RBE variations along the Bragg curve of a 200 MeV proton beam

Purpose: A lack of strong radiobiological datasets has resulted in the clinical adoption of a fixed, generic relative biological effectiveness (RBE) of 1.1 in current proton therapy (PT). However, in the distal area of the spread-out Bragg peak (SOBP), the RBE is certainly higher than 1.1 due to the rapid decrease in proton energy, resulting in an increased linear energy transfer (LET). Therefore, the RBE was quantified at different positions of the depth-dose profile for the 200 MeV clinical proton beam at iThemba LABS. Material and methods: V-79 fibroblasts were irradiated as monolayers at the plateau, proximal, middle and distal positions, as well as in the distal edge (32% of the maximum dose) of a 7 cm SOBP. At the same time, V-79 cells were also irradiated with 60Co γ-rays as reference radiation. α and β values were determined from the cell survival curves and the 95% confidence ellipses of these covariant parameters were compared in the analysis. Mean inactivation dose (MID) values were calculated and used for the RBE calculations. Results: A large overlap in the 95% confidence ellipses was observed for proton plateau and 60Co γ-rays, so there is no statistical significant difference in radiation quality. The MID decreases with depth from 3.65 Gy at the entrance plateau, to 3.52 Gy, 3.40 Gy and 3.15 Gy for the proximal, middle and distal position along the SOBP respectively. Since the entrance plateau results were not significantly different from 60Co γ-rays, RBE was calculated based on the plateau MID as a reference. This resulted in RBE values of 1.04, 1.07 and 1.16 for the proximal, middle and distal positions respectively. Furthermore, a clear separation was observed between the 95% confidence ellipses for the three positions in the SOBP. An RBE increase up to 1.46 was determined for the distal fall-off position. Conclusion: The results obtained in the current study with V-79 fibroblast cells confirm the expected increase in RBE along the proton Bragg curve