likelihood of systematic errors, additional calculations were made by the NRC using the EGSnrc code [21]. The chamber model used was independently constructed and was not as detailed as that simulated for the PENELOPE calculations. Nevertheless, all of the components thought to be significant were included, but the chamber sleeve in Figure 1(c) was modelled in a cylindrical geometry. One common element of the BIPM and NRC calculations is the phase-space files of incident spectra at 90 cm from the source. These were supplied by the NRC, generated using the BEAMnrc code [22].

The results are summarised in Table 5, where the figures in parentheses represent the statistical standard uncertainties only. The values in bold represent the values for Cw,c and have been used for this comparison. The uncertainties of these values (in parentheses) represent the combined standard uncertainty based on the uncertainty analysis presented in [13] and include components arising from the simulation geometries, input spectra, radiation transport mechanisms and cross-section data used.

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Table 5. Results of the calculations of the conversion factor Cw,c for the BIPM calorimeter using the Monte Carlo codes PENELOPE [16] at the BIPM and EGSnrc [21] at the NRC, the latter using a simplified geometry. The statistical standard uncertainty in the last digit(s) for each result is given in parenthesis. Results are shown for the use of different graphite stopping powers. The values in bold represent the values used for the present comparison and their combined standard uncertainties based on the uncertainty analysis presented in [13].

Graphite I-value / eV

Graphite density for

Sc

MC code 6 MV 10 MV 25 MV

78.0 bulk PENELOPE - - 1.1440 (7) 78.0 bulk EGSnrc 1.1204 (9) 1.1304 (7) 1.1458 (6) 82.5 bulk PENELOPE 1.1187 (6) 1.1296 (5) 1.1464 (6) 82.5 grain PENELOPE 1.1208 (6) 1.1312 (6) 1.1504 (6) 82.5 grain EGSnrc 1.1225 (9) 1.1339 (7) 1.1514 (6) 82.5 grain 1.1208(26) 1.1312(26) 1.1504(26)

A number of conclusions can be drawn from this table. As expected, the transfer chamber does not behave as a thick-walled instrument at these energies, particularly at 25 MV, and this is reflected in some dependence of Cw,c on the graphite stopping power used. Indeed, this is the largest component of uncertainty and the uncertainty analysis presented in [13] reflects this. Comparing the results using I = 82.5 eV and the graphite grain density, the BIPM (PENELOPE) and NRC (EGSnrc) calculations show a maximum difference of 2.4 parts in 103. This is thought to arise, at least in part, from the simplified modelling of the waterproof envelope for the transfer chamber in the EGSnrc calculations. Additional EGSnrc calculations with no sleeve showed that it has an effect of around 7 parts in 103 on the cavity dose at 25 MV. This result is not inconsistent with that measured by McEwen [7] for non-waterproof cylindrical ionization chambers.

It is evident from the PENELOPE calculations for I = 82.5 eV and the bulk graphite density that the dominant effect at 25 MV is the choice of density rather than I-value. This is supported by the results of additional EGSnrc calculations, not shown, using the default parameters at the NRC, I = 78.0 eV and the graphite grain density, which yield values within 1 part in 103 of those calculated using I = 82.5 eV and the same graphite density.

4. RESULTS In total, eleven working days were allocated for the comparison. Typically, after setting up a given BIPM device (calorimeter, transfer chamber in graphite or transfer chamber in water), selection of the radiation quality and pre-irradiation, the NRC thimble monitor was used to calibrate the internal and external transmission monitors. Following the measurements with the BIPM device, the transmission monitors were

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then re-calibrated. Each measurement series normally involved ten measurements. For a given BIPM device, the radiation quality was alternated between 6 MV, 10 MV and 25 MV, and often one of these qualities was repeated, before switching to a new BIPM device. In total, each quality was measured at least twice for each device. For calorimetric measurements, the calorimeter was set up on a Friday evening to benefit from the weekend for temperature stabilization and to obtain a good vacuum.

4.1. Monitoring stability A complete series of measurements was accomplished after seven working days. For these measurements, the standard deviation of the distribution of the ratio of the two transmission monitors was less than 1 part in 103.

A series of repeat measurements was planned for the end of the comparison, to study the reproducibility. Unfortunately, a problem arose with the accelerator and, while still meeting clinical specifications, the ratio of the responses of the various beam monitors had changed and was considered to no longer fulfil the demanding requirements of primary standard measurements. Consequently, data accumulated after this time is not included in the present analysis. This problem was later identified as degradation in the part of the monitor chamber that monitors the beam steering and beam energy (a recognized failure mode of this chamber type).

