1999 Volume 2 Issue 3 Pages 238-242
In an irradiation with a proton beam, the relatively small lateral scattering is one of the physical characteristics, but primary proton beams and secondary charged-particles participate in a evaluation of the absorbed dose. We performed the dosimetry of proton beams in water using plane parallel ionization cham-ber. The maximum energy of the proton beams used in the present study is 70 MeV pro-duced from an isochronous cyclotron in the National Institute of Radiological Sciences. The output factor for some range shifter was extrapolated to zero field size and the influence of lateral secondary charged-particles was excluded. The dose ratio was defined as relative dose for field sizes. The ratio of average dose for secondary charged-particles in some range shifter was calculated by the dose ratio using the fluence of primary proton beams and the share of scatter dose. From the results, the ratio of average dose for secondary charged-particles increase with the decrease of field size and is changed by the depth of Spread-Out-Bragg-Peak(SOBP). There was a possibility that the secondary charged-particles generated by collimator influ-enced the absorbed dose.
