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In heavy-ion radiotherapy, an accelerated beam is modified to realize a desired dose distribution in patients. The setup of the beam-modifying devices in the irradiation system is changed according to the patient, and it is important to check the depth dose distributions in the patient. In order to measure dose distributions realized by an irradiation system for heavy-ion radiotherapy, a multi-layer ionization chamber(MLIC) was developed. The MLIC consists of 64 ionization chambers, which are stacked mutually. The interval between each ionization chamber is about 4.1 mm water. There are signal and high voltage plates in the MILC, which are used as electrodes of the ionization chambers and phantom. Depth dose distribution from 5.09 mm to 261.92 mm water can be measured in about 30 seconds using this MLIC. Thus, it is possible to check beam quality in a short amount of time.

The feasibility of heavy ion computed tomography was investigated. A cylindrical polyethylene sample of 50 mm in diameter was used to establish the pencil beam scanning method and estimate the performance of the system. The sample was irradiated with carbon ions of 290 MeV/u and 400 MeV/u, and helium ions of 150 MeV/u. The residual beam energy was measured by a BGO scintillator in coincidence with two small plastic scintillators placed in front of and behind the sample to restrict the beam path and improve the spatial resolution in the CT reconstruction. The projection data were obtained by moving the sample only in the transverse direction because of its symmetrical structure. A Lucite sample with many holes of different diameters was used to demonstrate the spatial resolution of this CT system. From the reconstructed images the spatial resolution was estimated to be less than 2 mm under the conditions in this work. The electron density ratios of ethyl alcohol, water, and Lucite to polyethylene were obtained, which are in good agreement with the calculated values.