Abstract An indirect comparison of the standards for reference air kerma rate for

Transcription

1 Comparison BIPM.RI(I)-K8 of high dose-rate Ir-19 brachyerapy standards for reference air kerma rate of e VSL and e BIPM J.T. Alvarez 1, J. A. de Pooter, C. Andersen 3, A.H.L. Aalbers, P.J. Allisy-Roberts 4, C Kessler 4 1 ININ, Ocoyoacac, Mexico (IAEA Fellow at e BIPM) VSL, Dutch Metrology Institute, Delft, The Neerlands 3 Risø High Dose Reference Laboratory, Roskilde, Denmark 4 Bureau International des Poids et Mesures, F-931 Sèvres Cedex Abstract An indirect comparison of e standards for reference air kerma rate for 19 Ir high dose rate (HDR) brachyerapy sources of e Dutch Metrology Institute (VSL), The Neerlands, and of e Bureau International des Poids et Mesures (BIPM) was carried out at e VSL in November 009. The comparison result, based on e calibration coefficients for a transfer standard and expressed as a ratio of e VSL and e BIPM standards for reference air kerma rate, is wi a combined standard uncertainty of Introduction The Brachyerapy Standards Working Group (BSWG), created under e recommendation made by e Consultative Committee for Ionizing Radiation CCRI(I), proposed at eir meeting of November 005 to start a comparison of primary standards for reference air kerma rate (RAKR) of 19 Ir. To meet e needs of e National Metrology Institutes (NMIs), a new ongoing key comparison was registered in e BIPM key comparison data base (KCDB 014) under e reference BIPM.RI(I)-K8. As no primary facility for brachyerapy is available at e BIPM, e BIPM results are based on measurements using an NE 571 imble-type transfer ionization chamber in e 60 Co reference beam at e BIPM. The Dutch Metrology Institute (VSL) is e first NMI to participate in is new ongoing key comparison. The comparison result is given in terms of e ratio of e calibration coefficients of e imble chamber determined at e VSL in e 19 Ir radiation beam and at e BIPM in e 60 Co reference radiation beam, e latter corrected by a calculated factor to account for e energy dependence of e NE 571 chamber type between 60 Co and 19 Ir (Mainegra-Hing et al 006). Measurements were also made using a well-type ionization chamber of e BIPM. The response of is chamber, connected to its own electrometer, was determined relative to e BIPM RAKR realized in e NMI 19 Ir beam using e NE 571 (and e NMI electrometer system). Such a calibration of e well chamber at each NMI enables its future use in a comparison wi an NMI at does not calibrate e NE571 chamber type. The long-term stability of e well chamber is established by measurements at e BIPM using a 166m Ho source. The comparison result, approved by e CCRI, is analysed and presented in terms of degrees of equivalence for entry in e BIPM key comparison database. 1/10

2 . Characteristics of e transfer instruments Two transfer instruments, belonging to e BIPM, are used to undertake e comparisons: an NE 571 imble chamber and a Standard Imaging HDR1000 well chamber. The main characteristics of e transfer instruments are listed in Table 1. Table 1. Characteristics of e BIPM transfer chambers Characteristic/Nominal values NE Standard Imaging HDR 1000 Plus Insert Dimensions Inner diameter / mm Cavity leng / mm Wall mass ickness / g cm Tip to reference point / mm Bottom to reference point / mm (sweet-spot) Electrode Leng / mm Diameter / mm Air cavity Volume / cm Wall Material Graphite - Density / g cm Build-up cap Material Delrin - Potential of HV electrode wi respect to collecting electrode Thickness / g cm Determination of e VSL reference value As e VSL has no primary standard for high dose-rate (HDR) 19 Ir brachyerapy sources, e reference air kerma rate (RAKR) is determined using an NE 571 ionization chamber as a transfer instrument. The determination of e calibration coefficient for 19 Ir is based on a linear interpolation between e air kerma calibration coefficients for a 50 kv x-ray beam, where e primary standard is a free-air chamber, and a 137 Cs -ray beam, where e primary instrument is a graphite cavity chamber, and a correction factor determined using e meod described by Petersen et al (1994) and van Dijk et al (004). In e Petersen study, e energy response of a NE 571 was determined by calibrating e chamber against e primary standards in a series of 9 ISO narrow x-ray, 137 Cs and 60 Co beams. To obtain e air kerma calibration coefficient N K for 19 Ir, each 19 Ir spectral line is treated individually; e air kerma calibration coefficient is obtained for each spectral line by interpolation on e energy response curve and weighting em according to e intensity of each spectral line. The 19 Ir spectrum was simulated using Monte Carlo techniques. The calibration coefficient us obtained was compared to e one obtained using a linear interpolation between 50 kv x-ray and 137 Cs beams. From is study, it was concluded at a correction of has to be applied to e linear interpolation result. The VSL standards have been compared against e BIPM primary standards in medium-energy x-rays and 60 Co -rays and eir main characteristics are described in e corresponding comparison reports (Burns et al 009, Allisy-Roberts et al 007). For e VSL graphite cavity chamber, a new value for e wall correction factor has been determined recently (van Dijk 007) /10

