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Intercomparison of ionisation chamber measurements from 125I seeds
ARTICLE IN PRESS
Applied Radiation and Isotopes 65 (2007) 517–518 www.elsevier.com/locate/apradiso
Technical note
Intercomparison of ionisation chamber measurements from
125
I seeds
J.B. Daviesa,c,, K.F. Enarib, C. Baldockc a
Australian Nuclear Science and Technology Organisation, New Illawarra Road, Building 23, Lucas Heights, NSW 2234, Australia b Cancer Care Centre, St George Hospital, Kogarah, NSW 2217, Australia c Institute of Medical Physics, University of Sydney, NSW 2006, Australia Received 16 October 2006; received in revised form 8 November 2006; accepted 8 December 2006
Abstract The reference air kerma rates of a set of individual 125I seeds were calculated from current measurements of a calibrated re-entrant ionisation chamber. Single seeds were distributed to seven Australian brachytherapy centres for the same measurement with the user’s instrumentation. Results are expressed as the ratio of the reference air kerma rate measured by the Australian Nuclear Science & Technology Organisation (ANSTO) to the reference air kerma rate measured at the centre. The intercomparison ratios of all participants were within 75% of unity. r 2007 Elsevier Ltd. All rights reserved. Keywords: Brachytherapy; Dosimetry; Comparison
1. Introduction It is important for brachytherapy centres when comparing results of clinical trials, to ensure that the same physical quantity is being determined, in this case absorbed dose to human tissue. Independent calibration checks of brachytherapy sources prior to treatment are necessary to verify the air kerma rate stated by the manufacturer (International Atomic Energy Agency, 2002). An intercomparison of ionisation chamber measurements between a standards laboratory and hospitals is one way to gauge the centre’s ability to conduct accurate measurements (Woods, 1983). Australian Nuclear Science & Technology Organisation (ANSTO) maintains direct measurement traceability to the primary standard maintained by the National Institute of Standards and Technology (NIST), Gaithersburg USA, for the air kerma strength measurement of 125I brachytherapy seed model 6711 (Oncoseed, Amersham Health, USA) (Davies et al., 2005). The International Atomic Energy Agency recommends the International Commission on Corresponding author. Australian Nuclear Science and Technology Organisation, New Illawarra Road, Building 23, Lucas Heights, NSW 2234 Australia. Tel.: +61 2 97179002; fax: +61 2 97179325. E-mail address: [email protected] (J.B. Davies).
0969-8043/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2006.12.002
Radiation Units and Measurements (ICRU) quantity reference air kerma rate, KR, defined as the air kerma rate at a reference distance of 1 m, corrected for air attenuation and scattering (International Atomic Energy Agency, 2002). Since the reference air kerma rate is defined at 1 m, it is numerically equivalent to the air kerma strength at 1 m (Seltzer et al., 2003). An intercomparison was conducted between ANSTO and seven brachytherapy centres to estimate the accuracy of the measurement system employed. 2. Methodology Individual model 6711 seeds were removed from a RAPID Strand (Oncura, Plymouth USA). Current measurements were made using a re-entrant ionisation chamber (high-dose-rate (HDR) 1000 Plus, Standard Imaging, Middleton USA) connected to a calibrated electrometer (Keithley 35040, Cleveland USA). This measurement system was calibrated previously (Davies et al., 2005), establishing traceability to the primary standard measurement of air kerma strength for a 6711 seed maintained by NIST. The standard uncertainty in the ionisation chamber calibration coefficient was previously determined to be 1.1%. The KR (mGy h1) of each seed was calculated from these ionisation chamber current measurements at ANSTO.
