ABSTRACT
Passive scattering is a delivery technique in which scattering and range-shifting materials spread the proton beam. The proton beam that reaches the treatment room is monoenergetic and has a lateral spread of only a few millimeters. Clinical use of the proton beam requires both spreading of the beam to a useful uniform area in the lateral direction as well creating a uniform dose distribution in the depth direction. The main function of the treatment head, or nozzle , is shaping the proton beam into a clinically useful three-dimensional (3D) dose distribution. The number of peaks that is added proximally can be varied, varying the extent of the uniform region in depth. The simplest scattering system is a single, flat scatterer that spreads a small proton beam into a Gaussian-like profile. A better efficiency can be achieved by scattering more of the central protons to the outside and creating a flat profile.