A second issue related to beam monitoring was backscatter. The ratio of the two transmission monitors was evaluated for measurements using either a water phantom (NRC or BIPM) or the BIPM calorimeter phantom (containing either the calorimeter or the transfer chamber). The results show that the external transmission monitor response decreases relative to the internal accelerator monitor when the calorimeter phantom replaces a water phantom. This change, which varies in relative terms from around 3.0 × 10–3 at 6 MV to around 1.5 × 10–3 at 25 MV, is interpreted as a decrease in backscatter as seen by the external monitor, whose exposed position allows back-scattered radiation to be detected. In contrast, a thin lead plate shields the internal monitor from backscatter. Nevertheless, due to the symmetry of the measurement geometries this effect will cancel when determining R (see Equations 1 and 7). A similar argument is made regarding backscatter from the lead block used for the thimble monitor measurements. Although cancellation might be less than perfect because of the proximity of the lead block and small variations in its positioning, this effect is included in the statistical uncertainties.

4.2. Beam Profile A correction factor, krn, for the radial non-uniformity of the radiation field is required because the BIPM dose determination is over a diameter of 45 mm while the NRC determination is on axis. This correction can be significant for clinical accelerators where the uniformity for a 10 cm × 10 cm field can be compromised somewhat to meet uniformity specifications for all field sizes. A PTW StarcheckTM (PTW, Freiburg) ion chamber array was used to characterize each accelerator beam before and after the comparison. This 2-D detector array has 4 arrays of detectors (vertical, horizontal and two at 45 degrees) with a minimum spacing of 3 mm. The array was positioned in a Virtual WaterTM phantom at a water equivalent depth of 10 g cm–2. The array had been previously commissioned by comparison with a scanned ion chamber and GafChromic

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EBT film [23]. An example of a beam profile at 10 MV is shown in Figure 4. The values of krn determined for the BIPM calorimeter are given in Table 6, with a relative standard uncertainty of 1.4 × 10–3. Correction factors were also obtained for the NE2571 chamber used as the NRC dose reference.

Table 6. Measured correction factors for radial non-uniformity, krn, for a diameter of 45 mm at SSD = 900 mm. The standard uncertainty is estimated to be 1.4 × 10–3. Data is also given for the NE2571 chamber (thimble length 25 mm, diameter 7 mm).

Accelerator energy / MV krn, BIPM krn, NE2571

6 1.0004 0.9990

10 0.9932 0.9984

25 0.9911 0.9996

position (mm)

-40 -20 0 20 40

Dos

e (n

orm

aliz

ed)

0.92

0.94

0.96

0.98

1.00

1.02

Up (neg) to Down (pos)Left (neg) to Right (pos) Up-Left (neg) to Down-Right (pos)Left-Down (neg) to Up-Right (pos)

Figure 4. Beam uniformity data acquired using PTW Starcheck 2-D ion chamber array at 10 MV. The orientations given are as seen from the source. The design of the flattening filters leads to the ‘hole’ at the centre of the dose distribution for the 10 cm × 10 cm field. The asymmetry seen here is not unexpected and data acquired prior to this comparison indicate that the distribution does not vary significantly with time.

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4.3. Determination of Dc, Qw, and Qc The graphite absorbed dose Dc was obtained by taking the mean of the temperature rise detected by two thermistor bridges (the third bridge was not in operation at the time of the comparison), correcting equation (3) for the influence on cp of the impurity materials in the calorimeter core, kimp = 1.0004:

impc )( kTTcD p ⋅Δ⋅= . (8)

The mean values for Dc, Qw and Qc and their statistical uncertainties, relative to the thimble monitor charge Qth, are listed for each beam quality in Table 7. Values were determined using each transmission monitor, internal and external, and are indicated separately for each beam quality in Table 7. The measured charges for the various chambers were normalized to the reference air density.

Table 7. Experimental results obtained for the BIPM calorimeter and transfer chambers, relative to the charge measured for the NE2581 thimble monitor, Qth. The ratios are evaluated using the internal and external transmission monitors (on alternate lines), the difference for each pair (around 5 parts in 10–2) being related to backscatter into the external monitor. The standard uncertainty in the last digit is given in parenthesis.