ARTICLE IN PRESS J.B. Davies et al. / Applied Radiation and Isotopes 65 (2007) 517–518
Subsequently, one seed was sent to each participant in the intercomparison. Participants were requested to perform ionisation chamber measurements and calculate the reference air kerma rate based on their existing calibration coefficient for the reference air kerma rate (or air kerma strength) of a 6711 seed. Information requested from participants to verify the accuracy of the intercomparison included the type of equipment used, the ionisation chamber calibration coefficient, its uncertainty, and the temperature and pressure it refers to, the date and time measurements were made (or corrected to), and the ambient temperature and pressure at which measurements were made (or corrected to). Only one centre involved in the comparison used a sealed ionisation chamber making air density corrections not applicable. All other participants used re-entrant ionisation chambers open to the atmosphere. Results reported by each participant were corrected to the same conditions as the ANSTO measurements which were to a temperature of 20 1C and air pressure of 101.325 kPa, where applicable, and for radioactive decay of 125I to a reference time. The half-life of 125I used was 59.407 days (Be et al., 2004). 3. Results and discussion The intercomparison ratio, RK, is expressed as the quotient of the reference air kerma rate determined by ANSTO, KR (ANSTO), and that determined by the participating centre, KR (centre);
Table 1 Results of the intercomparison Reference air kerma rate (mGy h1)
Seed ID
1 2 3 4 5 6 7
ANSTO
Centre
0.422870.012 0.425670.011 0.414070.010 0.429070.011 0.433270.011 0.441370.011 0.422670.011
0.421070.012 0.411970.014 0.411070.012 0.417970.012 0.428270.013 0.434070.013 0.416570.014
Ratio, RK
1.00470.040 1.03370.043 1.00770.039 1.02770.040 1.01270.041 1.01770.039 1.01570.042
Uncertainties stated are the expanded uncertainties (using k ¼ 2). 1.10
1.05
RK
518
1.00
0.95
0.90
0
1
2
3
4 5 Centre No.
6
7
Fig. 1. Data points correspond to the intercomparison ratios, RK. Error bars indicate an estimate of the expanded uncertainty (using k ¼ 2).
RK ¼ K R ðANSTOÞ =K R ðcentreÞ
4. Conclusion
Table 1 displays the results of the ANSTO and centre measurements and RK for each comparison. Intercomparison ratios are shown in Fig. 1. Ionisation chamber current readings from individual seeds were low, typically 1.8 pA. This contributed to the overall uncertainty in the comparison. Uncertainties in measurements were calculated in accordance with specifications of the International Organization for Standardization (1993). The expanded uncertainty was obtained by multiplying the combined standard uncertainty by a coverage factor (k) of two, providing an interval with a level of confidence of approximately 95%. The expanded uncertainties in the ANSTO measurements of all seeds were between 2.5% and 2.8%. Expanded uncertainties in ionisation chamber calibration coefficients provided from participating centres were between 2.5% and 3.0%. Considering other factors such as the accuracy of the electrometer (0.2%), variation in current readings (0.5%), linearity of the ionisation chamber response (0.5%), the expanded uncertainties in the centre measurements, for all seeds involved in the intercomparison, were estimated to be between 2.9% and 3.3% as given in Table 1. The expanded uncertainties in the intercomparison ratios were then approximately 4%, as shown in Fig. 1.
Reference air kerma rate measurements of model 6711 seeds were compared between ANSTO and seven brachytherapy centres in Australia. Results of the intercomparison demonstrate general agreement in ionisation chamber measurements for the purposes of brachytherapy procedures in Australia.
References Be, M.-M., Chiste, V., Dulieu, C., Browne, E., Chechev, V., Kuzmenko, N., Helmer, R., Nichols, A., Schonfeld, E., Dersch, R., 2004. Table of radionuclides, Monographie BIPM-5. Davies, J.B., Mo, L., Alexiev, D., 2005. Development of an Australian secondary standard for the reference air kerma rate measurement of 125 I seeds. Aust. Phys. Eng. Sci. Med. 28, 196–199. International Atomic Energy Agency, 2002. IAEA-TECDOC-1274, Calibration of photon and beta ray sources used in brachytherapy. International Organization for Standardization, 1993. Guide to the expression of uncertainty in measurement. Seltzer, S.M., Lamperti, P.J., Loevinger, R., Mitch, M.G., Weaver, J.T., Coursey, B.M., 2003. New national air-kerma-strength standards for 125 I and 103Pd brachytherapy seeds. J. Res. Natl. Inst. Stand. Technol. 108, 337–358. Woods, M.J., 1983. Intercomparison of radionuclide calibrator measurements in U.K hospitals. Int. J. Nucl. Med. Biol. 10 (2/3), 103–105.
