This book provided a design and simulation method for a passive-scattering nozzle in proton beam radiotherapy. The double-scattering system including the range modulation wheel in a proton treatment nozzle was designed by using the NEU code. The Monte Carlo particle-transport code MCNPX was used in predicting dose distributions in this book. In the passive-scattering treatment technique, a rotating range modulation wheel is used to generate a spread- out Bragg peak depth-dose distribution. This dynamic geometry cannot be modeled in the simulation of MCNPX. This book provided an application of the "replacing dynamic by static" method to solve this problem. A script "mcnp_pstudy," written in Perl programming language, was used to set up a series of MCNPX input files, each of which contains the parameters of a specified step of the wheel, and invoke MCNPX to simulate each problem successively. The other Perl script "merge_mctal" was used to merge all the separate simulation results into one file containing the simulation results that are equivalent to a "dynamic" simulation.