A method for planning a radiation therapy treatment of a body represented by a volume of voxels determines a radiation dose matrix representing a spatial distribution of a radiation dose of beams of radiation irradiating the volume with homogeneous stopping power. The beams are collimated and are shifted copies of each other at each depth of the volume, and the spatial distribution is determined by calculating cumulative beam-axial doses in a single matrix-matrix multiplication and redistributing the cumulative beam-axial doses to all voxels in the volume using a convolution. Next, a set of correcting operations is applied to the radiation dose matrix to produce a cumulative voxel radiation dose of the volume. Each correcting operation is linear, independent from another correcting operation and has a transform.