The use of a single central axis causes the divergence of radiation at the abutting boundaries to be parallel, even in cases for which the abutting boundary does not coincide with the beam central axis. Lateral diffusion of scattered electrons from an area treated with one energy into the neighboring area is reciprocated to provide a smooth change from the dose at4 any given depth near the center of one area to that near the center of the other. This allows treatment of deep levels of tissue in one portion of a field, while sparing deep level structures in an adjacent portion of the field.