Single cell S-value calculations of a stochastic cascade model for Auger-electron emitting radionuclides

Objectives Auger-electron (AE) emitting radionuclides could be exploited for therapeutic purposes due to the high local energy deposition by low-energy Auger electrons or may deliver an unintentional mean absorbed dose burden when used as medical imaging agents. Emission spectra of AE emitting radionuclides are essential for dosimetric calculations to quantify the biological damage to the target. The BrIccEmis code, a Monte Carlo model of the Auger cascade is used here to provide radiation spectra data for 14 AE emitting radionuclides. The aim of this study is to compare MIRD single cell S-values with S-values derived from BrIccEmis¹ generated radiation spectra.

Methods Radiation spectra of ⁶⁷Ga, ^80mBr, ⁸⁹Zr, ⁹⁰Nb, ^99mTc, ¹¹¹In, ^117mSn, ¹¹⁹Sb, ¹²³I, ¹²⁴I, ¹²⁵I, ¹³⁵La, ^195mPt and ²⁰¹Tl was taken from the unabridged nuclear decay data of the MIRD RADTABS program² and also generated using the BrIccEmis code. Cellular S-values, that is, the mean absorbed dose to the target region (T) per unit cumulated activity in the source region (S), were calculated from an event-by-event MC simulation of the complete radiative and non-radiative spectra of each radionuclide with the general-purpose Monte Carlo code PENELOPE³. Taking the nucleus as the target, simulations were run assuming uniformly distributed activity in the nucleus (N←N), in the cytoplasm (N←Cy) or on the cell surface (N←CS).

Results Auger yields from MIRD are consistently higher than the values calculated using the BrIccEmis code. The methodology used to produce data in MIRD did not account for variations in binding energies during atomic relaxation, thus overestimated the intensity of low-energy electrons. BrIccEmiss determined S-values were generally lower compared with MIRD S-values with the greatest differences noted when the source is far from the target region, i.e. for the N←Cy and N←CS configurations.

Conclusions Realistic modelling of atomic relaxation following nuclear decay is essential for producing the radiation spectra needed for the calculations of S-values. ¹ B Lee et al. Comp Math Meth Med 2012; 651475. ² KF Eckerman and A Endo, MIRD Radionuclide Data and Decay Schemes 2008. ³F Salvat, PENELOPE-2011; NEA 6416 $$graphic_761DF89D-C03A-471D-9FEB-E6A9133890F0$$