Applied Radiation and Isotopes 65 (2007) 517–518 www.elsevier.com/locate/apradiso
Technical note
Intercomparison of ionisation chamber measurements from
125
I seeds
J.B. Daviesa,c,, K.F. Enarib, C. Baldockc a
Australian Nuclear Science and Technology Organisation, New Illawarra Road, Building 23, Lucas Heights, NSW 2234, Australia b Cancer Care Centre, St George Hospital, Kogarah, NSW 2217, Australia c Institute of Medical Physics, University of Sydney, NSW 2006, Australia Received 16 October 2006; received in revised form 8 November 2006; accepted 8 December 2006
Abstract The reference air kerma rates of a set of individual 125I seeds were calculated from current measurements of a calibrated re-entrant ionisation chamber. Single seeds were distributed to seven Australian brachytherapy centres for the same measurement with the user’s instrumentation. Results are expressed as the ratio of the reference air kerma rate measured by the Australian Nuclear Science & Technology Organisation (ANSTO) to the reference air kerma rate measured at the centre. The intercomparison ratios of all participants were within 75% of unity. r 2007 Elsevier Ltd. All rights reserved. Keywords: Brachytherapy; Dosimetry; Comparison
1. Introduction It is important for brachytherapy centres when comparing results of clinical trials, to ensure that the same physical quantity is being determined, in this case absorbed dose to human tissue. Independent calibration checks of brachytherapy sources prior to treatment are necessary to verify the air kerma rate stated by the manufacturer (International Atomic Energy Agency, 2002). An intercomparison of ionisation chamber measurements between a standards laboratory and hospitals is one way to gauge the centre’s ability to conduct accurate measurements (Woods, 1983). Australian Nuclear Science & Technology Organisation (ANSTO) maintains direct measurement traceability to the primary standard maintained by the National Institute of Standards and Technology (NIST), Gaithersburg USA, for the air kerma strength measurement of 125I brachytherapy seed model 6711 (Oncoseed, Amersham Health, USA) (Davies et al., 2005). The International Atomic Energy Agency recommends the International Commission on Corresponding author. Australian Nuclear Science and Technology Organisation, New Illawarra Road, Building 23, Lucas Heights, NSW 2234 Australia. Tel.: +61 2 97179002; fax: +61 2 97179325. E-mail address: [email protected] (J.B. Davies).
0969-8043/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2006.12.002
Radiation Units and Measurements (ICRU) quantity reference air kerma rate, KR, defined as the air kerma rate at a reference distance of 1 m, corrected for air attenuation and scattering (International Atomic Energy Agency, 2002). Since the reference air kerma rate is defined at 1 m, it is numerically equivalent to the air kerma strength at 1 m (Seltzer et al., 2003). An intercomparison was conducted between ANSTO and seven brachytherapy centres to estimate the accuracy of the measurement system employed. 2. Methodology Individual model 6711 seeds were removed from a RAPID Strand (Oncura, Plymouth USA). Current measurements were made using a re-entrant ionisation chamber (high-dose-rate (HDR) 1000 Plus, Standard Imaging, Middleton USA) connected to a calibrated electrometer (Keithley 35040, Cleveland USA). This measurement system was calibrated previously (Davies et al., 2005), establishing traceability to the primary standard measurement of air kerma strength for a 6711 seed maintained by NIST. The standard uncertainty in the ionisation chamber calibration coefficient was previously determined to be 1.1%. The KR (mGy h1) of each seed was calculated from these ionisation chamber current measurements at ANSTO.
