A geometrically based method of treatment planning system for gamma knife stereosurgery optimization was developed to find the best number of shots

shot locations and collimator sizes for treatment planning. The problem is similar to the problem of filling an arbitrary 3D object with spheres--the goal is that small spheres fill the sharp corners and large spheres fill more open regions so that the total number of spheres is minimized while the target is fully filled. In our approach

firstly the distance map was generated

then the end points of medial axis of the object were detected directly from distance map. Choose one of the end points as the first shot position

the sphere centered at the shot was removed from the target volume. The distance map of the remainder of the target was generated

the location of one of the cross points whose distance to the up shot is shortest was chosen as the current shot

the sphere centered at the shot was removed from the target. The process is repeated until the maximum distance value of the remainder target was less than 80% of half of the minimum collimator size (4 mm). The above steps were repeated until all end points were processed

thus binary trees corresponding to each start shot can be got to denote all possible solutions

and one of the path of all binary trees which maximized the measure of our objective was chosen as the initial optimization solution consists of the start points

the measure considered both the coverage percentage of tumor and that of normal tissue

also the sensitive of the coved normal tissue of each shot. The experimental results showe that the approach is more effective and less time consumed than the existing methods.