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 ...