A proton beam therapy system was installed at the cyclotron facility of National Institute of Radiological Science. The system consists of a beam port for horizontal proton beam therapy, a spot scanning control apparatus, and a proton therapy irradiation system. The beam in monitored by three monitoring ionization chambers. The spot scanning is controlled by a TOSBAC-40 C computer. The irradiation system comprises a range modulator, a multi-leaf collimator, and applicator, and a position-defining light source. The scanning of beam spot is made by a horizontally and a vertically deflecting electromagnets. The scanning speed at the irradiation position is about 1 cm/m sec. The uniformity of irradiation field is kept for the domain of 18 cm squares. The depth distribution of irradiation dose in water was investigated, and the dose distribution was uniform to the depth of 3.6 cm. This spot scanning system can make the irregular field irradiation, which can be made by the TOSBAC-40 C computer. The trial use of the system has been made since October, 1979. (Kato, T.). Kouchi,-Kiyomitsu; Kanai,-Tatsuaki; Matsuzawa,-Hideo (National Inst. of Radiological Sciences, Chiba (Japan)); Inada,-Tetsuo.
