Purpose: The magnitude of dose enhancement ratio (DER) induced by GNPs is sensitive to the beam quality, such as particle type and energy spectrum - traversing matter changes the energy spectrum. Monte Carlo simulations were used to assess the impact of depth-induced spectrum change on the micro-dosimetry induced by GNPs irradiated using megavoltage (MV) beams.
Methods: Simulations were carried out using TOPAS-nBio. The micro-dosimetry calculation was split into three distinct steps. Step one: a simulated 10648cm3 cubic water phantom was irradiated by 6 MV x-rays, and phase space data (PSD) of the particles passing through a 5cm² plane at different depths (0.25cm to 10cm) of phantom were recorded. Step 2: the recorded PSD was used as a source of GNP irradiation. For increasing interaction probability, the PSD acquired in the first step was shrunk to be nanoscopic (16nm²). Ejected secondary particles were recorded as spherical PSD around the GNP. Step 3: the spherical PSD recorded in the step two was used to calculate the dose distribution in nanoscale – utilizing track structure-based physics models within TOPAS-nBio. The dose was scored in bins of 1nm for from 0nm-100 nm from the GNP center, 10 nm from 100nm-1μm and 100 nm from 1μ-100μm.
Results: Simulations were validated using known characteristics of 6 MV clinical beams in water and found to be accurate. The fraction of low energy particles (<500 keV) in the incident beam increased from 6.56% to 8.44% as depth increased from 0.25cm to 10 cm. GNP-mediated DER was very localized around the NP regardless of depth, however, a depth dependency is still apparent.
Conclusion: The incident photon spectrum does show a depth dependency, which should be accounted for when considering clinical GNP aided radiation therapy. The optimal energy for GNP-mediated DER needs to be further refined before clinical use.
Not Applicable / None Entered.