[Elucidation of the energy absorption spectra of the (11)C beam in a plastic scintillator.].

Heavy ion therapy using the energetic (12)C beam is successfully under way at HIMAC, Japan. The method is more advantageous than traditional radiation therapy in dose concentration owing to the Bragg peak and high relative biological effectiveness. A research study using the (11)C beam for heavy ion therapy in the future has been carried out in order to develop the capability of monitoring the dose distribution. Our group has examined the total energy absorption spectrum of the (11)C beam in a plastic scintillator. We could clearly observe the total absorption peak of (11)C in the energy spectrum and, in addition, we found a broad bump structure was associated with the peak. The bump area occupies 37% of the total spectrum and it probably affects the dose calculation for an accurate treatment planning. We elucidated the mechanism that leads to the structure of the total energy absorption spectra given by (11)C and (12)C in a block of plastic scintillator. This paper describes the method in detail and gives experimental analysis results which deal with the bump structure. We could explain the bump structure using the energy spectra caused by the fragmentation reactions.

[Performance evaluation of X-ray CT using visible scintillation light].

We proposed a new method of performance evaluation for X-ray CT using visible scintillation light and examined its usefulness in this study. When we scanned a plastic scintillator disk in a gantry opening of the X-ray CT, we could observe visible scintillation light. The rotation of the light-emitting area of the disk corresponded to that of the X-ray tube. We were able to record the scintillation light by digital video camera. By analyzing the area of visible scintillation light, the rotation speed of the X-ray tube, angular spread of the X-ray beam, uniformity of the incident X-rays, and change in X-ray energy were measured. No other method is available to obtain the above parameters of X-ray CT during a single CT scan. In the measurements of the uniformity of incident X-rays and change of X-ray energy, our method showed good accuracy in detecting the attenuation caused by the couch between the X-ray tube and the plastic scintillator disc. The proposed method is inexpensive and easy-to-use. We conclude that the method is a useful tool for performance evaluation as well as a maintenance tool for X-ray CT.

[Measurement of depth dose distribution using plastic scintillator].

We propose a new method to measure the depth dose distribution in matter. The method is to use organic plastic scintillator as a phantom, which has the effective atomic number close to that of human soft tissue, and to measure the position distribution of the scintillation light by a wavelength analyzer through a thread of plastic optical fiber. The purpose of this study is to examine possibility to measure the depth dose distribution with the proposed method. We carried out the following three measurements. First, we measured the effective field of view (FOV); the plastic optical fiber accepts the light from the FOV. Then, we measured the dose dependence of the amount of emission light. Finally, the light distribution in the depth direction by the present method was compared to the depth dose distribution by the ionization chamber. The results are as follows. The angular range of FOV is 7.03 degrees +/- 0.21 degrees The dose dependence shows a good linearity. In other words, the amount of emission light is proportional to the absorbed dose. Both of the light and the dose distributions are proportional each other, and they show the same attenuation in the region deeper than 15 mm. From these results, we conclude that the present method can be used for the measurement of the depth dose distribution in matter.