TOXICITY AND THERAPEUTIC EFFECTS OF LOW-ENERGY ELECTRONS

The biologic consequences of tissue-incorporated radionuclides that de cay by electron capture and internal conversion are often unpredictabl e because of their strong dependence on the microscopic distribution o f energy. These radionuclides differ from those that decay by beta--em issions in that nuclear processes leave primary vacancies in the inner electronic shells of the daughter atoms. The complex series of vacanc y cascades that follow are composed of both radiative and nonradiative (Auger) transitions and result in the copious emission of low-energy electrons that dissipate their energy typically within nanometer dista nces from the decay site. The highly localized nature of the energy ab sorbed may lead to severe damage of molecular structures in the immedi ate vicinity of the decaying atom. If these intracellular structures a re essential for the viability of the cell, lethal effects can follow, depending on the ability of the cell to repair such damage. According ly, both experimental and theoretical approaches have been used to ass ess the radiotoxicity and therapeutic potential of these radionuclides . Studies at the molecular, cellular, and tissue levels have demonstra ted that the major factors contributing to observed biologic effects i nclude the intracellular concentration and distribution of the radionu clide in the mammalian cell, its proximity to nuclear DNA, the spectra of its Auger electrons and their pattems of energy deposition. This p aper reviews and discusses some of the recent in-vitro and in-vivo fin dings in this field.