**2.2 Auger electrons**

Auger electrons are generated from suborbital transitions. They are typically very short-range emissions, of the order of 1–1000 nm, depending on their emission energy. Auger electron is intermediate (4–26 keV/μM) of LET [3, 22, 23]. These emissions could be highly cytotoxic if the RPT drug localizes within the cell nucleus [36, 37]. Bromine-77, indium-111, iodine-123, and iodine-125 are the most commonly used Auger electron emitters. In vitro studies had shown highly effective and specific tumor cell killing when they labeled with targeting vehicles that can localize these subcellular-range radiations close to cellular DNA [38, 39]. Human studies using locoregional administration showed promise regarding tumor cell incorporation of the Auger emitters [2]. However, Auger electron-emitter RPT has not been widely adopted yet. Besides, auger electron agents must be incorporated into the DNA, and their unfavorable pharmacokinetics might be the reasons for the lack of efficacy. Technological developments that could overcome those factors will interest RPT development [2].
