Microdosimetry-based relative biological effectiveness calculations for radiotherapeutic electron beams: a FLUKA-based study.

Based on FLUKA, the present study is aimed at calculating the microdosimetric distributions of electron beams (6, 12 and 18 MeV) for radiotherapy as a function of depth in water at a site-size of 1 μm using a tissue-equivalent proportional counter (TEPC). Using the calculated microdosimetric distributions, the depth-specific relative biological effectiveness (RBE) of electron beams used in radiotherapy is calculated based on the theory of dual radiation action (TDRA) and the microdosimetric kinetic model (MKM). The TDRA-based calculation shows the variation of RBE of an electron beam with the absorbed dose and depth in water. In this study, we compared the RBE values calculated based on the TDRA and MKM. The FLUKA-based microdosimetric distributions in water obtained using the pre-calculated electron fluence spectra resulted in an improvement in the computational efficiency by a factor of 110 when compared with a full simulation. Depending on the beam energy and depth of water, RBE was in the range 0.67-0.78. RBE at 10% survival of HSG tumor cells was 0.84, which was nearly independent of the depth and beam energy.