Nanoparticle radiosensitization: from extended local effect modeling to a survival modification framework of compound Poisson additive killing and its carbon dots validation

Objective. To construct an analytical model instead of local effect modeling for the prediction of the biological effectiveness of nanoparticle radiosensitization. Approach. An extended local effects model is first proposed with a more comprehensive description of the nanoparticles mediated local killing enhancements, but meanwhile puts forward challenging issues that remain difficult and need to be further studied. As a novel method instead of local effect modeling, a survival modification framework of compound Poisson additive killing is proposed, as the consequence of an independent additive killing by the assumed equivalent uniform doses of individual nanoparticles per cell under the LQ model. A compound Poisson killing (CPK) model based on the framework is thus derived, giving a general expression of nanoparticle mediated LQ parameter modification. For practical use, a simplified form of the model is also derived, as a concentration dependent correction only to the α parameter, with the relative correction (α″/α) dominated by the mean number, and affected by the agglomeration of nanoparticles per cell. For different agglomeration state, a monodispersion model of the dispersity factor η = 1, and an agglomeration model of 2/3 < η < 1, are provided for practical prediction of (α″/α) value respectively. Main results. Initial validation by the radiosensitization of HepG2 cells by carbon dots showed a high accuracy of the CPK model. In a safe range of concentration (0.003–0.03 μg μl−1) of the carbon dots, the prediction errors of the monodispersion and agglomeration models were both within 2%, relative to the clonogenic survival data of the sensitized HepG2 cells. Significance. The compound Poisson killing model provides a novel approach for analytical prediction of the biological effectiveness of nanoparticle radiosensitization, instead of local effect modeling.

Incorporation of Low Concentrations of Gold Nanoparticles: Complex Effects on Radiation Response and Fate of Cancer Cells

(1) Background: In oncology research, a long-standing discussion exists about pros and cons of metal nanoparticle-enhanced radiotherapy and real mechanisms behind the tumor cell response to irradiation (IR) in presence of gold nanoparticles (GNPs). A better understanding of this response is, however, necessary to develop more efficient and safety nanoparticle (NP) types designed to disturb specific processes in tumor cells. (2) Aims and Methods: We combined 3D confocal microscopy and super-resolution single molecule localization microscopy (SMLM) to analyze, at the multiscale, the early and late effects of 10 nm-GNPs on DNA double strand break (DSB) induction and repair in tumor cells exposed to different doses of photonic low-LET (linear energy transfer) radiation. The results were correlated to different aspects of short and long-term cell viability. SkBr3 breast cancer cells (selected for the highest incidence of this cancer type among all cancers in women, and because most breast tumors are treated with IR) were incubated with low concentrations of GNPs and irradiated with ⁶⁰Co γ-rays or 6 MV X-rays. In numerous post-irradiation (PI) times, ranging from 0.5 to 24 h PI, the cells were spatially (3D) fixed and labeled with specific antibodies against γH2AX, 53BP1 and H3K9me3. The extent of DSB induction, multi-parametric micro- and nano-morphology of γH2AX and 53BP1 repair foci, DSB repair kinetics, persistence of unrepaired DSBs, nanoscale clustering of γH2AX and nanoscale (hetero)chromatin re-organization were measured by means of the mentioned microscopy techniques in dependence of radiation dose and GNP concentration. (3) Results: The number of γH2AX/53BP1 signals increased after IR and an additional increase was observed in GNP-treated (GNP(+)) cells compared to untreated controls. However, this phenomenon reflected slight expansion of the G2-phase cell subpopulation in irradiated GNP(+) specimens instead of enhanced DNA damage induction by GNPs. This statement is further supported by some micro- and nano-morphological parameters of γH2AX/53BP1 foci, which slightly differed for cells irradiated in absence or presence of GNPs. At the nanoscale, Ripley’s distance frequency analysis of SMLM signal coordinate matrices also revealed relaxation of heterochromatin (H3K9me3) clusters upon IR. These changes were more prominent in presence of GNPs. The slight expansion of radiosensitive G2 cells correlated with mostly insignificant but systematic decrease in post-irradiation survival of GNP(+) cells. Interestingly, low GNP concentrations accelerated DSB repair kinetics; however, the numbers of persistent γH2AX/53BP1 repair foci were slightly increased in GNP(+) cells. (4) Conclusions: Low concentrations of 10-nm GNPs enhanced the G2/M cell cycle arrest and the proportion of radiosensitive G2 cells, but not the extent of DNA damage induction. GNPs also accelerated DSB repair kinetics and slightly increased presence of unrepaired γH2AX/53BP1 foci at 24 h PI. GNP-mediated cell effects correlated with slight radiosensitization of GNP(+) specimens, significant only for the highest radiation dose tested (4 Gy).

