A method is disclosed for controlling the delivery of radiation therapy to a tumor of a patient from one or more beams of ionizing radiation, to conform to a prescribed dosage of radiation for each of predetermined plural respectively shaped portions of the tumor according to the shape and other characteristics of the tumor. A radiation beam is selectively generated from different directions with respect to travel of the beam along a plurality of spatial paths including oscillating and arcuate movements. Parameters of the beam are calculated from conditions of distribution of a target dose, and the cross-section of the beam is adjusted so as to deliver the prescribed dosage of radiation to each of the respectively shaped portions of the tumor on which the beam impinges. The cross-section of the beam is constantly adjusted according to a predetermined area of the tumor which is to receive radiation therapy, and adjusted cross-sections of the beam are moved along selected individual ones of the spatial paths at least one time. Also, movements of the beam along individual spatial paths are split according to the plural portions of the tumor which are to receive different radiation doses. Travel of the beam is controlled along the selected individual paths so as to deliver radiation therapy within the prescribed dosage to each of the plural portions of the tumor in a minimum amount of time. In one embodiment, a micro multi-leaf collimator is placed between the beam and the tumor, and the collimator leaves are adjusted to change the cross-section of the beam impinging on a specified portion of the tumor according to the shape of the specified portion.