Abstract: In a radiation therapy device isodose curves are adjusted both by a movable plate that is controlled during the irradiation and by a non-movable filter body in the radiation path. The filter body is located in such a manner that it has decreasing absorbability in the opening direction of the plate. On the other hand, the plate produces a decreasing effective dose in its opening direction. by superimposing the two effects, it is possible to have the isodose curve in the object of irradiation rise or fall in the opening direction, so that a wide range of variation in the possible isodose curves is obtained, without having to change the fixed filter body.