Microdosimetric measurements and estimation of human cell survival for heavy-ion beams.

The microdosimetric spectra for high-energy beams of photons and proton, helium, carbon, neon, silicon and iron ions (LET = 0.5-880 keV/microm) were measured with a spherical-walled tissue-equivalent proportional counter at various depths in a plastic phantom. Survival curves for human tumor cells were also obtained under the same conditions. Then the survival curves were compared with those estimated by a microdosimetric model based on the spectra and the biological parameters for each cell line. The estimated alpha terms of the liner-quadratic model with a fixed beta value reproduced the experimental results for cell irradiation for ion beams with LETs of less than 450 keV/microm, except in the region near the distal peak.

Application of heavy-ion CT.

In treatment planning for hadron therapy, information about the relative stopping power in a patient's body is used to calculate the range of incident ions. This information is obtained from computed tomography (CT) images using a conversion table from x-ray CT numbers into stopping powers relative to the stopping power of water. In treatment planning at the National Institute of Radiological Sciences (NIRS), the conversion table has been created based on the polybinary tissue model. However, it has not been fully verified that the model is accurate enough for use in real animal tissues. In order to irradiate heavy ions more precisely in radiotherapy, we have to evaluate the accuracy of the polybinary tissue calibration in animal tissues. We have measured animal tissue samples with a heavy-ion CT (HICT) and an x-ray CT. The x-ray CT image was converted to an image of relative stopping power by using the table derived from the polybinary tissue calibration (polybinary-tissue-model CT (PTCT) image). On the other hand, with HICT, the two-dimensional distribution of relative stopping power can be obtained directly. A comparison between PTCT and HICT images enabled us to verify the accuracy of the conversion table derived from the polybinary tissue calibration. Consequently, it was found that the agreement between the relative stopping powers of PTCT and HICT is 1.6% for fat, muscle and bone.