  There is considerable interest in the use of heavy atom nanoparticles as
theranostic contrast agents due to their high radiation cross-section compared
to soft tissue. However, published studies have primarily focused on
applications of gold nanoparticles. This study applies Monte Carlo radiation
transport modelling using Geant4 to evaluate the macro- and micro-scale
radiation dose enhancement following X-ray irradiation with both imaging and
therapeutic energies on nanoparticles consisting of stable elements heavier
than silicon. An approach based on the Local Effect Model was also used to
assess potential biological impacts. While macroscopic dose enhancement is well
predicted by simple absorption cross-sections, nanoscale dose deposition has a
much more complex dependency on atomic number, with local maxima around
germanium (Z=32) and gadolinium (Z=64), driven by variations in secondary Auger
electron spectra, which translate into significant variations in biological
effectiveness. These differences may provide a valuable tool for predicting and
elucidating fundamental mechanisms of these agents as they move towards
clinical application.
