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 pro ton beam of the therapy unit of the Paul Scherrer Institut (Switzerland) we re 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 tak en 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 distributio ns 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 assumptio n that clustered DNA damage is a relevant step of the process leading to ce ll inactivation. Experiments on clonogenic survival of V79 cells were perfo rmed at PSI and the results were compared with the simulations, showing ver y good agreement. Simulated and experimental results consistently confirm t hat the RBE along most of the SOBP is approximate to 1,2, whereas it increa ses dramatically at the distal part. The peak in biological effect is there fore shifted downstream from the physical dose peak, (C) 1999 Elsevier Scie nce B.V. All rights reserved.