The method involves defining control planes for controlling dosage of rays, respectively, where each plane divides an objective volume (4) into two sub volumes. Isolated computing of local partial radiation doses of all rays assigned to the planes is performed for one side of the planes so that sum of local partial radiation doses of the rays assigned to the side of the planes provides sub volume-total radiation dose and sum of local partial radiation doses of the remaining rays assigned to the planes provides difference between the sub volume-total radiation dose and total radiation dose. An independent claim is also included for a radiation therapy system.
