A method for applying a radiation treatment with an arbitrary isodose profile is disclosed. The treatment apparatus has a radiation source which generates a radiation beam with an axis. The apparatus also includes a collimator having a plurality of independently movable plates disposed in the path of the radiation beam and oriented in a direction perpendicular to said beam axis. Each set has two movable plates. Control signals are generated. Two plates within a set of plates are actuated independently in orthogonal directions during the treatment, in response to the control signals, causing the beam to change in width. The intensity of the beam is changed as a function of the position of the plates to generate an arbitrary beam profile. Exemplary profiles which may be generated included rotationally symmetric parabolic profiles and flat profiles.