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Experimental determination of radiation quality correction factors for frequently used types of ionization chambers



In modern radiation therapy, tumours are treated using high-energy photon and electron radiation which is generated by clinical linacs. The fundamental precondition for the treatment to be successful (i.e. curing the tumour) and to prevent undesired side-effects (damage due to radiation) is to know the exact dose that a linac generates under certain standardized conditions - the so-called "reference conditions".

For dose measurement in clinical practice, ionization chambers are used which are calibrated to indicate the absorbed dose to water in a 60Co radiation field. If such a dosemeter is used in a high-energy photon or electron field generated by an accelerator, the change in the responsivity of the dosemeter due to the change in the type of radiation or energy has to be taken into account. The exact procedure for dose measurement is described in detail in so-called "dosimetry protocols" such as, e.g. the German standard DIN 6800-2 [1] or the international protocol IAEA TRS-398 [2]. Thereby, the influence of the radiation type and energy - the combination of these two properties is designated as the "radiation quality" - is accounted for by applying a radiation quality correction factor kQ. The numerical value of this factor depends, on the one hand, on the type and energy of the radiation and, on the other hand, it is also influenced by the design, the dimensions and the materials of the ionization chamber - i.e. it depends on the type of ionization chamber.

The major part of the dosimetry protocols currently relies on values of the correction factor kQ which are based on model calculations and exhibit a standard uncertainty of approx. 1 %. Compared to the uncertainty which can be attained today when calibrating ionization chambers (0.2 % … 0.3 %), this uncertainty of the correction factor kQ frequently has a limiting effect on the total uncertainty of a dose measurement.

For seven types of ionization chambers which are frequently used in clinical practice, kQ factors have therefore been determined experimentally in high-energy photon fields; their relative standard measurement uncertainty merely amounts to 0.4 % (k = 1). These values are taken into account in a revised version of the standard DIN 6800-2, so that it will then be possible to carry out a routine measurement of the absorbed dose to water with a lower uncertainty.


The value of the correction factor kQ for a certain ionization chamber and a certain radiation quality results from the ratio of the calibration factors which are determined for this particular ionization chamber in a 60Co radiation field and in the corresponding high-energy radiation field.

For each of the types PTW 30012, PTW 30013, PTW 30015, PTW 30016, PTW 31010, PTW 31013 and PTW 31015, 3 ionization chambers were investigated. Each of these ionization chambers was calibrated both in a 60Co reference radiation field of PTB and in several high-energy photon radiation fields. The measurements necessary for this were carried out at the clinical linacs of the type "Elekta Precise" which are available at PTB and have nominal acceleration voltages of 4 MV, 6 MV, 8 MV, 10 MV, 15 MV and 25 MV, respectively, under the reference conditions defined in IAEA TRS-398 [2] in a water phantom whose surface was located at a distance of 100 cm from the radiation source. The field size at the phantom's surface was 10 cm × 10 cm; the ionization chambers were positioned in the water phantom in such a way that their reference point was located on the axis of the radiation field at the reference depth of 10 cm. All the chamber readings were normalized to the reading of a large-area transmission ionization chamber which was located in the radiation field, at the shadow tray of the radiation head [3].

Both the calibrations in the 60Co radiation field and the measurements in the high-energy accelerator fields are directly traceable to the German primary standard for the measurement of the absorbed dose to water - a water calorimeter [4] - and therefore exhibit the smallest possible measurement uncertainty.


For each of the investigated ionization chambers, the individual kQ factors for different radiation qualities were calculated from the experimentally determined calibration factors. It turned out that for one and the same radiation quality, the kQ factors of different ionization chambers of the same type deviated from each other by no more than 0.2 %. This observation, which is in agreement with previous investigations [5], justifies the indication of type-specific values of the correction factor kQ that are valid for all chambers of the respective type.

