A GPU-accelerated Monte Carlo code, RT2for coupled transport of photon, electron/positron, and neutron

Objective.This work aims to develop a graphics processing unit (GPU)-accelerated Monte Carlo code for the coupled transport of photon, electron/positron and neutron over a broad range of energies for medical applications.Approach.By separating the MC evolution of radiation into source, transport, and interaction kernels, the branch divergence was alleviated. The memory coalescence was achieved by vectorizing the access pattern in which the secondary particles were archived. To accelerate further particle tracking, ray-tracing hardware acceleration in the Nvidia OptiXTMframework was applied. For photon and electron/positron, the EGSnrc interaction modules were ported as a GPU-optimized configuration. For neutron, a group-wised transport based on NJOY21 preprocessed data was implemented. The developed code was validated against CPU-based FLUKA. Neutron, x-ray and electron beams incident on water and ICRP phantoms were simulated. The neutron energy group and the transport parameters of photon and electron were set to be the same in both codes. A single Nvidia RTX 4090 card was used in this code while all 20 threads of a single Intel Core i9-10900K node were used in FLUKA.Main results.The number of histories was set to ensure that statistical uncertainties lower than 2% for all voxels whose doses were larger than 20% of the maximum. In all cases, the dose differences in the voxels between the codes were within 2.5%. For photons and electrons, the developed code was 150-300 times faster than FLUKA in both geometries. For neutrons, the code was respectively 80 and 135 times faster in the water and ICRP phantoms than FLUKA.Significance.This study offers an appropriate solution for uncoalesced memory access and branch divergence commonly encountered in coupled MC transport on the GPU architecture. The formidable acceleration in computing times and accuracy shown in this study can promise a routine clinical use of MC simulations.

Review of photon interaction cross section data in the medical and biological context

The probability of a photon (x-ray, gamma-ray, bremsstrahlung, etc) of a given energy E undergoing absorption or scattering when traversing a layer of material Z can be expressed quantitatively in terms of a linear attenuation coefficient mu (cm(-1)). Since mu is dependent on the material's density, rho (g cm(-3)), which can be variable, the quantity usually tabulated is the mass attenuation coefficient mu/rho (cm2 g(-1)) in which the dependence on the density has been removed. Mu/rho, in turn, can be obtained as the sum of the different types of possible interactions of photons with atoms of the material. For photon energies below 1 MeV the major interaction processes to be considered are incoherent (Compton) scattering, coherent (Rayleigh) scattering and atomic photoeffect absorption. Above 1 MeV one must also include nuclear-field pair production and atomic-field (triplet) production, and above 5 MeV one in principle should include photonuclear absorption, although the latter is neglected in data tabulations up to the present time. This review includes a selective history of measurements and theory relating to mu/rho from the turn of the century up to the present time, and is intended to provide a basis for further calculations and critical tabulations of photon cross section data, particularly as required by users in radiation medicine and biology. The mass energy-absorption coefficient mu(en)/rho is also briefly discussed.