THE EFFECT OF INHOMOGENEITIES ON DOSE DISTRIBUTIONS OF HIGH‐ENERGY ELECTRONS

The evaluation of dose distributions of high-energy electrons behind and near small areas of high density or void causes considerable problems for the therapy physicists. The same can indeed be said about the distribution in areas of such a complicated structure as the head and neck, including the bone structures and voids a t the sinus maxillaris and sinus sphenoidalis. We have started our studies of the effect of inhomogeneities with these special cases in mind, whereas the influence of large areas of lowor high-density tissue, such as the lungs and the regular bone layer of the skull, have not been covered by our investigations. Those problems, however, have been treated in other papers in this monograph. In the special situations of our study, the scattering of the electrons plays a dominant role. Let me therefore start with a fairly simple consideration of the disturbance of a beam of strongly scattered particles by obstacles placed in the beam. We may assume that the particles are only scattered elastically, but not absorbed. We consider first the shielding effect of a wall introduced in the beam of such radiation. We may take as an example a wall of lead used as a shield in a very broad, essentially parallel, beam of neutrons (FIGURE 1). If the lateral dimensions of the wall are just large enough to cover a small object to be shielded, the reduction of the intensity of the radiation may be considerable in the shadow of the wall. Although no particles are absorbed and no reduction of their energy takes place in the elastic collisions with the atoms of the obstacle, their number in the shadow of the wall is still smaller. The reason is that they are scattered out from the parallel beam, i.e., they are removed from the parallel beam to a diverging bundle of rays. One may use the concept of a removal cross section for a quantitative evaluation of the effect. It goes without saying that the particles that were removed will contribute t o the intensity a t both sides of the shielded objects. What happens when the lateral dimensions of the wall are increased? Particles are now removed from a part of the beam, which originally passed the object a t some distance from it. Consequently, some of these will now be scattered into the position of the object, where they will cause a n increase of the intensity. If the lateral dimensions of the wall are increased to infinity, the wall will finally have n o