Applications and possible generalisations of a method tested at the OPTIS facility, for analysing physical and radiobiological properties of therapeutic proton beams

A method developed to analyse the physical and biophysical properties of therapeutic proton beams and more in general to estimate mixed field effects, previously tested at the OPTIS facility (Paul Scherrer Institut, Switzerland), was applied to a 160 MeV proton beam, modulated to obtain a therapeutic Spread-Out Bragg Peak (SOBP). The spatial distribution of dose in a water phantom was calculated with the condensed-history MC code FLUKA. The yield of DNA Complex Lesions (CL, which were found to play a relevant role in cell inactivation and other radiobiological endpoints), reflecting the radiation clustering properties, had been previously calculated using an event-by-event code and integrated in FLUKA. The code this way obtained provides the spatial distribution of CL per cell, which can be regarded as a ‘biological dose’. The contribution of the secondary hadrons to the biological dose was found to be much more relevant with respect to the case of the physical dose and therefore cannot be neglected. An RBE of approx 1.2 was found along the plateau and in most of the SOBP (due to secondary hadrons), with a sharp increase in the distal part (due to the presence of low energy protons). The ‘biological peak’ resulted to be shifted towards larger depths with respect to the physical peak. The results are in good agreement with experimental data reported in the literature.