Quantification of the radiosensitization effect of high-Z nanoparticles on photon irradiated cells: combining Monte Carlo simulations and an analytical approach to the local effect model.

In vitro experiments show significant reduction in the survival fraction of cells under irradiation treatments assisted with high-Z nanoparticles (NPs). In order to predict the radiosensitization effect of NPs, a modification of the local effect model, in which the energy deposition from NPs is assessed by Monte Carlo (MC) radiation transport codes, has been employed in the past. In this work, a combined framework that splits the consideration of the radiosensitization effect into two steps is proposed. The first step is the evaluation of the radial dose distribution around a single NP ionized by a photon beam with given energy spectrum using MC simulation. Thereafter, an analytical approach based of the local effect model and the calculated radial dose distribution (RDD) is used for evaluation of the average dose and the average number of lethal lesions in a cell target due to a set of ionized NPs. The explicit expressions were derived for the case of a spherical cell target and the radial dose distribution describing by the power law function. RDDs around gold NPs of different radii were simulated using the MC technique and fitted by a power law function 2.21. The fitted RDD and the derived expressions were applied for calculation of the survival curves and relative biological effectiveness of a spherical cell loaded with gold NPs and irradiated with monoenergetic photons of 10 - 150 keV. The proposed framework provides a practical alternative to time-consuming MC simulations, enabling the assessment of the response of cell cultures to an irradiation treatment assisted with NPs for a wide variety of cell geometries, NP distributions and irradiation schemes.