Implementing Geant4 on GPU for medical applications

Monte Carlo simulation (MCS) plays a key role in medical applications, especially for emission tomography (ET) and radiotherapy (RT). Unfortunately MCS is also associated with long calculation times that prevent for using it in routine clinical practice. Actually, a solution based on the use of computer clusters to solve the intensive computational issues is not realistic within routine clinical environment. Recently graphics processing units (GPU) became in many domains a cheap solution for the acquisition of a high power computation. The objective of this work was to develop an efficient framework for the implementation of MCS on GPU architectures. Geant4 was used as the MCS engine for targeting medical imaging and radiotherapy applications. We propose the definition of a global strategy and associated structures for such a GPU based simulation. The different steps needed for a Geant4 simulation were implemented on GPU. The first validations have shown equivalence in the underlying photon physics processes between the Geant4 and the GPU codes. Based on these simplistic simulations, we are expecting a speedup factor of over 200 for a complete simulation in emission tomography or in radiotherapy dosimetry.