Consider an electron beam which is targeted towards some object with a certain depth. The concept of dose refers to the amount of energy expended by the electrons in collisions with atoms of the object. Naturally the electrons kinetic energy is maximal at the surface, hence this is where they can expend the most energy in collisions, and therefore the dose is maximal at the surface. Ionization on the other hand, refers to the charge that is liberated by ionizing radiation (the electron beam). If the energy expended on collisions is maximal at the surface, then shouldn't the amount of ionization also be maximal at the surface? The charge is liberated in these collisions after all.