3 which resulted in a decrease of 0.4 % in e air kerma determination. These modifications have been taken into account in e determination of e RAKR of e HDR 19 Ir source. The main characteristics of e VSL 19 Ir source and e transfer instrument used to determine e RAKR are listed in Tables and 3, respectively. Table. Characteristics of e VSL 19 Ir source After-loader unit Nucletron NLF 01 D36B8588 Manufacturer of source Mallinckrodt Medical BV Apparent activity of source 386 GBq at 0/04/010 Table 3. Characteristics of e VSL transfer chamber Characteristic/Nominal values NE 571 Dimensions Inner diameter / mm 6.4 Wall mass ickness / g cm Cavity leng / mm 4.1 Tip to reference point / mm 13.0 Electrode Leng / mm 0.6 Diameter / mm 1.0 Air cavity Volume / cm Wall Material Graphite Density / g cm Voltage applied Polarity to inner electrode / V -50 Ambient conditions The ionization current for each chamber is corrected to e reference conditions of K and kpa; relative humidity is controlled at (50 5) % and no humidity correction is applied. 4. Determination of e BIPM reference values The BIPM does not possess an 19 Ir source. The reference value for e imble chamber used to evaluate e comparison result is based on measurements performed in e 60 Co -ray beam, wi supporting measurements in 50 kv x-rays. The reference value for e well chamber is based on measurements made wi e 19 Ir source of e participating labs, wi supporting measurements using a reference source of 166m Ho. 4.1 Reference values for e BIPM imble chamber The NE 571 is calibrated periodically at e BIPM in e 60 Co -ray beam, under e reference conditions described by Allisy-Roberts et al (009). The chamber is also calibrated periodically in e CCRI 50 kv x-ray beam to verify at its response at lower energies remains stable. The calibration coefficient of e imble chamber N K, BIPM for 19 Ir is derived from e calibration coefficient determined in e BIPM 60 Co -beam and a calculated correction factor k en to take into account e energy dependence of is type of ionization chamber (Mainegra-Hing et al 006): N,Co-60 K, BIPM NK,BIPM ken (1) 3/10

4 The value for k en is taken to be 1.000, wi an estimated relative standard uncertainty of (e statistical standard uncertainty of e calculated value is ). The BIPM mean values for e imble chamber made around e period of e comparison are compared to e long-term value to verify e stability of e chamber. The long-term reproducibility in e BIPM 60 Co beam is in relative value (and for e 50 kv x-ray beam). Considering e long-term stability for e 60 Co beam and e 50 kv x-ray beam a value of is included in e BIPM uncertainty budget. 4. Reference values for e BIPM well chamber Until now, e sealed source used to monitor e stability of e well-type HDR1000 ionization chamber is a low activity source (about 1.3 MBq) of 166m Ho. The long-term reproducibility of e well chamber established using is source is about in relative value. It is possible at some of is variation is due to e low activity and for is reason a higher activity source (about 1.7 GBq) of 137 Cs will be implemented for future comparisons. To derive a reference value for e well chamber, e BIPM determines its calibration coefficient at each NMI using e NMI 19 Ir source as w N K, BIPM K R, BIPM () I where K R, BIPM is e RAKR for e NMI source evaluated from e current determined by e NMI using e BIPM NE 571 chamber (for e usual NMI reference conditions) and e BIPM N determination. The well chamber current I, measured at e sweet-spot, is appropriately K,BIPM corrected to e reference conditions of measurements and normalized to e reference ambient conditions. 5. Comparison measurements at e VSL 5.1 Comparison measurements wi e BIPM imble chamber The calibration coefficient N K, VSL K R,VSL IVSL K R, VSL N K, VSL for e BIPM imble chamber at e VSL is given by (3) where is e VSL reference air kerma rate and I VSL is e ionization current of e BIPM imble chamber measured by e VSL. The relative standard uncertainty of e mean ionization current was estimated to be 10 4 (two calibrations; for each calibration, ree series of 8 measurements wi source repositioning, as described below) Determination of e BIPM calibration coefficient is described in Section 4.1. The chamber was calibrated before and after e measurements at e VSL; e relative standard uncertainty of e mean is taken to be parts in The ionization current in each case is corrected for e influence factors described below. 4/10

