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International comparison of the primary standards for the realization of the unit of absorbed dose to water of beta-brachytherapy sources


With the completion of a new primary standard in 2005 - the multi-electrode extrapolation chamber - PTB has been able to realize a spatially resolved 3D dose distribution absolutely in the clinically relevant near field of beta-brachytherapy sources without having to rely on a supplementary secondary dosimetric system. In 2006, a bilateral comparison with NIST was successfully performed in the near field of a 90Sr/90Y line source [1] (an identical construction is used in vascular radiation therapy). In the course of further efforts made to prove the comparability of the primary standards, an international comparison of the absorbed dose to water in the near field of a 106Ru planar source was carried out in 2008. Similar types of sources are used in eye tumor radiation therapy. For the comparison, an especially prepared planar source was mounted into a holder and measured at a water-equivalent depth of 1 mm, at PTB, at NIST and at the Dutch NMi.

At PTB, the source was measured with the new primary standard, a multi-electrode extrapolation chamber (MEK) [2,3]. This measuring device is based on an extrapolation chamber with a segmented collector electrode, which was manufactured on a Si wafer by means of lithographic methods in PTB’s Clean Room Centre. This novel collector electrode allowed measurements to be performed simultaneously on 28 measurement points with a spatial resolution of 1 mm x 1 mm. At NIST and at NMi, the water absorbed dose in the dose maximum was also determined absolutely with an extrapolation chamber (diameter of the collector electrode: 4 mm at NIST and 1 mm at NMi). To obtain a better spatial resolution, a two-dimensional dose distribution with radiochrome film was additionally determined at NIST (see figure). Averaged over a surface of 4 mm in diameter, the comparison of the water absorbed rate in the dose maximum yielded a relative difference of 1 % between PTB and NIST, and - averaged over a surface of 1 mm in diameter - a relative difference of 4 % between PTB and NMi. The relative standard uncertainty of PTB and of the measuring device of NMi - determined in accordance with GUM - amounted to 3 %, that of NIST to 3.5 %.

Figure : Isodoses of the 106Ru planar source at a water depth of 1 mm. The measurement was performed at NIST with radiochromic film. (Intervals of the isodoses: 5 mGys-1)


  1. M. Bambynek, C.G. Soares:
    Intercomparison of primary standards at PTB and NIST for the realization of the unit of absorbed dose to water for beta particle brachytherapy sources,
    Medical Physics Vol. 34, No. 6, 2463 (2007).
  2. M. Bambynek:
    Development of a multi-electrode extrapolation chamber as a prototype of a primary standard for the realization of the unit of the absorbed dose to water for beta brachytherapy sources,
    Nuclear Instruments and Methods in Physics research A 492, 264 – 275 (2002).
  3. M. Bambynek, H. Drehsler, A. Essmann, H.K. Drösemeyer:
    Primary standard measuring device for the realization and dissemination of the unit of the water absorbed energy dose of beta brachy therapy sources,
    DGMP Proceedings Regensburg (2006).