[Improvement of dose distributions at NIRS's 70 MeV proton eye treatment beam course]

In order to improve dose distributions at NIRS's 70 MeV proton eye treatment beam course, we introduced finer bar ridge filters, and examined the effects of range compensators. The pitch of new bar ridge filters was 5mm in contrast to 15mm pitch of old ones. A NC-machine recently available enabled this refinement. The spread out Bragg peak (SOBP) widths were 10, 15, 20 and 30 mm. The new ridge filters improved the field uniformity considerably. In the ridge filter design we assumed parallel beam condition in which the mono-energetic proton should proceed in parallel with the central axis, and bar ridges only changes the proton ranges. We searched empirically for the optimum wobbler radius in view of field flatness and depth dose distribution. Range shifter and compensator did not affect the field flatness and depth dose distribution at the optimum condition thus searched. We measured dose distribution in a phantom using a compensator of stairs-shape, which fairly modulated the beam. A 50% isodose line almost coincided with the compensator shape, and these results suggested that improvements of dose distributions should be possible using compensators. However width between 50% and 80% isodose lines depended on the thickness of phantom. This might be due to scattering in the compensator and suggests that it is necessary to calculate dose distribution taking account of such effects.