A Monte Carlo study of energy deposition at the sub-cellular level for application to targeted radionuclide therapy with low-energy electron emitters

Optimizing targeted radionuclide therapy for patients with circulating malignant cells (e.g. blood-related cancers) or a micrometastatic spread requires quantification of various dosimetric parameters at the single-cell level. We present results on the energy deposition of monoenergetic electrons of initial energy from 100 eV to 20 keV – relevant to Auger emitting radionuclides – distributed either uniformly or at the surface of spherical volumes of radii from 10 nm to 1 lm which correspond to critical sub-cellular targets. Calculations have been carried out by our detailed-history Monte Carlo (MC) code which simulates event-by-event the complete slowing down (to 1 Ry) of both the primary and all subsequent generations of electrons, as well as, by the continuous-slowing-down-approximation (CSDA) using analytic range-energy relationships. The latter method has been adopted by the MIRD committee of the Society of Nuclear Medicine for dosimetry at the cellular level (>1 lm). Differences between the MC and CSDA results are up to 50% and are expected to be even larger at higher energies and/or smaller volumes. They are attributed to the deficiencies of the CSDA method associated with the neglect of straggling and d-ray transport. The results are particularly relevant to targeted radiotherapy at the genome level by Auger emitters.