Three-dimensional LET calculations for treatment planning of proton therapy.

The clinical impact of a variable relative biological effectiveness (RBE) in proton therapy is still under investigation. As the RBE depends on the linear energy transfer (LET), we present a fast method for three-dimensional calculations of the dose-averaged LET. The corresponding algorithm, based on an analytical expression for the LET and on Monte Carlo simulations, accounts for tissue inhomogeneities and allows LET calculations for realistic treatment plans and patient geometries given by computed tomography (CT) data sets. LET distributions were calculated for different proton spot scanning techniques, in particular for the full three-dimensional (3D) modulation and for the distal edge tracking technique (DET). In the case of 3D modulation, a more homogeneous distribution of LET in the planning target volume was observed than for the DET. Consequently, the proposed method allows to assess the impact of potential variations in RBE for various dose delivery techniques.