E / MV transmission monitor Dc/Qth [Gy µC–1] Qw /Qth Qc /Qth

6 internal 18.51(2) 4.471(5) 4.562(5) external 19.67(1) 4.732(6) 4.842(2)



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4.4. Combined Uncertainty

The significant uncertainties in the determination of Dw,NRC and Dw,BIPM in high energy photon beams are listed in Tables 1, 2, 3 and 5. The combined uncertainties, including those arising from the beam monitoring, are listed in Table 8. Table 8. Standard uncertainty components in the determination of the comparison ratio Dw,NRC/Dw,BIPM

Relative standard uncertainty component u(y)/y / 10–3

calibration of thimble monitor in terms of Dw,NRC 4.1 calibration of thimble monitor in terms of Dc,BIPM (cf. Table 2) 2.3 Qw/Qc (cf. Table 3) 0.9 Cw,c (cf. Table 5) 2.3 SDD correction (110 cm → 100 cm) 0.5 day-to-day transfer using NE2581 chamber 1.4 combined relative standard uncertainty [uc(y)/y] / 10–3: 5.5

4.5. Comparison of Dw determinations of the NRC and the BIPM The comparison results Dw,NRC/Dw,BIPM are presented in Table 9, derived as the ratio of the given values of the NRC and BIPM calibrations of the thimble monitor, Dw,NRC/Qth and Dw,BIPM/Qth, respectively. The latter is the product of the stated values for the BIPM transfer chamber calibration in graphite, Dc/Qc, the BIPM measurements in water relative to the thimble monitor, Qw/Qth, the Monte Carlo dose conversion factor Cw,c and the radial non-uniformity correction factor krn,BIPM. For each beam quality, a result is derived using each of the internal and external transmission monitors.

In addition, a series of measurements was made at 6 MV using an NRC reference thimble chamber in the BIPM water phantom (due to time constraints and technical difficulties, this measurement could not be made at the other two beam qualities). Its calibration coefficient ND,w had been determined by the NRC before the comparison. Thus four comparison values are recorded at 6 MV. The results are depicted graphically in Figure 5.

The final comparison result is taken as the mean result for each radiation quality. These are listed in Table 10 and shown in graphical form as a function of TPR20,10 in Figure 6.

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Table 9. Absorbed dose to water calibration of the thimble monitor determined by the NRC, Dw,NRC/Qth, and the BIPM, D w,BIPM/Qth, and the quotient Dw,NRC/Dw,BIPM. Also given are the measured values for Dc/Qc, the Monte Carlo conversion factors Cw,c, the measured radial non-uniformity correction factors krn evaluated for the BIPM standard, and the measured ratios Qw/Qth for the BIPM transfer chamber in water.

Nominal radiation quality

/MV

monitor Dc/Qc

[Gy/µC] krn,BIPM Cw,c Qw /Qth D w,BIPM /

Qth [Gy/µC]

D w,NRC / Qth

[Gy/µC]

Dw,NRC/Dw,BIPM

6 internal 4.057 1.0004 1.1208 4.471 20.34 20.33 0.999 external 4.062 1.0004 1.1208 4.732 21.56 21.51 0.998 internal 4.057 1.0004 1.1208 4.471 20.34 20.27 0.997 external 4.062 1.0004 1.1208 4.732 21.56 21.45 0.995

10 internal 4.011 0.9932 1.1312 4.862 21.91 22.00 1.004 external 4.023 0.9932 1.1312 5.138 23.23 23.17 0.998

25 internal 3.919 0.9911 1.1504 5.652 25.26 25.14 0.995 external 3.921 0.9911 1.1504 5.883 26.30 26.12 0.993

0.985

0.990

0.995

1.000

1.005

1.010

Dw

(NR

C)/D

w(B

IPM

)

internal external Figure 5. The individual comparison results for different beam monitoring possibilities. The blue diamonds represent 6 MV, the red squares 10 MV and the yellow triangles 25 MV. The four data points to the left are obtained using the internal transmission monitor of the accelerator while the four to the right are determined using the external transmission monitor constructed by the NRC.

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0.98

0.99

1.00

1.01

1.02

0.66 0.68 0.7 0.72 0.74 0.76 0.78 0.8 0.82

TPR(20,10)

R(N

RC

/BIP

M)

Figure 6. Results of the comparison in accelerator photon beams of the calorimetric standards for absorbed dose to water of the NRC and the BIPM, shown as a function of TPR20,10. The uncertainty bars represent the combined standard uncertainty of each comparison result.

Table 10. Final comparison results for the three radiation qualities at the NRC.