5 Positioning At e BIPM, e BIPM imble chamber is positioned wi e stem perpendicular to e beam direction and wi e appropriate marking on e stem (engraved line) facing e source; e build-up cap being used for bo 60 Co and 50 kv x-rays. At e VSL e BIPM imble chamber is set up at e centre of e irradiation jig between two caeters at are each 10 cm from e chamber, as illustrated in Figure 1, and wi e appropriate marking on e stem (engraved line) in e upward direction. The charges measured in ese two arrangements are averaged, reducing e uncertainty related to positioning. A set of at least ree runs wi repositioning of e source in between was made for each caeter. Figure 1. Set-up for e NE 571 imble chamber The NE 571 imble chamber in e VSL irradiation jig, equidistant from each lateral source position. Applied voltage and polarity At e BIPM, a collecting voltage of 50 V (negative polarity) is applied to e outer electrode of e chamber at least 30 min before any measurements are made. At e VSL, a collecting voltage of 50 V (positive polarity) is applied to e central electrode of e chamber at least 30 min before any measurements were made. Consequently, no corrections were applied at eier laboratory for polarity. Volume recombination Volume recombination is negligible at dose rates less an 15 mgy s 1 for e chamber at is polarizing voltage, and e initial recombination loss will be e same in e two laboratories. Consequently, no correction for recombination was applied at eier laboratory. Charge and leakage measurements At e BIPM, e charge Q, collected by e transfer instrument is measured using a Keiley electrometer, model 64. The radiation source is operational during e entire exposure series and e charge is collected for e appropriate, electronically controlled, time interval. At e VSL, e charge collected is measured similarly wi a Keiley model 6517A electrometer. At e BIPM, pre-irradiation was for at least 30 min ( 10 Gy) before any measurements were made and similarly at e VSL e chamber was pre-irradiated wi at least 10 Gy. The measured ionization current was corrected for e leakage current at bo laboratories. This correction was less an in relative value at e BIPM and less an 1 10 at e VSL; e latter value being reproducible at e level. 5/10

6 Ambient conditions During e measurements, e air temperature was stable to better an 0.01 C at bo laboratories. The measurements are normalized to K and kpa. Relative humidity is controlled at (50 5) % at e BIPM and (50 5) % at e VSL. Consequently, no correction for humidity is applied to e measured ionization current. Radial non-uniformity correction At e BIPM, e correction applied to e ionization current for e radial non-uniformity would only be for e transfer instrument, wi an uncertainty of However for is comparison no correction for radial non-uniformity is made at e BIPM. At e VSL, no nonuniformity correction is applied, since e imble chamber is of e same type as e transfer standard used to calibrate e HDR 19 Ir source (e radial non-uniformity correction cancels in evaluating e calibration coefficient at e VSL). Consequently, e VSL calibration coefficient is valid for a uniform radiation field. However, an uncertainty of 10 4 is included in e VSL uncertainty budget. Stem and room scatter Since e imble chamber transfer instrument is of e same type as e transfer standard used at e VSL to calibrate e HDR 19 Ir source, e stem and room scatter corrections cancel in evaluating e calibration coefficient at e VSL. Consequently, e VSL calibration coefficient is effectively scatter-free. 5. Comparison measurements wi e BIPM well chamber The HDR1000 well chamber, togeer wi its electrometer and probes for temperature, pressure and humidity, is used as a transfer system to determine a comparison result for ose NMIs at do not provide calibrations of imble-type ionization chambers. The well chamber was calibrated at e VSL wi respect to e BIPM determination of RAKR, as described in Section 4.. The essential details of e current measurements are reproduced here. Sweet-spot At e VSL, measurements at a series of dwell positions for e 19 Ir source showed e sweetspot of e well chamber to be at 49.4 mm, measured from e inside base plate of e well chamber. Charge and leakage measurements For each dwell position, ree series of 10 measurements of 60 s were made, e source being retracted and repositioned between each series. For each series, e first measurement was discarded to ensure at e chamber response had stabilized. Measurements were corrected by leakage, which was measured at each dwell position. This correction was, in relative value, less an Ambient conditions The measurements are normalized to K and kpa. No humidity correction is applied. Decay correction The measurements are corrected for e decay of e source to e reference date of The half-life used by e VSL for 19 Ir is days wi u c = days, taken from Woods et al (199). 6/10

