1. A method for dose planning for a radiation therapy system, the radiation therapy system comprising a radiation therapy unit having a fixed radiation focus point, wherein a spatial dose distribution surrounding the focus point can be changed by adjusting collimator settings, said collimator having a plurality of collimator passage inlets directing radiation emanating from radioactive sources of a source carrier arrangement of the therapy system to said focus point, said method comprising: a) obtaining a target volume of a region of a patient to be treated during a treatment of a patient in a radiation therapy unit, said target volume being modeled as a three-dimensional voxel representation; b) selecting an isodose level for a planned treatment; c) determining shots to be delivered during said treatment, each shot being modelled by a spatial dose volume distribution of radiation represented by a three-dimensional voxel representation, the shape of said spatial distribution depending on a specific collimator setting and said selected isodose level; d) selecting a first layer of shots in a decreasing volume order for filling a first space immediately adjacent to an outer boundary of the target volume represented by the three-dimensional voxel representation, the first space extending along the entire outer boundary of the target volume, comprising the sub-steps of: d1) initiating a position search for a specific shot to find an accepted position within the first space of said target volume; d2) iteratively mapping different positions for the voxel representation of a current shot in the first space of the voxel representation of the target volume; and d3) checking whether predetermined conditions for each specific position are satisfied, comprising the sub-steps of: d3i) if the predetermined conditions are satisfied for a specific position, defining that position as an accepted position and continuing with steps d1)-d3) for a shot having a similar shape; and d3ii) if the predetermined conditions are not satisfied for any position within the target volume, continuing with steps d1)-d3) for a subsequent shot in said decreasing volume order; e) selecting a second layer of shots in a decreasing volume order for filling a second space immediately adjacent to the first space in a radially inward direction, an outer boundary of the second space being defined by an inner boundary of the first layer of shots, comprising the sub-steps of: e1) initiating a position search for a specific shot to find an accepted position within the second space of said target volume; e2) iteratively mapping different positions for the voxel representation of a current shot in the second space of the voxel representation of the target volume; and e3) checking whether predetermined conditions for each specific position are satisfied, comprising the sub-steps of: e3i) if the predetermined conditions are satisfied for a specific position, defining that position as an accepted position and continuing with steps e1)-e3) for a shot having a similar shape; or e3ii) if the predetermined conditions are not satisfied for any position within the target volume, continuing with steps e1)-e3) for a subsequent shot in said decreasing volume order, wherein the step e) of selecting the second layer of shots is performed after no more shots can be placed within the first space through the sub-steps of d1)-d3).