Nominal radiation quality / MV TPR20,10 R uc(R)/R 6 0.681 0.997 0.006

10 0.731 1.001 0.006 25 0.800 0.994 0.006

4.6. Degrees of equivalence The analysis of the results of BIPM comparisons in terms of degrees of equivalence is discussed in [24]. Following a decision of the CCRI, the BIPM determination is taken as the comparison reference value. It follows that for each laboratory i having a BIPM comparison result xi with combined standard uncertainty ui, the degree of equivalence with respect to the reference value is the relative difference Di = (Dwi – DwBIPM,i) / DwBIPM,i = xi – 1 and its expanded uncertainty Ui = 2 ui. The results Di and Ui for the present comparison can be evaluated from Table 10 as R – 1 and 2uc(R)/R, respectively.

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5. DISCUSSION The NRC and BIPM calorimetric standards for absorbed dose to water in accelerator photon beams are in agreement at the level of one standard uncertainty (6 parts in 103). There is no significant trend with energy seen in the results and the standard deviation of the three results of 3.3 parts in 103 is probably largely due to the uncertainties associated with beam monitoring and radial non-uniformity.

Several observations can be made. The measured correction factor for radial non-uniformity, krn, for the BIPM standard is large at 10 MV (0.9932) and at 25 MV (0.9911). The uncertainty of these values is estimated to be 1.4 parts in 103. However, given the ‘hole’ in the beam centre and the limited data available to characterize this effect in two dimensions, it might be that the uncertainty is under-estimated. Further, the krn factors used were measured before the comparison. The measured value at 25 MV changed by 3 parts in 103 after the accelerator incident noted in Section 4.1. For future comparisons it would therefore be of interest to measure the beam profile before, during and after the comparison to verify its stability. A transportable system for such measurements has now been established.

For this first comparison, the BIPM measurements were made with the detector at 100 cm from the source, while the NRC standard has been characterized with the phantom surface at 100 cm. A correction was included to account for this difference, introducing an additional uncertainty of 5 parts in 104. This could be avoided in future comparisons by carrying out all measurements under the reference conditions normally used by the NMI.

A difference of 6 parts in 103 can be observed in the comparison ratio for 10 MV depending on which transmission monitor is used. Furthermore, the ratio Dw,NRC/Dw,BIPM is smaller for the external monitor than for the internal monitor, for all three accelerator energies (Fig. 5). While no firm explanation has been found, these effects are thought to be related, at least in part, to backscatter. The combination of measurements that has been made ensures that backscatter effects cancel as long as the geometry is reproducible. However, the monitoring at 10 MV shows less reproducibility than at 6 MV and 25 MV, which might explain the observed difference between the two results at 10 MV. The importance of rigorous beam monitoring is illustrated by the fact that the combined standard uncertainty arising from the beam monitoring amounts to approximately 2 parts in 103, with an indication that this might be an underestimate for 10 MV.

The contribution to the absorbed dose arising from photoneutron production has been considered [25]. Photon beams above 8 MV can generate neutrons with mean energies between 1 MeV and 2 MeV. The neutron equivalent dose at different depths in a PMMA-slab phantom has been measured in photon beams of 6 MV and 18 MV by Vanhavere et al. [26]. At 18 MV the neutron contribution to the absorbed dose at the measurement depth is a few parts in 104. As this small contribution will be similar in graphite and in water no correction is made to the BIPM standard for the neutron contribution.

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6. CONCLUSION This comparison is the first in the ongoing BIPM key comparison BIPM.RI(I)-K6. The comparison result, reported as a ratio of the NRC and the BIPM evaluations of absorbed dose to water, is 0.997 at 6 MV, 1.001 at 10 MV and 0.994 at 25 MV, each with a relative standard uncertainty of 6 × 10−3. The results will be included in the BIPM key comparison database [2] as degrees of equivalence with respect to the BIPM value, as a function of TPR20,10. As new comparisons are made, degrees of equivalence between pairs of participating laboratories will be evaluated, based on an interpolation, if necessary, in terms of TPR20,10.

REFERENCES [1] Picard S, Burns D T and Roger P 2009 Construction of an Absorbed-Dose

Graphite Calorimeter, Rapport BIPM-09/01.

