Comparing 2 Monte Carlo Systems in Use for Proton Therapy Research

Purpose: Several Monte Carlo transport codes are available for medical physics users. To ensure confidence in the accuracy of the codes, they must be continually cross-validated. This study provides comparisons between MC2 and Tool for Particle Simulation (TOPAS) simulations, that is, between medical physics applications for Monte Carlo N-Particle Transport Code (MCNPX) and Geant4. Materials and Methods: Monte Carlo simulations were repeated with 2 wrapper codes: TOPAS (based on Geant4) and MC2 (based on MCNPX). Simulations increased in geometrical complexity from a monoenergetic beam incident on a water phantom, to a monoenergetic beam incident on a water phantom with a bone or tissue slab at various depths, to a spread-out Bragg peak incident on a voxelized computed tomography (CT) geometry. The CT geometry cases consisted of head and neck tissue and lung tissue. The results of the simulations were compared with one another through dose or energy deposition profiles, r90 calculations, and γ-analyses. Results: Both codes gave very similar results with monoenergetic beams incident on a water phantom. Systematic differences were observed between MC2 and TOPAS simulations when using a lung or bone slab in a water phantom, particularly in the r90 values, where TOPAS consistently calculated r90 to be deeper by about 0.4%. When comparing the performance of the 2 codes in a CT geometry, the results were still very similar, exemplified by a 3-dimensional γ-analysis pass rate > 95% at the 2%–2-mm criterion for tissues from both head and neck and lung. Conclusion: Differences between TOPAS and MC2 were minor and were not considered clinically relevant.

Gasdynamic description of electron-beam flying-off in a plasma

The propagation of one and two electron beams in a plasma is considered in the case when the source is time-dependent. When the electron-plasmon interaction time is assumed to be much less than the electron flying-off time, the main gasdynamic equations are obtained on the basis of the quasilinear theory of weak turbulence. The solution for a monoenergetic beam is a beam–plasma structure moving with constant velocity. In general, two beams propagate as two beam–plasma structures with constant velocities. However, the second electron beam influences the flying-off dynamics of both beams. This peculiar interaction modifies the shapes of the structures.

The Repulsive Part of the Potential Between Hg Atoms and Light Atomic Projectiles

Interatomic potentials are studied by means of a crossed beam apparatus consisting of a target chamber in which a monoenergetic beam of H, He, B, and N ions of energies E < 70 keV interacts with a thermal beam of Hg atoms. Angular distributions for the scattering of all charge states of 1H, 4He, 11B, and 14N are measured in the range of 17° < θL < 60°. All measured cross sections are in good agreement with the theory for scattering in a Thomas–Fermi potential to which Lindhard's theoretical estimates are shown to be quite accurate approximations. A Padé approximant is derived for the Thomas–Fermi potential and is used to calculate tables of Thomas–Fermi scattering parameters.