A therapy system (20) determines an aperture shape (102,152) based on a diameter (106, 156) of a milling bit. The system (20) includes at least one processor (68, 70) programmed to receive an aperture shape (102, 152) of a treatment plan for a patient and a diameter (106, 156) of a milling bit. The aperture shape (102, 152) is regularized with respect to the diameter (106, 156) of the milling bit by at least one of over segmenting (150) a first portion of the aperture shape (102, 152) based on the diameter (106, 156) of the milling bit and under segmenting (100) a second portion of the aperture shape (102, 152) based on the diameter (106, 156) of the milling bit.