[2] Key Comparison BIPM.RI(I)-K6 / BIPM Key Comparison Database http://kcdb.bipm.org/

[3] Calorimetric Comparison of Absorbed Dose to Water at High Energies 2009 BIPM Key Comparison BIPM.RI(I)-K6, Protocol 1.1 – CCRI(I).4

[4] Ross C K, Seuntjens J P, Klassen N V and Shortt K R 2000 “The NRC Sealed Water Calorimeter: Correction Factors and Performance” (Proc. Workshop on Recent Advances in Calorimetric Absorbed Dose Standards, NPL Report CIRM 42) (National Physical Laboratory, Teddington, UK, 2000)

[5] Shortt K R, Shobe J and Domen S R 2000 Comparison of dosimetry calibration factors at the NRCC and the NIST Med. Phys. 27 1644-54

[6] Shortt K, Ross C, Seuntjens J, Delaunay F, Ostrowsky A, Gross P and Leroy E 2001 Comparison of dosimetric standards of Canada and France for photons at 60Co and higher energies Phys. Med.Biol. 46 2119-42

[7] McEwen M R 2010 Measurement of ionization chamber absorbed dose kQ factors in megavoltage photon beams Med. Phys. 37 2179-93

[8] Seuntjens J P, Ross C K, Shortt K R, and Rogers D W O 2000 Absorbed-dose beam quality conversion factors for cylindrical chambers in high energy photon beams Med. Phys. 27 2763-79

[9] Ross C K, McEwen M R and Klassen N V 2007 Vessel Designs and Correction Factors for Water Calorimetry, Proc. Absorbed Dose and Air Kerma Primary Standards Workshop Paris 2007 (LNE-LNHB: Saclay)

[10] Krauss A 2006 The PTB water calorimeter for the absolute determination of absorbed dose to water in 60Co radiation Metrologia 43 259-72

[11] Picard S, Burns D T and Roger P 2007 Determination of the Specific Heat Capacity of a Graphite Sample Using Absolute and Differential Methods Metrologia 44 294-302

4 This is the version used for the comparison. An updated version is now available on the KCDB web site.

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https://www.bipm.org/utils/common/pdf/rapportBIPM/2009/01.pdf

http://kcdb.bipm.org/

http://www.iop.org/EJ/abstract/0026-1394/44/5/005/

http://kcdb.bipm.org/AppendixB/appbresults/bipm.ri(i)-K6/BIPM.RI(I)-K6_Technical_Protocol.pdf


[12] Picard S, Burns D T and Roger P 2008 Measurement of the Specific Heat Capacity of Synthetic Sapphire (α-AI2O3) from 293 K to 301 K Rapport BIPM-08/05

[13] Burns D T 2010 The dose conversion procedure for the BIPM graphite calorimeter standard for absorbed dose to water, in preparation

[14] Radiation Oncology Physics, A handbook for teachers and students, Ed. Podgorsak E B, IAEA, Vienna, 2005

[15] Mijnheer B J 1985 Variations in response to radiation of a nylon-walled ionization chamber induced by humidity changes Med. Phys. 12 625-6

[16] Salvat F, Fernandez-Varea J M, Acosta E and Sempau J 2008 PENELOPE, - a code system for Monte Carlo simulation of electron and photon transport Electronic manual supplied with PENELOPE 2008

[17] Burns D T 2006 A new approach to the determination of air kerma using primary-standard cavity ionization chambers Phys. Med. Biol. 51 929-42

[18] MacPherson M S 1998 Accurate measurements of the collision stopping powers for 5 to 30 MeV electrons Technical Report PIRS-626 (Ottawa, Canada: National Research Council of Canada)

[19] Burns D T 2009 A re-evaluation of the I-value for graphite based on an analysis of recent work on W, sc,a and cavity perturbation corrections Metrologia 46 585–90

[20] 1984 Stopping power for electrons and positrons ICRU Report 37 (Washington D.C.: International Commission on Radiation Units and Measurements)

[21] Rogers D W O, Kawrakow I, Seuntjens J P, Walters B W and Mainegra-Hing E 2003 NRC user codes for EGSnrc Technical Report PIRS-702 (RevB) (Ottawa, Canada: National Research Council of Canada)

[22] Rogers D W O, Faddegon B A, Ding G X, Ma C-M and Wei J 1995 BEAM: A Monte Carlo code to simulate radiotherapy treatment units Med. Phys. 22 503-24

[23] McEwen M and Xu L 2009 SU-FF-T-385: Commissioning of a PTW Starcheck 2-D Ion Chamber Array Med. Phys. 36 2610 (abstract)

[24] Allisy P J, Burns D T, Andreo P 2009 International framework of traceability for radiation dosimetry quantities, Metrologia 46 S1-8

[25] Greene D and Williams P C 1983 Central axis depth dose data for use in radiotherapy Brit. J. Radiol. Supplement 17 61-86

[26] Vanhavere F, Huyskens D and Struelens L 2004 Peripheral neutron and gamma doses in radiotherapy with an 18 MV linear accelerator Rad. Prot. Dos. 110 607-12

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