The possible radiobiological impact of dose rate on Leksell Gamma Knife treatments has sometimes been discussed. In a normal treatment plan with Leksell Gamma Knife® there is a variation in dose of about a factor 2 within a target. However, there is in general a larger variation in dose rate especially if there are many isocenters in the plan. Dose rate histograms quantify the distribution of dose rates of the voxels in the target volume but the dose rate history of each voxel gives the possibility to localize regions where the dose rate has been consistently low.

     The purpose of the study is therefore to extract dose rate history from clinical treatment plans and investigate, by using biological models of cell survival incorporating dose rate effects, if there is a correlation to the radiobiological response.

      

     Several clinical treatment plans have been studied and for each of them dose rate histograms for each isocenter including voxels in the target and close to the target are generated. The dose rate history of each voxel is also derived and time spent in changing treatment positions or changing the collimator – when the dose rate is zero – is incorporated. Different biological models of cell survival are applied to the dose rate history of each voxel to determine the cell survival probability. For each target, plan and biological model, iso-biological effect surfaces are drawn and compared to iso-dose surfaces. Sensitivity of the cell survival probability to variations in the biological parameters is also analyzed.

     The suggested method makes it possible to discriminate plans with similar dosimetric metrics (coverage, selectivity, dose gradients, etc) but with different dose rate histograms and different dose rate histories.

     This is a retrospective study.

     There are some indications that dose rate may affect the radiobiological response.

     However, the uncertainty in the biological modeling is large making definite conclusions uncertain.