Purpose: In some studies of radiotherapy enhancement with gold nanoparticles, cell kill per Gy exceeds that predicted by models considering only gross excess energy deposition due to nanoparticle interactions. It has been proposed that the fine structure of energy deposition around the nanoparticles leads to an explanation of excess cell kill, marking this fine structure as a surrogate for dose enhancement. Here we investigate the energy deposition around nanoparticles other than gold to identify possible dose enhancement opportunities.
Methods: Simulations were performed using the Geant4 Monte Carlo toolkit and Geant4-DNA extension. Simulation geometry consisted of a focused, mono-energetic 40keV photon beam impinging perpendicularly on a 10uM cube of water containing a single 20nm nanoparticle of varying composition. Experiments consisted of 10⁸ primary events and were repeated for statistical purposes. For each nano-material radial dose deposition was scored in concentric shells nanoparticle to a distance of 300nm.
Results: Interactions in the gold nanoparticle produced the largest number of secondary particles per ionization (21.0 + 0.29), followed by bismuth (13.7+ 0.49), iodine (13.2 + 0.30), and iron oxide (9.60 + 0.22). Normalized radial dose functions for gold and bismuth were qualitatively similar, displaying greater dose near the nanoparticle compared to iodine and Fe2O3. However, iodine produced a dose an order of magnitude higher than gold at a distance of 300nm, indicating a higher fractional dose due to low energy photons.
Conclusion: The qualitative similarity of the normalized radial dose functions indicates that the efficacy of nanoparticle enhanced radiotherapy may be most sensitive to bioconcentration and limited by potential cytotoxicity in normal tissues. The observed similarity between gold and bismuth radial dose functions suggests equivalent dynamics of biological effect, while the extended range of dose from iodine could allow for an effectiveness less dependent upon biological distribution of the nanoparticles within the cell.
Not Applicable / None Entered.