Monitoring Ion Beam Therapy With a Compton Camera: Simulation Studies of the Clinical Feasibility

As more and more particle therapy centers are being built world-wide, there is increasing need in treatment monitoring methods, ideally in real time. This article investigates the clinical applicability of a Compton camera design by means of Monte Carlo simulations. The Compton camera performance has been studied with the simulation of point-like source and beam irradiation of a polymethyl methacrylate (PMMA) phantom. The system absolute photon detection efficiency, measured via source irradiation, varies in the range [1, 4] $\times 10^{-4}$ with energy variations in the prompt gamma (PG) energy range and source position shifts with respect to the center of the camera. With proton and carbon beams impinging on a PMMA cylindrical phantom, the number of detected PG coincidences related to various beam time structure has been studied. Finally, the accuracy of the camera in identifying the dose profile fall-off position has been estimated and two different event reconstruction methods have been compared for this purpose; one based on analytical calculation with a line-cone technique, the second relying on an iterative maximum likelihood expectation maximization algorithm. Both methods showed the possibility to reconstruct the beam depth-dose profile and to retrieve the dose fall-off with millimeter precision on a spot basis.