Improvement of depth dose distributions by addition of small doses from laterally incident beams

A three beam irradiation technique is described, in which two beams add a relatively small amount of dose to the depth dose distribution of another beam. The result is a nearly flat depth dose distribution on the interval from the surface to an arbitrarily chosen depth and a steep dose decrease to a lower dose level beyond that interval. Dose addition to a single photon beam depth dose is mainly performed with adding wedged beams. The volume in which dose homogeneity is obtained, is independently influenced by the field shape of the adding beams and the field shape of the primary beam. The final distribution from dose addition to high energy electron beams generally has an increased therapeutic range. The use of photon wedged beams is less suitable to simultaneously increase the therapeutic range and the steepness of the depth dose decrease in these cases. The properties of the derived dose distributions and the influence of some variables are discussed.