Design and Simulation of a Passive-Scattering Nozzle in Proton Beam Radiotherapy

Proton beam radiotherapy is an emerging treatment tool for cancer. Its basic principle is to use a high-energy proton beam to deposit energy in a tumor to kill the cancer cells while sparing the surrounding healthy tissues. The therapeutic proton beam can be either a broad beam or a narrow beam. In this research, we mainly focused on the design and simulation of the broad beam produced by a passive double-scattering system in a treatment nozzle. The NEU codes package is a specialized design tool for a passive double-scattering system in proton beam radiotherapy. MCNPX is a general-purpose Monte Carlo radiation transport code. In this research, we used the NEU codes package to design a passive double-scattering system, and we used MCNPX to simulate the transport of protons in the nozzle and a water phantom. We used "mcnp_pstudy" script to create successive input files for different steps in a range modulation wheel for SOBP to overcome the difficulty that MCNPX cannot be used to simulate dynamic geometries. We used "merge_mctal" script, "gridconv" code, and "VB script embedded in Excel" to process the simulation results. We also invoked the plot command of MCNPX to draw the fluence and dose distributions in the water phantom in the form of two-dimensional curves or color contour plots. The simulation results, such as SOBP and transverse dose distribution from MCNPX are basically consistent with the expected results fulfilling the design aim. We concluded that NEU is a powerful design tool for a double-scattering system in a proton therapy nozzle, and MCNPX can be applied successfully in the field of proton beam radiotherapy