Spot scanning using radioactive beams for heavy–ion radiotherapy (in Japanese)

In this thesis we describe a technique for determining the spatial distribution of beams at the Heavy Ion Medical Accelerator in Chiba (HIMAC). A narrow beam of ions will be moved to various positions to irradiate a three‐dimensional conformal field in a patient body. Since is a positron emitter, one can determine the spatial distribution of where the ions terminated with a PET scanner. The intensity of the beam is less than 1% of the beam. We used the more efficient spot scanning for the dose delivery. The spot beam of (346 MeV/n) is produced by a projectile‐fragmentation process of accelerated (430 MeV/n) beams in a Be target 51 mm in thickness. The depth and lateral size of the spot beam are chosen by selection of its momentum and focusing, respectively. In order to deliver the desired dose to the irradiation volume, the depth dose and lateral dose distribution of the spot beam were corrected by taking into consideration its wide momentum spread (2%). A spot beam consists of monochromatic beam fractions of various momenta, which lead to various ranges. Each beam fraction has a different lateral beam size because of an aberration of the lens system used in beam delivery. The dose distribution is measured by a multistrip parallel‐plate ionization chamber by changing the thickness of upstream PMMA slabs. The calculated dose distribution of the irradiation volume of in the PMMA phantom of was in good agreement (±0.2%) with measurements.