Development of a simple control system for uniform proton dose distribution in a dual-ring double scattering method.

In proton radiotherapy with high focusing of irradiation on the tumour, it is important to obtain treatment beams with a highly uniform dose distribution. Uniform dose distribution in the clinical irradiation field can be obtained by the dual-ring double scattering method. This method is superior to the wobbler method, which uses electromagnetic deflection of the proton beams, because of the absence of the temporal structure of irradiation distribution. However, in the dual-ring double scattering method the condition of incident proton beams entering the scatter, especially the accuracy of the position of the incident proton beams with respect to the scatter, markedly affects the uniformity of the beam distribution in the irradiation field. In this study, to ensure the uniformity of dose distribution during treatment, we developed a control system equipped with an automatic fine adjustment of the beam axis and a mechanism for moving the second dual-ring scatter of the double scatters to the optimal position. Using this system, we achieved uniform dose distribution in the irradiation field during proton radiotherapy, with symmetry within +/-1% and flatness within 2%.