To implement RBE calculations in treatment planning systems based on the Microdosimetric Kinetic Model (MKM) upon analytical calculations of dose-mean lineal energy (yD). MKM relies on the patterns of energy deposition in sub-nuclear structures called domains, whose radii are cell-specific and need to be determined.
Methods and material
The radius of a domain (rd) can be determined from the linear-quadratic (LQ) curves from clonogenic experiments for different cell lines exposed to X-ray and proton beams with known yD. In this work, LQ parameters for two different human lung cell lines (H1299 and H460) are used, and yD among cells is calculated through an analytical algorithm. Once rd is determined, MKM-based calculations of RBE are implemented in a treatment planning system (TPS). Results are compared to those produced by phenomenological models of RBE, such as Carabe and McNamara.
Differences between model-based predictions and experimentally determined RBE are analyzed for yD=5 keV/μm. For the H1299 line, mean differences in RBE are 0.13, −0.29 and −0.27 for our MKM-based calculation, Carabe and McNamara models, respectively. For the H460 line, differences become −0.044, −0.091 and −0.048, respectively. RBE is computed for these models in a simple plan, showing MKM the best agreement with the experimentally obtained RBE, keeping deviations below 0.08.
Microdosimetry calculations at the TPS-level provide tools to improve predictions of RBE using the MKM with actual values of yD instead of LET. The radius of the characteristic domain needs to be determined to tailor the RBE prediction for each cell or tissue.