Palladium Nanosheets as Safe Radiosensitizers for Radiotherapy

Many noble metal-based nanoparticles have emerged for applications in cancer radiotherapy in recent years, but few investigations have been carried out for palladium nanoparticles. Herein, palladium nanosheets (Pd NSs), which possess a sheetlike morphology with a diameter of ∼14 nm and a thickness of ∼2 nm, were utilized as a sensitizer to improve the performance of radiotherapy. It was found that Pd NSs alone did not decrease the cell viability after treatment for as long as 130 h, suggesting the excellent cytocompatibility of the nanoagents. However, the viability of cancer cells treated with X-ray irradiation became lower, and the viability became even lower if the cells were co-treated with X-ray and Pd NSs, indicating the radiosensitization effect of Pd NSs. Additionally, compared with X-ray irradiation, the combined treatment of Pd NSs and X-ray irradiation induced the generation of more DNA double-stranded breaks and reactive oxygen species within cancer cells, which eventually caused elevated cell apoptosis. Moreover, in vivo experiments also verified the radiosensitization effect and the favorable biocompatibility of Pd NSs, indicating their potential for acquiring satisfactory in vivo radiotherapeutic effect at lower X-ray doses. It is believed that the present research will open new avenues for the application of noble metal-based nanoparticles in radiosensitization.

Increased local tumor control through nanoparticle-mediated, radiation-triggered release of nitrite, an important precursor for reactive nitrogen species

The efficacy of dose-enhancing gold nanoparticles (AuNPs) is negatively impacted by low tumor uptake, low cell membrane penetration, limited diffusion distance, and short lifetime of radiation-induced secondary particles. To overcome these limitations, we have developed a novel AuNP system capable of radiation-triggered release of nitrite, a precursor of reactive nitrogen species, and report here on the in vivo characterization of this system. AuNPs were functionalized through PEGylation, cell-penetrating peptides (CPP; AuNP@CPP), and nitroimidazole (nIm; AuNP@nIm-CPP). Mice with subcutaneous 4T1 tumors received either AuNP@nIm-CPP or AuNP@CPP intraperitoneally. Tumor and normal tissue uptake were evaluated 24 h post AuNP administration. A separate cohort of mice was injected and irradiated to a single-fraction dose of 18 Gy in a 225 kVp small animal irradiator 24 h post NP administration. The mice were followed for two weeks to evaluate tumor response. The mean physical and hydrodynamic size of both NP systems were 5 and 13 nm, respectively. NP nIm-loading of 1 wt% was determined. Tumor accumulation of AuNP@nIm-CPP was significantly lower than that of AuNP@CPP (0.2% vs 1.2%, respectively). In contrast, AuNP@nIm-CPP showed higher accumulation compared to AuNP@CPP in liver (16.5% vs 6.6%, respectively) and spleen (10.8% vs 3.1%, respectively). With respect to tumor response, no differential response was found between non-irradiated mice receiving either saline or AuNP@nIm-CPP alone. The combination of AuNP@CPP+ radiation showed no differential response from radiation alone. In contrast, a significant delay in tumor regrowth was observed in mice receiving AuNP@nIm-CPP+ radiation compared to radiation alone. AuNP functionalized with both CPP and nIm exhibited an order of magnitude less tumor accumulation compared to the NP system without nIm yet resulted in a significantly higher therapeutic response. Our data suggest that by improving the biokinetics of AuNP@nIm-CPP, this novel NP system could be a promising radiosensitizer for enhanced therapeutic response following radiation therapy.

Biological dose-enhancement analysis with Monte Carlo simulation for Lipiodol for photon beams

Background

Previously, the physical dose-enhancement factor (D_physEF) enhancement was introduced. However, the dose enhancement considering the biological effectiveness was not shown.

Purpose

The aim of the current study was to evaluate the biological dose-enhancement factor (D_bioEF) by the dose rate and to compare the D_physEF and the D_bioEF in Lipiodol for liver Stereotactic Body Radiation Therapy (SBRT).

