Monte Carlo calculation of 119Sb microscale absorbed dose using cascaded and averaged Auger electron spectra

Radionuclides decaying by electron capture or internal transition produce a large number of Auger electrons in a cascade that follows their radioactive decay. A shortlist of the most potent Auger electron-emitters has appeared in the literature including ^103mRh, ¹⁰³Pd, ¹¹¹In, ¹¹⁹Sb, ¹²³I, ¹²⁵I, ¹⁶⁵Er, and ¹⁹⁷Hg. Among them, ¹¹⁹Sb has been identified as the most potent for targeting micrometastases, yielding several tens of Auger electrons per decay with energies from a few eV up to 30 keV. In this paper, we recalculate Auger, Coster-Kronig, and super Coster-Kronig yields and transition probabilities as subshell-normalized relative transition probabilities and develop a new method to create radionuclide sources in TOPAS Monte Carlo, the code for which has been made publicly available. We then apply our method to encode the Auger electron spectra of ¹¹⁹Sb from MIRD RADTABS and EADL into TOPAS and calculate the absorbed dose to water volumes of radius 10 nm up to 10 μm, finding that the averaged MIRD Auger electron spectrum underestimates the absorbed dose by a factor of 20 to 50 on this scale. We show that this result is not isolated to ¹¹⁹Sb and conclude that either the cascaded MIRD or EADL spectrum should be used for accurate microscale dosimetry. We compare with results obtained using the built-in Geant4 Atomic Relaxation for ¹¹⁹Sb in TOPAS and find an unexpected continuum of low-energy electrons but no excess absorbed dose relative to either MIRD or EADL. We show that 119Sb does not produce more absorbed dose in microscale volumes than ^103mRh, ¹⁰³Pd, ¹¹¹In, ¹²³I, ¹²⁵I, ¹⁶⁵Er, or ¹⁹⁷Hg, warranting future microdosimetry calculations of RBE and DNA damage to understand whether ¹¹⁹Sb is the most potent Auger electron-emitter, as claimed in the literature.