Gold Nanoparticle Dose Enhancement of Inverse-Compton Based Monoenergetic Photon Beams: A Monte Carlo Evaluation

Abstract

To investigate the dosimetric enhancement of an Inverse-Compton Scattering (ICS) based radiation source by gold nanoparticles. ICS light sources provide a novel method of producing high energy photons of tunable wavelength in the 100keV range. The highly collimated, near-monoenergetic photons produced by such a source are well-suited for gold nanoparticle dose enhancement in comparison with the broad-band spectra produced by traditional orthovoltage and megavoltage radiotherapy accelerators. The ICS source, treatment geometry, and various levels of gold uptake were simulated to determine the optimal source and treatment configuration for realistic nanoparticle uptake scenarios. The spectrum of the source was simulated using a 3D time and frequency domain code, producing a phase-space distribution of the Compton scattered X-rays of energy 91keV with a 1milliradian divergence. The Monte Carlo particle transport toolkit Geant4 v10.03 was used to simulate dose deposition of ICS beam in a head phantom. The phantom comprised of a 15.3cm sphere of ICRU brain material surrounded by a 7mm thick layer of compact bone. The central 4cm diameter region of brain was simulated with 0g/g, 10ug/g, 100ug/g, 1mg/g, 10mg/g, and 100mg/g gold uptake by volume. A treatment comprising of a rotationally symmetric conformal arc was simulated. A microdosimetric simulation was also performed, and dose enhancement and delocalization due to the nanoparticles were tabulated in kernels allowing for evaluation of the biological effect as a function of nanoparticle location within a cell (i.e. adjacent to nucleus or within cytoplasm). For a single beam, the dose at treatment depth relative to maximum dose in the skull was 55%. A rotationally symmetric beam configuration increases the dose to treatment depth to 95% of the skull maximum dose. Physical dose enhancement of +0%, +<1%, +9.6%, +89.6%, and +363% was observed for 10ug/g, 100ug/g, 1mg/g, 10mg/g, and 100mg/g uptake respectively. In comparison previous studies of gold nanoparticles irradiated by synchrotron and LDR brachytherapy sources, our results suggest a potential factor of 7-15 increase in biological effect beyond the physical dose enhancement. The ICS X-ray photon source provides a promising method for producing treatment beams well-suited to take advantage of the dose enhancement offered by gold nanoparticles, while minimizing dose due to off-resonance photon absorption.