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Carbon ions against cancer

Radiation measurements in cooperation with PTB create a basis for more precise dosage

PTB-News 3.2017
05.09.2017
Especially interesting for

radiation therapy

dosimetry

Tumor irradiation by carbon ions is clearly becoming more precise. With the help of investigations carried out by the Physikalisch-Technische Bundesanstalt (PTB), the accuracy of the absorbed dose measurement could be improved by a factor of three. The German Cancer Research Center (DKFZ) and the Heidelberg Ion Beam Therapy Center (HIT) were also involved in this research collaboration.

Schema des Raster-Scan-Verfahrens, das am HIT verwendet wird. Der Ionenstrahl (grün) wird mithilfe von Magneten über ein vordefiniertes Raster gescannt, wobei an jeder Position die erforderliche Zahl der Teilchen bestimmt wird. Durch Variation der Energie wird zusätzlich die Eindringtiefe der Ionen in den Tumor (das Gebilde auf der rechten Seite des Bildes) variiert. So kann der Tumor insgesamt sehr exakt bestrahlt werden. (Abb.: HIT)

Tumor radiation with carbon ions can offer an additional healing effect compared to conventional irradiation methods, since together with the raster scan method used in this case, the radiation dose can be positioned very precisely and the surrounding tissue is best preserved. This therapy is particularly interesting in the case of deep-seated tumors as well as of those which are surrounded by particularly radiation-sensitive tissue. To date, however, the dosimetry of such ion beams has not been as precise as the dosimetry of conventional, high-energy X-rays.

In order to measure the absorbed dose of the radiation used, calibrated detectors, usually so-called ionization chambers, are used in clinical dosimetry. However, ionization chambers respond differently to different types of radiation. Therefore, a correction factor must be taken into account depending on the type of radiation. This factor previously had an uncertainty of approximately 3 % when using carbon ions, which is approximately three times higher than in x-radiation. The goal of the cooperation project was to reduce the uncertainty to 1 %.

To date, only a theoretically determined correction factor has been used in everyday clinical practice, as there is no reliable experimental data. This project has now shown for the first time that this factor can indeed be measured with much less uncertainty. This was achieved by means of a portable water calorimeter, which was used at the HIT facility to exactly measure the absorbed dose to water and to experimentally determine the correction factor for different ionization chambers.

The next step will be to carry out investigations also with other ion beams, e. g. with helium or oxygen ions.

Contact

Achim Krauss
Department 6.2
Dosimetry for Radiation Therapy and Diagnostic Radiology
Phone: +49 (0)531 592-6230
Opens window for sending emailachim.krauss(at)ptb.de

Scientific publication

J.-M. Osinga-Blättermann, S. Brons, S. Greilich, O. Jäkel, A. Krauss: Direct determination of kQ for Farmer-type ionization chambers in a clinical scanned carbon ion beam using water calorimetry. Phys. Med. Biol. 62, 2033–2054 (2017)