Study of the Clinical Proton Beam Relative Biological Effectiveness at the JINR Phasotron, Dubna

Proton clinical beams contain particles with high linear energy transfer (LET). Secondary heavy charged particles produced from nuclear interactions and degraded protons at the Bragg peak region are particles with high LET. These particles could enhance the Relative Biological Effectiveness (RBE) of the proton beam. We have carried out two radiobiological experiments to investigate the RBE of 150 MeV clinical proton beam. The irradiation of the Chinese Hamster V79 cells were performed at two points of the depth-dose distribution - at the beam entrance and at the Bragg peak. The contribution of the high LET particles to dosimetric and microdosimetric characteristics in the various depth of proton beam was also experimentally studied using the CR-39 track etched detectors. The LET spectra between 10 and 700 keV/{\mu}m were measured by means of track detectors and the automatic optical image analyzer LUCIA-II. The relative contribution of the high LET particles to ab! sorbed dose increases from several per cent at the beam entrance to several tens of per cent at the Bragg peak region. The value of the RBE increases from 1.02 at the beam entrance to 1.23 at the Bragg peak. The values of RBE obtained by two independent methods are in a good agreement