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Determination of the radial anisotropy of the radiation field of Ir-192 and CO-60 brachytherapy sources

29.09.2008

As explained in the Annual Report of 2006, PTB has fundamentally renewed its calibration measuring set-up for 192Ir and 60Co brachytherapy sources and has thus considerably reduced the uncertainties of the calibration factors for HDR (High Dose Rate) brachytherapy sources for the measurand "Reference Air Kerma Rate" (RAKR) [1]. The RAKR is the air kerma rate free in air at a distance of 100 cm from the radiator's focus of activity(1). On some sources which have since then been submitted to PTB for calibration it turned out that in the case of repeat measurements of the Reference Air Kerma Rate, the reproducibility was not completely in line with the uncertainties to be expected. The higher standard deviation of the calibration factors of single sources was found to be caused by the inhomogeneous activity distribution in the 192Ir sources. Due to the high absorption capability of the iridium-palladium alloy of the source, this inhomogeneous distribution causes a more or less strong anisotropy of the photon radiation field, both in polar and in radial direction. As the RAKR is defined in a point vertical to the central axis and is also determined in this way, the radial anisotropy leads to the poor reproducibility of the calibration factor mentioned above. As a rotation of the source during calibration is not possible for technical reasons, the measurements are repeated several times, and the mean value of these results is indicated as the calibration factor. As a result of the running-in and running-out of the afterloading system, the source is turned randomly during the repeat measurements (see [2]), so that the standard deviation of the mean value is then mainly determined by the inhomogeneity of the radiation field of the source.

In addition, the relative radial anisotropy of the radiation field is exactly determined by a separate measurement, in the case of which an ionization chamber of the type Exradin A4 is rotated in steps of 15 degrees around the source at a constant distance from it, with the aid of the industrial robot which was also used for calibration. For this purpose, the source is positioned centrally on the central axis of a lead cylinder, as shown in Figure 1. The lead cylinder has a conical opening on one side, which allows a narrowly collimated beam to emerge. The ionization chamber is located in the central beam of this radiation beam, rigidly connected to the lead cylinder. While the lead cylinder - including the ionization chamber - is rotated with the aid of the robot, the radiation source always remains in the same position due to the hose catheter which is firmly attached to the afterloader. Figure 2 shows the relative signal of the ionization chamber as a function of the exit angle of the radiation from the radiation source and thus the relative radial change of the Reference Air Kerma Rate. In the case of the source indicated here, the relative change of the RAKR over the range from 0° to 360° amounts to ± 0.5%. As no information about the orientation of the source during irradiation is available when the source is used in the hospital, this uncertainty must possibly be taken into account, in addition, in the irradiation planning. For this reason, the radial anisotropy of the radiation field has - since the beginning of 2008 - been determined and indicated in the calibration certificate for each calibration of a 192Ir or 60Co brachytherapy source.

Figure 1 : Measurement set-up for the determination of the radial anisotropy of the radiation field of 192Ir and 60Co HDR brachytherapy sources

Figure 2 : Radial anisotropy of the radiation field of a 192Ir HDR brachytherapy source

Literature

  1. International Commission on Radiation Units and Measurements, 2004. Report 72,
    Dosimetry of Beta Rays and Low-Energy Photons for Brachytherapy with sealed sources.
    Journal of the ICRU, 4(2)
  2. Selbach, H.-J.:
    New Calibration Facility for 192Ir and 60Co Brachy Therapy Radiation Sources.
    Annual Report of the Physikalisch-Technische Bundesanstalt (2006)

(1) Note: The measurand "characteristic dose rate" as described in DIN 6809 T2, basically corresponds to the "Reference Air Kerma Rate" (RAKR) normally used outside Germany, with the difference that the characteristic dose rate also contains "unavoidable scattering fractions" from the air and the environment as well as the influence of the air attenuation. For irradiation planning, however, only the fraction corrected by the scatter and the attenuation is important.