A processing device for determining a position and/or orientation for a grid template with respect to a human or animal body in a medical interventional procedure, wherein said grid template comprises a plurality of holes that define a corresponding plurality of grid hole trajectories and wherein the grid template is adapted for supporting and guiding at least one interventional tool along such grid hole trajectory when inserted through at least one of said holes into the body in said interventional procedure, the processing device comprising: - an anatomical spatial information processing unit for receiving and/or processing data representative of at least one target spatial volume in the body; - a grid position sampler for generating a plurality of candidate positions and/or orientations of the grid template with respect to the at least one target spatial volume in the body; - a quality calculator for calculating, for each candidate position and/or orientation of the plurality of candidate positions and/or orientations, at least one quality metric representative of the suitability of the candidate position of the grid template for the interventional procedure, and - a position selector for selecting a position and/or orientation from the plurality of candidate positions and/or orientations based on the at least one quality metric, wherein said quality calculator is adapted for determining, for each candidate position and/or orientation, a spatial relationship between each grid hole trajectory of the grid template, when positioned in accordance with the candidate position and/or orientation, and said at least one target volume, wherein said at least one quality metric comprises a value indicative of a treatment efficacy measure of the medical interventional procedure when constrained by said determined spatial relationship, characterised in that said treatment efficacy measure being representative of a radiation dose or ablation effect received in said at least one target spatial volume when one or more radiation sources or ablators are positioned along said grid hole trajectories. - an anatomical spatial information processing unit for receiving and/or processing data representative of at least one target spatial volume in the body; - a grid position sampler for generating a plurality of candidate positions and/or orientations of the grid template with respect to the at least one target spatial volume in the body; - a quality calculator for calculating, for each candidate position and/or orientation of the plurality of candidate positions and/or orientations, at least one quality metric representative of the suitability of the candidate position of the grid template for the interventional procedure, and - a position selector for selecting a position and/or orientation from the plurality of candidate positions and/or orientations based on the at least one quality metric, wherein said quality calculator is adapted for determining, for each candidate position and/or orientation, a spatial relationship between each grid hole trajectory of the grid template, when positioned in accordance with the candidate position and/or orientation, and said at least one target volume, wherein said at least one quality metric comprises a value indicative of a treatment efficacy measure of the medical interventional procedure when constrained by said determined spatial relationship, characterised in that said treatment efficacy measure being representative of a radiation dose or ablation effect received in said at least one target spatial volume when one or more radiation sources or ablators are positioned along said grid hole trajectories. wherein said quality calculator is adapted for determining, for each candidate position and/or orientation, a spatial relationship between each grid hole trajectory of the grid template, when positioned in accordance with the candidate position and/or orientation, and said at least one target volume, wherein said at least one quality metric comprises a value indicative of a treatment efficacy measure of the medical interventional procedure when constrained by said determined spatial relationship, characterised in that said treatment efficacy measure being representative of a radiation dose or ablation effect received in said at least one target spatial volume when one or more radiation sources or ablators are positioned along said grid hole trajectories.