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Key comparison of the primary standards for the unit "absorbed dose to water" in high-energy photon fields between PTB and BIPM

11.04.2012

For many years, the Physikalisch-Technische Bundesanstalt (PTB) has been operating a water calorimeter as a primary standard for the realization of the unit "absorbed dose to water" in the 60Co radiation field. The principle of such a calorimeter is based on the very precise measurement of the radiation-induced temperature increase of water and the subsequent conversion into the absorbed dose to water (see [1]). A copy of this calorimeter is meanwhile also used for routine measurements of the absorbed dose to water in high-energy photon fields which are generated by the two clinical linear accelerators of PTB. This allows dosemeters to be calibrated on accelerators also in high-energy photon fields or to perform reference radiations with a measurement uncertainty of approx. 0.3 %, which is about as small as in the case of corresponding measurements in the 60Co radiation field.

Figure 1 : PTB's water calorimeter in front of a clinical linac.

To guarantee the worldwide uniformity of dosimetry and the international recognition of PTB calibrations, the German primary standard (i.e. the water calorimeter) must be compared at regular intervals with the other worldwide available primary standards for the absorbed dose to water. Whereas such comparisons have already been carried out for a long time for 60Co radiation (see, e.g., [2]), such a comparison has, for the first time, been launched for high-energy photon radiation on accelerators in 2009 by the Bureau International des Poids et Mesures (BIPM). In addition to PTB and the BIPM, the national metrology institutes of Australia, Italy, France, Japan, Great Britain, Canada, Switzerland, the USA and the Netherlands are taking part in this comparison which has been entered into the "Key Comparison Data Base" (KCDB) of the BIPM with the abbreviation "BIPM.RI(I)-K6)". The comparison is organized in a star-type way so that - first of all - each national metrology institute compares itself with the BIPM; from these bilateral comparisons, conclusions can then be drawn about the differences between two participants.

PTB participated as the second national metrology institute in this comparison (after the Canadian NRC). The measurements on the accelerators of PTB had already been carried out in 2010. In the course of the year 2011, the measurements were evaluated and the final report of this key comparison between the BIPM and PTB is now available [3].

The comparison is based on the measurement of the absorbed dose to water in one and the same radiation field with the primary standard of the BIPM Dw,BIPM and the water calorimeter of PTB Dw,PTB. The result of the comparison is the ratio of the two dose values

                                              

together with the associated measurement uncertainty u(R). If both primary standards furnish exactly the same value for the absorbed dose to water, R = 1 is valid. A difference between the primary standards is characterized by a value deviating from 1, which does not imply any statement about which standard measures "more correctly". Equivalence of the two primary standards exists if the value of R does not deviate by more than twice the measurement uncertainty 2•u(R) from 1. This is why the determination of the measurement uncertainty u(R) is of particular importance for such key comparisons.

For the measurements, the primary standard of the BIPM for the unit "absorbed dose to water" (a graphite calorimeter [4]), including the associated measuring electronics and other required measuring instruments, was brought to PTB in Braunschweig in order to carry out the measurements in the same high-energy photon radiation fields as the primary standard of PTB (see Figure 2).

Figure 2 : The graphite calorimeter of the BIPM, in measuring position in front of one of the two clinical linacs of PTB.

The measurements were carried out in a total of three different photon radiation fields with nominal accelerating voltages of 6 MV, 10 MV and 25 MV, whereby the measurement conditions always complied with the reference conditions for dosimetry in radiation therapy (see, e.g., [5] or [6]). The performance and evaluation of the measurements as well as the determination of the measurement uncertainty are described in detail in the final report of this comparison [3].

The results of this comparison (i.e. the dose ratio R and the associated standard measurement uncertainty u(R)) for the three photon radiation fields used are given in Table 1 and Figure 3. Figure 3 shows, in addition, the results of the comparison between the BIPM and the NRC (Canada) [7] which also indicate the differences between the primary standards of PTB and the NRC.

Nominal acceleration
voltage
in MV
Ru(R)
61.0010.005
101.0030.006
251.0020.006
Table 1:
Results of the comparison of the primary standards of PTB and the BIPM in high-energy photon fields on accelerators.

Figure 3 : Results of the comparison of the primary standards of PTB and of the BIPM (red) as well as of the NRC and the BIPM (blue).

The results show that within the scope of the measurement uncertainty, there is very good equivalence of the primary standards of PTB, the BIPM and the NRC for the unit "absorbed dose to water" in high-energy photon fields on accelerators.

At present, the BIPM.RI(I)-K6 comparison is continued with further participants so that - in the course of the next years - the results of further national metrology institutes will gradually be added to Figure 3.

Literature

  1. A. Krauss:
    The PTB water calorimeter for the absolute determination of absorbed dose to water in 60Co radiation.
    Metrologia 43 (2006), 259–272
  2. C. Kessler, P. J. Allisy, D. T. Burns, A. Krauss, R.-P. Kapsch:
    Comparison of the standards for absorbed dose to water of the PTB, Germany and the BIPM for 60Co γ rays.
    Metrologia 43 (2006), Tech. Suppl. 06005
  3. S. Picard, D. T. Burns, P. Roger, P. J. Allisy-Roberts, R.-P. Kapsch, A. Krauss:
    Key comparison BIPM.RI(I)-K6 of the standards for absorbed dose to water of the PTB, Germany and the BIPM in accelerator photon beams.
    Metrologia 48 (2011), Tech. Suppl. 06020
  4. S. Picard, D. T. Burns, P. Roger:
    The BIPM Graphite Calorimeter Standard for Absorbed Dose to Water.
    Standards, Applications and Quality Assurance in Medical Radiation Dosimetry (IDOS), Proceedings of an International Symposium, Vienna, 9-12 November 2010, Intaernational Atomic Energy Agency (2011), 55–65
  5. Deutsches Institut für Normung: DIN 6800-2. Dosismessverfahren nach der Sondenmethode für Photonen- und Elektronenstrahlung – Teil 2: Dosimetrie hochenergetischer Photonen- und Elektronenstrahlung mit Ionisationskammern. Beuth Verlag Berlin (2008)
  6. International Atomic Energy Agency: Absorbed Dose Determination in External Beam Radiotherapy. Technical Reports Series No. 398, IAEA, Wien (2000)
  7. S. Picard, D. T. Burns, P. Roger, P. J. Allisy-Roberts, M. R. McEwen, C. D. Cojocaru, C. K. Ross:
    Comparison of the standards for absorbed dose to water of the NRC and the BIPM for accelerator photon beams.
    Metrologia 47 (2010), Tech. Suppl. 06025