Abstract: The medial axis analysis of an object can be used to effectively guide and optimize radiosurgery treatment planning. In this method, a fast Euclidean medial axis transformation in three dimensions based on dynamic grassfire simulation and ridge extraction is presented. A ridge occurs when fire fronts collapse during grassfire propagation. Iso-contours(2D) or iso-surfaces(3D) can be obtained from dynamic grassfire transforms. They are locally smooth everywhere except at ridge locations. Ridges are detected by measuring local curvature at each point. This process is invariant under spatial translations and rotations. In radiosurgery treatment planning, optimal shots are only placed on the medial axis of the 3D target, which reduces optimization time and complexity.