S1982
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Digital Poster 4188
Parameter-specific doses as a way to handle ambiguous biologically weighted doses in the RayStation treatment planning system Elias Coniavitis 1 , Rasmus Bokrantz 2 , Jakob Ödén 2 , Daniel Simon Colomar 1 , Gustav Lidberg 1 , Lars Glimelius 1 1 Physics Department, RaySearch Laboratories, Stockholm, Sweden. 2 Research Department, RaySearch Laboratories, Stockholm, Sweden Purpose/Objective: Radiotherapy dose computation is performed on a discretized voxel grid, where each voxel is assumed to consist of a single material. For biologically weighted dose computation each voxel must also have a single set of biological parameters (e.g. α / β for EQD2; RBE- model parameters for RBE-weighted dose). Typically these are specified per ROI, and voxels are uniquely associated to a single ROI during parameter assignment. This unambiguous assignment is necessary for dose computation, resulting in a single biologically weighted dose for each voxel.Biologically weighted dose distributions represented using unique voxel values can, however, have artificial cold- or hot- spots in ROI-level dose statistics and DVHs, as illustrated in Figure 1 showing EQD2 for a prostate treatment. Discrepancies arise because voxels belonging to multiple ROIs (e.g. on ROI boundaries or overlap regions) are assigned the parameters of one ROI (that ROI is “prioritized”), leading to incorrect evaluation for the other(s). These inaccuracies are fundamentally due to volume discretization—in reality some cells inside the voxel volume are of one kind, and some of another. We present a solution for this, implemented in RayStation.
Material/Methods: We define parameter-specific dose as a biologically weighted dose associated with specific biological parameters, computed by assigning these parameters uniformly to all voxels. Thus the same voxel has different dose values for different parameter-specific doses. RayStation computes parameter-specific doses for every distinct set of biological parameters assigned to any ROI. When evaluating dose statistics, DVHs, clinical goals and prescription values for a given ROI, the parameter-specific dose corresponding to that ROI’s parameters is used. Thus, voxels enter these computations with dose corresponding to the ROI’s parameters, rather than potentially those of adjacent or overlapping ROIs. Results: Parameter-specific doses are most significant for situations with multiple sets of substantially different biological parameters. Examples include EQD2 of neighboring structures with different α / β , and RBE- weighted doses for BNCT with different boron uptake in adjacent tissues. Table 1 quantifies EQD2 dose- statistics for parameter-specific doses and doses with unique voxel values, for different fractionation schedules. Considerable discrepancies arise in the hypofractionation regimens for the PTV D98 for OAR- prioritized unique voxel values, and for the OARs D1cc for PTV-prioritized assignment of ambiguous voxels.
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