The type-specific kQ factors of the types of ionization chambers investigated (mean values of the individual kQ factors) are indicated for the photon radiation fields used in Table 1 (the chambers were not all investigated in all photon radiation fields).

Table 1: Type-specific kQ factors of the types of ionization chambers investigated here.

acceleration voltage
4 MV6 MV8 MV10 MV15 MV25 MV
Radiation quality index Q0.6380.6830.7140.7330.7600.799
PTW 300120.99040.98300.9641
PTW 300130.98390.97440.9570
PTW 300150.99010.98010.9649
PTW 300160.99440.98970.98600.98080.97600.9653
PTW 310100.98660.97810.9573
PTW 310130.98680.97740.9601
PTW 310150.99590.98820.98260.98080.97360.9611

The relative standard measurement uncertainty of the kQ factors listed in Table 1 was determined in accordance with the "Guide to the expression of uncertainty in measurement" (GUM); it amounts to 0.4 %.

The kQ factors determined here complement the database of kQ factors determined experimentally by other authors [6] or determined by means of modern Monte Carlo simulations of the radiation transport [7][8] of certain kQ factors. In the cases where the types of ionization chambers investigated and the radiation qualities are the same, there is excellent agreement of the kQ factors determined here with the values known from literature. To illustrate this, Figure 1 shows the example of the ionization chamber of the type PTW 31010.

Figure 1 : Comparison of the kQ factors determined in this work for chambers of the type PTW 31010 with values from literature. The grey-shaded area represents the measurement uncertainty of the kQ factors currently used in dosimetry  protocols.


For three ionization chambers, each of seven types that are frequently used in practice, individual kQ factors were determined in high-energy photon radiation fields. The relative standard measurement uncertainty of these experimentally determined kQ factors amounts to 0.4 % (k = 1) and is thus considerably lower than the uncertainty stated in DIN 6800-2 [1] or IAEA TRS-398 [2]. The results show that for the ionization chambers investigated, the specimen-specific variation of the kQ factors within the scope of the measurement uncertainty is negligible, which allows type-specific values for the correction factor kQ to be indicated. The experimentally determined values are in very good agreement with the results of recent Monte Carlo calculations and other published experimental values, and thus complement the existing database of experimentally determined kQ values.


  1. 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)
  2. International Atomic Energy Agency:
    Absorbed Dose Determination in External Beam Radiotherapy. An International Code of Practice for Dosimetry Based on Standards of Absorbed Dose to Water.
    Technical Reports Series No. 398, IAEA, Wien (2000),
  3. R.-P. Kapsch, A. Krauss:
    On the performance of monitor chambers to measure the output of medical linear accelerators for high-precision dosimetric investigations.
    World Congress on Medical Physics and Biomedical Engineering, Munich, September 7 - 12, 2009, IFMBE Proceedings 25/I, 85-88, Springer (2009)
  4. A. Krauss:
    The PTB water calorimeter for the absolute determination of absorbed dose to water in 60Co radiation.
    Metrologia 43 (2006), 259–272
  5. R.-P. Kapsch, C. Pychlau:
    Exemplarstreuung von kQ-Werten.
    In: B. Kollmeier (Hrsg.): Medizinische Physik 2008, Proceedings der Jahrestagung der Deutschen Gesellschaft für Medizinische Physik, Oldenburg (2008) [CD-ROM]
  6. M. R. McEwen:
    Measurement of ionization chamber absorbed dose kQ factors in megavoltage photon beams.
    Med. Phys. 37 (2010), 2179–2193
  7. J. Wulff, J. T. Heverhagen, K. Zink:
    Monte-Carlo-based perturbation and beam quality correction factors for thimble ionization chambers in high-energy photon beams.
    Phys. Med. Biol. 53 (2008), 2823–2836
  8. B. R. Muir, D. W. O. Rogers:
    Monte Carlo calculations of kQ, the beam quality conversion factor.
    Med. Phys. 37 (2010), 5939–5950