Dual-ring double scattering method for proton beam spreading

A dual-ring double scattering method has been proposed to obtain a large uniform beam field in the proton rotating gantry used for cancer treatment. This method makes it possible to reduce the distance for beam field formation and to use the larger part of the beam. A flat beam field can be formed at a position on the patient by scattering the beam more strongly at the inner part of the dual-ring second scatterer than the outer part. Using this method we can form the beam field of 20 cm in diameter with the uniformity of ±1.7% p-p using the first scatterer made of 2.6-mm-thick Pb and the second dual-ring scatterer (inner part: 4.6-mm-thick Pb; outer part: 12.6-mm-thick Al) with an additional 0.7-mm-thick iron sheet for the 230 MeV incident pencil-like proton beam at the distance of 3.000 m. It was found that precise beam alignment (typically 1 mrad) on the center of the second scatterer is required to obtain the uniform beam field. (author)