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Dosimetry in the secondary radiation field in C-12 radiation therapy


The therapy of solid tumours by means of high-energy protons or 12C ions is one of the most modern forms of cancer therapy for specific types of tumours. In Germany, 12C ion therapy has for many years been developed by the GSI (GSI Helmholtzzentrum für Schwerionenforschung GmbH) in Darmstadt, and will soon be performed in routine operation at the Ion Therapy Centre (HIT) in Heidelberg. In addition, further ion therapy centres with high-energy ion beams are being established in Marburg and Kiel.

It is the aim of radiation therapy to kill the tumour completely and to expose the surrounding healthy tissue to as little radiation as possible. However, and especially in the case of ion therapy, secondary radiation is also generated by the interaction of the ion beams with the tissue and leads to a low exposure of the healthy tissue. Meanwhile, many investigations are focussed on the dosimetric determination of the secondary radiation generated during radiation therapy. In ion beam therapy with high-energy ions - protons with up to 250 MeV and 12C ions with up to 400 MeV per nucleon of kinetic energy - high-energy secondary particles such as neutrons, protons, deuterons and helium nuclei are generated by nuclear reactions with atomic nuclei of the tissue [1]. For dosimetric measurements in such a complex radiation field, the experimental methods of microdosimetry are particularly suited.

For the measurements carried out at PTB on the 12C therapy beam of the GSI, a proportional counter filled with a tissue-equivalent gas was used (Tissue Equivalent Proportional Counter, TEPC) which has also been successfully used for the dosimetry of cosmic radiation at flight altitudes with PTB’s In-flight Dosimetry System (πDOS) [2]. By reducing the gas pressure to approximately 40 hPa, a tissue volume of 4 µm in diameter is simulated with a gas volume of 5.69 cm. With a TEPC, a pulse height distribution is measured which allows conclusions about the ionisation density - i.e. beam quality - to be drawn and which can be converted into a dose distribution. As it is based on the measurement of the dose in tissue and its microscopic distribution, this measurement procedure is suited for all components of the radiation field and very much approximates the definition of the equivalent dose.

The measuring arrangement was composed of a cylindrical water phantom approx. 15 cm in diameter which was irradiated with the 12C ion beam with an energy of 200 MeV per nucleon. The dose distribution around the phantom was measured under 0°, 20° and 90° at a distance of 2 m. The use of an additional veto detector made it also possible to distinguish between dose fractions contributed by neutral radiation particles - e.g. photons or neutrons - and charged radiation particles such as, for example, protons, deuterons or helium nuclei. The measured distribution of the ambient dose equivalent is shown in Figure 1 in comparison to other measurements.

The measurements are aimed at determining the additional dose outside the irradiated tumour to be able to estimate the additional dose by secondary radiation in the patient. If typical irradiation conditions during a tumour treatment are compared, e.g. the irradiation of a volume of 5 cm × 5 cm × 5 cm in the water phantom with 10 Gy, doses of a few hundred micro Sievert are measured in forward direction at a distance of 2 m. This means that in the vicinity of the irradiated tumour, the healthy tissue is exposed to an additional dose of several mSv.

Figure : Ambient dose equivalent per beam particle measured with the TEPC (πDOS). The charged component () consists in forward direction mainly of the helium nuclei and protons. Under larger angles, mainly neutrons are emitted (neutral component, ). The results of πDOS and the measurements of the neutron component with a WENDI-II neutron monitor are in good agreement. The differences to the measurements performed with a BaF2 detector system can be attributed to protons which are partly detected by the two systems, πDOS and WENEDI-II, and which cannot be distinguished from neutrons.


  1. K. Gunzert-Marx, H. Iwase, D. Schardt, R. S. Simon:
    Secondary Beam Fragments produced by 200 MeV/u 12C ions in water and their dose contributions in carbon ion radio therapy.
    New Journal of Physics 10 (2008) 075003
  2. F. Wissmann:
    Long-term Measurements of H*(10) at Aviation Altitudes in the Northern Hemisphere.
    Radiat. Prot. Dosimetry 121 (2006) 347