7 6. Results of e comparison 6.1 Thimble chamber comparison result The individual calibration coefficients of e imble chamber will not be disclosed as is transfer chamber will be calibrated by oer NMIs participating in is ongoing comparison. The comparison result is expressed as e ratio of e calibration coefficients of e imble chamber determined at bo laboratories, N K,VSL K, VSL N K,BIPM R (4) in which e average value of measurements made at e BIPM before and after ose made at e VSL is compared wi e mean of e measurements made at e VSL. For e VSL, e comparison result R is K 6. Additional results for e well chamber As explained in Section 4., e BIPM determines for each comparison e calibration coefficient w of e well chamber N K, BIPM using equation (). Alough not used for e main comparison result, w it is possible to derive a corresponding calibration coefficient N K R, VSL for e VSL as w N K, VSL KR, VSL (5) I where is e VSL reference air kerma rate. For e reasons stated above, e individual calibration coefficients are not disclosed. K R, VSL At e time of producing is report, e National Physical Laboratory (NPL) had also participated in e BIPM.RI(I)-K8 comparison. Taking e mean of e two values determined by e BIPM (at is, at e VSL and at e NPL) as a normalization, is possible to see how e NMIs compare wi each oer in terms of well-chamber calibrations coefficients, as shown in Figure. Figure. NMI results for e well-chamber NK,NMI / NK,mean VSL NMI NPL Relative calibration coefficients for e BIPM well chamber, as determined for each NMI 19 Ir source using e dedicated current measurement system of e well chamber. The uncertainty bars represent e combined standard uncertainties Note at e well-chamber results in Figure do not provide information at is independent of e main comparisons result using e imble chamber, but raer provide a means of 7/10

8 establishing a comparison result when no imble chamber calibration is made by a particular NMI. 7. Uncertainties 7.1 Thimble chamber The uncertainties associated wi e BIPM imble chamber calibration are listed in Tables 4 and 5 for e BIPM and e VSL, respectively (JCGM 008). Table 4. Uncertainties associated wi e imble chamber calibration at e BIPM Relative standard uncertainty u ia u ib 60 Co air kerma determination K Ionization current I Positioning Radial non-uniformity Long-term stability Energy dependence k en NK,BIPM for 19 Ir u ia represents e relative standard uncertainty estimated by statistical meods, type A u ib represents e relative standard uncertainty estimated by oer means, type B Table 5. Uncertainties associated wi e imble chamber calibration at e VSL Relative standard uncertainty u ia u ib 19 Ir air kerma determination Reference standard N K Ionization current VSL reference standard Calibraiton of e transfer chamber I ionization current transfer chamber k dec decay P pressure T temperature k air abs air absorption <1x10 7 k ion ion recombination k pol polarization k nu radial non-uniformity k stem stem scatter k scatter room scatter Lateral distance between source position Lateral and longitudinal positioning N K, VSL From e Tables 4 and 5, e combined standard uncertainty u c for e comparison result R is VSL K, 8/10

