Physical and biophysical characteristics of a fully modulated 72 MeV therapeutic proton beam: model predictions and experimental data.

The physical and radiobiological features of the fully-modulated 72 MeV proton beam of the therapy unit of the Paul Scherrer Institut (Switzerland) were analyzed in deep detail by adopting both an experimental and theoretical approach. The spatial distribution of the physical dose was calculated by using the FLUKA MC transport code; the role of nuclear interactions was taken into account and the geometry of the apparatus was faithfully reproduced. The contributions of the various beam components were analysed separately, The simulation results were compared with measured depth-dose distributions and very good agreement was found. The depth-dependence of cell survival along the completely spread-out Bragg peak (SOBP) was simulated with a biophysical model, based on the assumption that clustered DNA damage is a relevant step of the process leading to cell inactivation. Experiments on clonogenic survival of V79 cells were performed at PSI and the results were compared with the simulations, showing very good agreement. Simulated and experimental results consistently confirm that the RBE along most of the SOBP is approximate to 1,2, whereas it increases dramatically at the distal part. The peak in biological effect is therefore shifted downstream from the physical dose peak,