The Auger cascade and therapeutic applications of 125I

The Auger cascade consists of low-energy electrons which are emitted instead of characteristic X-rays when a vacancy in an inner electronic shell is filled. Such atomic vacancies are produced by orbital electron capture, internal conversion, and the photoelectric absorption of photons. The relative abundance of Auger electrons and their decrease with increasing atomic charge (Z) is explained. For iodine (Z = 53), about 30% of the energy is carried by Auger electrons and thus is dissipated locally. As the concept of average dose cannot always be applied to electron capture decay in cellular components, microdosimetric parameters, applied separately to each of the distinct types of low energy radiations involved, seem more appropriate. The development and applications of the relevant microdosimetric concepts are reviewed briefly. The approach explains the high biological effectiveness of 125I in different and unrelated systems, such as DNA molecules and the thyroid gland. The effects of 125I on thyroids of humans and experimental animals were compared to those produced by 131I by the Glasgow and the Beilinson groups. The experience of these teams in applying therapeutic doses of 125I in order to control thyrotoxic patients is evaluated.