ARTICLE IN PRESS J.B. Davies et al. / Applied Radiation and Isotopes 65 (2007) 517–518
Subsequently, one seed was sent to each participant in the intercomparison. Participants were requested to perform ionisation chamber measurements and calculate the reference air kerma rate based on their existing calibration coefficient for the reference air kerma rate (or air kerma strength) of a 6711 seed. Information requested from participants to verify the accuracy of the intercomparison included the type of equipment used, the ionisation chamber calibration coefficient, its uncertainty, and the temperature and pressure it refers to, the date and time measurements were made (or corrected to), and the ambient temperature and pressure at which measurements were made (or corrected to). Only one centre involved in the comparison used a sealed ionisation chamber making air density corrections not applicable. All other participants used re-entrant ionisation chambers open to the atmosphere. Results reported by each participant were corrected to the same conditions as the ANSTO measurements which were to a temperature of 20 1C and air pressure of 101.325 kPa, where applicable, and for radioactive decay of 125I to a reference time. The half-life of 125I used was 59.407 days (Be et al., 2004). 3. Results and discussion The intercomparison ratio, RK, is expressed as the quotient of the reference air kerma rate determined by ANSTO, KR (ANSTO), and that determined by the participating centre, KR (centre);
Table 1 Results of the intercomparison Reference air kerma rate (mGy h1)
Seed ID
1 2 3 4 5 6 7
ANSTO
Centre
0.422870.012 0.425670.011 0.414070.010 0.429070.011 0.433270.011 0.441370.011 0.422670.011
0.421070.012 0.411970.014 0.411070.012 0.417970.012 0.428270.013 0.434070.013 0.416570.014
Ratio, RK
1.00470.040 1.03370.043 1.00770.039 1.02770.040 1.01270.041 1.01770.039 1.01570.042
Uncertainties stated are the expanded uncertainties (using k ¼ 2). 1.10
1.05
RK
518
1.00
0.95
0.90
0
1
2
3
4 5 Centre No.
6
7
Fig. 1. Data points correspond to the intercomparison ratios, RK. Error bars indicate an estimate of the expanded uncertainty (using k ¼ 2).
RK ¼ K R ðANSTOÞ =K R ðcentreÞ
4. Conclusion
Table 1 displays the results of the ANSTO and centre measurements and RK for each comparison. Intercomparison ratios are shown in Fig. 1. Ionisation chamber current readings from individual seeds were low, typically 1.8 pA. This contributed to the overall uncertainty in the comparison. Uncertainties in measurements were calculated in accordance with specifications of the International Organization for Standardization (1993). The expanded uncertainty was obtained by multiplying the combined standard uncertainty by a coverage factor (k) of two, providing an interval with a level of confidence of approximately 95%. The expanded uncertainties in the ANSTO measurements of all seeds were between 2.5% and 2.8%. Expanded uncertainties in ionisation chamber calibration coefficients provided from participating centres were between 2.5% and 3.0%. Considering other factors such as the accuracy of the electrometer (0.2%), variation in current readings (0.5%), linearity of the ionisation chamber response (0.5%), the expanded uncertainties in the centre measurements, for all seeds involved in the intercomparison, were estimated to be between 2.9% and 3.3% as given in Table 1. The expanded uncertainties in the intercomparison ratios were then approximately 4%, as shown in Fig. 1.
Reference air kerma rate measurements of model 6711 seeds were compared between ANSTO and seven brachytherapy centres in Australia. Results of the intercomparison demonstrate general agreement in ionisation chamber measurements for the purposes of brachytherapy procedures in Australia.
References Be, M.-M., Chiste, V., Dulieu, C., Browne, E., Chechev, V., Kuzmenko, N., Helmer, R., Nichols, A., Schonfeld, E., Dersch, R., 2004. Table of radionuclides, Monographie BIPM-5. Davies, J.B., Mo, L., Alexiev, D., 2005. Development of an Australian secondary standard for the reference air kerma rate measurement of 125 I seeds. Aust. Phys. Eng. Sci. Med. 28, 196–199. International Atomic Energy Agency, 2002. IAEA-TECDOC-1274, Calibration of photon and beta ray sources used in brachytherapy. International Organization for Standardization, 1993. Guide to the expression of uncertainty in measurement. Seltzer, S.M., Lamperti, P.J., Loevinger, R., Mitch, M.G., Weaver, J.T., Coursey, B.M., 2003. New national air-kerma-strength standards for 125 I and 103Pd brachytherapy seeds. J. Res. Natl. Inst. Stand. Technol. 108, 337–358. Woods, M.J., 1983. Intercomparison of radionuclide calibrator measurements in U.K hospitals. Int. J. Nucl. Med. Biol. 10 (2/3), 103–105.