title: Monte Carlo Evaluation of Auger Electron-Emitting Theranostic Radionuclides.
abstract: Several radionuclides used in medical imaging emit Auger electrons, which, depending on the targeting strategy, either may be exploited for therapeutic purposes or may contribute to an unintentional mean absorbed dose burden. In this study, the virtues of 12 Auger electron-emitting radionuclides were evaluated in terms of cellular S values in concentric and eccentric cell-nucleus arrangements and by comparing their dose-point kernels.
  
  
  METHODS
  The Monte Carlo code PENELOPE was used to transport the full particulate spectrum of (67)Ga, (80m)Br, (89)Zr, (90)Nb, (99m)Tc, (111)In, (117m)Sn, (119)Sb, (123)I, (125)I, (195m)Pt, and (201)Tl by means of event-by-event simulations. Cellular S values were calculated for varying cell and nucleus radii, and the effects of cell eccentricity on S values were evaluated. Dose-point kernels were determined up to 30 μm. Energy deposition at DNA scales was also compared with an α emitter, (223)Ra.
  
  
  RESULTS
  PENELOPE-determined S values were generally within 10% of MIRD values when the source and target regions strongly overlapped, that is, S(nucleus←nucleus) configurations, but greater differences were noted for S(nucleus←cytoplasm) and S(nucleus←cell surface) configurations. Cell eccentricity had the greatest effect when the nucleus was small, compared with the cell size, and when the radiation sources were on the cell surface. Dose-point kernels taken together with the energy spectra of the radionuclides can account for some of the differences in energy deposition patterns between the radionuclides. The energy deposition of most Auger electron emitters at DNA scales of 2 nm or less exceeded that of a monoenergetic 5.77-MeV α particle, but not for (223)Ra.
  
  
  CONCLUSIONS
  A single-cell dosimetric approach is required to evaluate the efficacy of individual radionuclides for theranostic purposes, taking cell geometry into account, with internalizing and noninternalizing targeting strategies.
authors.affiliation: Department of Oncology, CR-UK/MRC Oxford Institute for Radiation Oncology, University of Oxford, Oxford, United Kingdom Department of Biomedical Science, Tshwane University of Technology, Pretoria, South Africa nadia.falzone@oncology.ox.ac.uk.
authors.forename: Nadia
authors.initials: N
authors.lastname: Falzone
authors.affiliation: Facultat de Física (ECM and ICC), Universitat de Barcelona, Barcelona, Spain; and.
authors.forename: José M
authors.initials: JM
authors.lastname: Fernández-Varea
authors.affiliation: Physics Department, Royal Marsden NHSFT, Sutton, Surrey, United Kingdom.
authors.forename: Glenn
authors.initials: G
authors.lastname: Flux
authors.affiliation: Department of Oncology, CR-UK/MRC Oxford Institute for Radiation Oncology, University of Oxford, Oxford, United Kingdom.
authors.forename: Katherine A
authors.initials: KA
authors.lastname: Vallis
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000528:radiation effects; D003198:Computer Simulation; D006801:Humans; D018499:Linear Energy Transfer; D008954:Models, Biological; D015233:Models, Statistical; D009010:Monte Carlo Method; D009369:Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D011829:Radiation Dosage; D011868:Radioisotopes / Q000627:therapeutic use*; D011877:Radionuclide Imaging; D012542:Scattering, Radiation
chemical_list: D011868:Radioisotopes
keywords: Auger electron emitters; Monte Carlo simulation; S values; cellular dosimetry; dose point kernels; targeted radiotherapy; theranostics