Broadband and Monochromatic X-ray Irradiation of Platinum: Monte Carlo Simulations for Dose Enhancement Factors and Resonant Theranostics

Introduction: X-ray irradiation of heavy-element (high-Z) nanomaterials as radiosensitizing agents should be extremely efficient for therapy and diagnostics (theranostics). However, broadband radiation from conventional X-ray sources results in unnecessary radiation exposure. Monochromatic X-ray sources are expected to be considerably more efficient. We present numerical simulations with platinum as an agent for killing cancerous cells via increased linear-energy-transfer (LET) and dose enhancement. We also describe a simple device for broadband-to-monochromatic (B2M) conversion. Materials and Methods: Monte Carlo simulations for X-ray energy absorption and dose deposition in tissues were carried out using the Geant4 code for 100 kV, 170 kV and 6 MV broadband X-ray sources. Dose enhancements were calculated using platinum as the radiosensitizing agent. A broadband 100 kV source is used to produce monochromatic fluxes in the K lines from a zirconium target. Results and Discussion: It is shown that X-ray energies in the range below 100 keV are most efficient in achieving both the required penetrative depths and deposition of energy. We confirm previous results that it is only the low-energy component around 100 keV from the 6 MV Linac that is most effective in dose-enhanced cell killing. Monochromatic x-rays such as obtained from B2M conversion should be most effective for theranostics, provided they can be tuned to resonant energies in the targeted material. Keywords— X-rays, monochromatic, broadband, platinum,