München 2009 – wissenschaftliches Programm

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ST: Fachverband Strahlen- und Medizinphysik

ST 6: Ion Radiation Therapy 1: Radiation fields and effects

ST 6.3: Vortrag

Mittwoch, 11. März 2009, 14:50–15:10, A021

Microdosimetry measurements for 300 MeV/u C-12 pencil beams stopping in water — •Giovanna Martino, Oksana Kavatsyuk, Marco Durante, and Dieter Schardt — GSI Biophysik, Darmstadt

Microdosimetry measurements have been carried out at GSI to investigate the radiation field produced by a 300 MeV/u C-12 pencil-like beam stopping in water and to study the dose fall-off as a function of distance from the beam axis. For this purpose, microdosimetric spectra were recorded moving a Tissue-Equivalent-Proportional-Chamber (TEPC) to 50 different positions inside a water phantom. As the lateral spread of heavy ions is very small the main contribution to the lateral dose comes from secondary fragments which are produced in nuclear reactions. The corresponding LET spectra are characterized by peaks around 10 keV/μm in a range extended up to around 50 keV/μm. Absorbed doses were obtained by integration of the microdosimetric spectra. Along lines parallel to the beam axis the dose curves, as a function of depth, show a typical grow-in part due to the build-up of secondary fragments, including neutrons. The dose characteristics along lines perpendicular to the beam axis show a steep exponential fall-off. At larger distances from beam axis the fall-off is slower. Here the main contribution comes from light secondary fragments, such as protons and neutrons, which have much broader angular distributions than the heavier fragments. These results confirm that the lateral dose characteristics of carbon ion beams used in radiotherapy shows an extremely steep decrease with distance from the beam axis.