9 7. Well chamber The uncertainties associated wi e well type chamber calibration are listed in Tables 6. Table 6. Uncertainties associated wi e well chamber calibration at e VSL Relative standard uncertainty 19 Ir air kerma determination BIPM VSL u ia u ib u ia u ib Reference air kerma K R NK,BIPM for 19 Ir Corrected ionization current NE571 I k i (1) Calibration of e well type chamber Ionization current well chamber I w Temperature, pressure correction Short-term stability w N K (1) k i are e VSL correction factors listed in Table 5, as I was measured using e VSL measuring system 8. Discussion The VSL meod to determine e calibration coefficient for its reference standard for 19 Ir (and us e RAKR) is briefly explained in Section 3 of is report and is fully described by van Dijk et al (004), where e result obtained using e meod adopted is compared wi e result obtained using a linear interpolation between 50 kv and 137 Cs beams. The VSL meod gives for a NE 571 chamber a calibration coefficient at is 0.3 % higher at e linear interpolation. To assess e effect of e different meods adopted at e VSL and e BIPM, e transfer chamber used for is comparison was also calibrated at e BIPM in e 137 Cs reference beam and a linear interpolation made wi e 50 kv calibration coefficient. Following e VSL procedure, is interpolated BIPM value for 19 Ir was increased by 0.3 % and compared wi e VSL reference value. Using is modified BIPM standard, e comparison result would be , raer an e actual comparison result R K = This result (0.9978) is closer to what would be expected given e VSL/BIPM comparison results for 60 Co (0.9985) and 50 kv (1.001). We might deduce, erefore, at most of e difference between e two standards is due to e different meods used. 9. Degrees of equivalence Comparison of a given NMI wi e key comparison reference value For each NMI i having a comparison result R K,i (denoted x i in e KCDB) wi combined standard uncertainty, u i, e degree of equivalence wi respect to e reference value is given by a pair of terms: K, NMI i K, BIPM K, BIPM K, i e relative difference D N N N R 1 i (6) R R R and its expanded uncertainty U i = u i. (7) The results for D i and U i, are expressed in mgy/gy. 9/10

10 Consequently, e degree of equivalence of e VSL wi e reference value is expressed as D i U i /(mgy/gy) VSL The degree of equivalence of NMI i wi respect to each NMI j is e difference, D ij = D i D j = x i x j (8) and its expanded uncertainty U ij = u ij, where, u ij u NMIi, corr 10. Conclusion u NMIj,corr f u u n n NMIi, n NMIj, n The VSL standard for e reference air kerma rate for 19 Ir gamma radiation compared wi e BIPM reference value gives a comparison result of wi a combined standard uncertainty u c of This is e first result in is new ongoing comparison, registered as BIPM.RI(I)-K8. The degrees of equivalence wi later participants will be published in due course. (9) References Allisy-Roberts P.J., Burns D.T., Kessler C., 007, Summary of e BIPM.RI(I)-K1 comparison for air kerma in 60 Co gamma radiation, Metrologia, 007, 44, Tech. Suppl., Allisy-Roberts P.J., Burns D.T., Kessler C., 009, Measuring conditions used for e calibration of national ionometric standards at e BIPM, 009, Rapport BIPM-09/04, 0 pp. Burns D.T., de Prez L.A., 009, Key comparison BIPM RI(I)-K3 of e air kerma standards of e VSL, Neerlands and BIPM, in medium-energy x-rays, Comparison report JCGM 008, Evaluation of measurement data Guide to e expression of uncertainty in measurement (GUM 1995 wi minor corrections) JCGM 100:008. KCDB 014, The BIPM key comparison data base ( Mainegra-Hing E, Rogers D., 006, On e accuracy of techniques for obtaining e calibration coefficient NK of 19Ir HDR brachyerapy sources. Med. Phys. 33, Petersen J.J., van Dijk E., Grimbergen T.W.M., Aalbers A.H.L.,1994, Absolute determination of e reference air kerma rate for MicroSelectron-HDR 19 Ir source serial number 098, NMi-VSL Report S-EI-94.0, 19pp. Petersen J.J., van Dijk E., Aalbers A.H.L., 1994, Comparison of meods for derivation of 19 Ir calibration factors for e NE 561 and NE 571 ionisation chambers, NMi-VSL Report S-EI , 14pp. van Dijk, E, 007, Wall correction factors for a cylindrical cavity chamber and 137 Cs radiation using Monte Carlo meods, NMi Report VSL-ESL-IO-007/3, 10pp. van Dijk E., Kolkman-Deurloo I.K., Damen, P.M.G., 004, Determination of e reference air kerma rate for 19 Ir brachyerapy sources and e related uncertainty, Med. Phys., 31, Woods M.J., Lucas SEM Reher DFG, Sibbens G., 199, The half live 19 Ir, Nucl. Instr. Phys. Res. A31, /10