The influence of the beam modulation technique on dose and RBE in proton radiation therapy.

Because a dose can be described as fluence times LET, it is evident that, in a mixed radiation field, similar doses can be achieved with different particle energy distributions. Isodose contours are iso-effect contours only if the energy spectra of the accompanying particles remain constant. Under this condition, the beam delivery technique used to build a spread-out Bragg peak (SOBP) can influence the relative biological effectiveness (RBE). We investigated the influence of the beam modulation method on the dose distribution and, taking into account the respective RBEs, on the biological dose distribution. For this, we first performed proton transport calculations in order to obtain the dose and the proton energy spectra at a given depth. Secondly, RBE values were calculated using the microdosimetric response function and the track structure model for two biological end points. We found an increasing RBE with depth within the SOBP. The higher the energy used for modulation the lower the average LET and the RBE and the higher the proton fluence. The RBE for an active beam modulation system behaves like the respective RBE of a passive system with similar initial beam energy.