Materials and methods

Flattening-filter-free (FFF) 6-MV (6MVX) and 10MVX beams were delivered by TrueBeam. A virtual inhomogeneity phantom and a liver SBRT patient-treatment plan were used. The D_physEF and lineal energy distribution ( y ) distribution was calculated from Monte Carlo simulations. Using a microdosimetric-kinetic (MK) model that is estimated based on the linear-quadratic formula for Lipiodol using human liver hepatocellular cells (HepG2), the biological dose and biological dose enhancement factor (D_bioEF) were calculated. The dose rate in the simulation was changed from 0.1 to 24 Gy/min.

Results

The D_bioEF (DR:2Gy/min) and D_physEF with 10MVX FFF beam were 23.2% and 19.1% at maximum and 12.8% and 11.1% on average in the Lipiodol. In the comparison of the D_bioEF between 0.1-24 Gy/min, the D_bioEF was 21.2% and 11.1% with 0.1 Gy/min for 6MVX and 10 MVX, respectively. The D_bioEF was larger than DEF for the 6MVX and 10MVX FFF beams. In clinical cases with the 10MVX FFF beam, the D_bioEF and D_physEF in the Lipiodol region can increase the in-tumor dose by approximately 11% and 10%, respectively, without increasing the dose to normal tissue.

Conclusions

The lower-energy and higher-dose-rate beams were contributed to the biological dose. The Lipiodol caused the enhancement of the physical dose and biological effectiveness.

Advances in knowledge

The biological dose enhancement (D_bioEF) should be considered in the high-density material such as the Lipiodol.

Copper Oxide Nanoparticles Induce Enhanced Radiosensitizing Effect via Destructive Autophagy

Emerging nanotechnologies for radiotherapy are attracting increasing interest from researchers in recent years. To improve the radiotherapeutic performance, developing nanoparticles that can efficiently generate toxic reactive oxygen species (ROS) under X-ray irradiation are highly desirable. Here, we investigate the potential of copper oxide nanoparticles (CuO NPs) as nanoradiosensitizers. Increased cancer cell inhibition is observed in colony formation assay and real-time cell analysis after the combined treatment with CuO NPs and X-ray irradiation, whereas the CuO NPs alone do not have any negative influence on cell viability, indicating the radiosensitization effect of CuO NPs. Importantly, the significantly increased ROS level in cells contributes to the enhanced damage to cancer cells under the combined treatment. Besides, the cell cycle is regulated to the X-ray-sensitive phase (G₂/M phase) by CuO NPs, which may also account for the inhibited proliferation of cancer cells. Furthermore, results from Western blot analysis and colony formation assay reveal that the increased cell death may be mainly attributed to the excessive autophagy induced by both CuO NPs and X-ray irradiation. Moreover, in vivo experiments verify the radiosensitization of CuO NPs and their favorable biosafety. The current study suggests that CuO NPs can be utilized as nanoradiosensitizers for increasing the efficiency of cancer radiotherapy.

Targeting chemo-proton therapy on C6 cell line using superparamagnetic iron oxide nanoparticles conjugated with folate and paclitaxel

PURPOSE

This report presents novel nanoparticle-based drug delivery system (NPDDS) aiming to targeting chemo-proton therapy (TCPT) to improve the therapeutic efficacy on brain cancer treatments.

MATERIALS AND METHODS

A NPDDS, superparamagnetic iron oxide nanoparticles conjugated with folate and paclitaxel, was synthesized and applied to C6 brain cancer cell line that was prepared for TCPT. The characterization of NPDDS was analyzed by transmission electron microscope (TEM) and Fourier transform infrared (FTIR) spectroscopy. The uptake of NPDDS into the cytoplasm of C6 cells was observed by confocal laser scanning microscopy (CLSM). The therapeutic efficacy of proton beam was quantitatively evaluated by flow cytometry and clonogenic assay at various radiation dose.

RESULTS

NPDDS was synthesized in the uniform size distribution with a mean diameter of 5.44 ± 0.70 nm, and it showed no significant cytotoxicity at the concentration lower than 200 ng/mL. Radiosensitization enhancement factors of PTX, D-SPIONs and FA-PTX-D-SPIONs were 1.35, 1.16 and 1.52, respectively.

CONCLUSIONS

It was demonstrated that TCPT improved the therapeutic efficacy of the proton beam therapy when the synthesized novel NPDDS was administrated. The improvement in therapeutic efficacy was achieved by the synergetic effect of drug delivery increased by FA, radiosensitivity increased by PTX and absorption of proton energy increased by SPIONs.

Relative biological effectiveness study of Lipiodol based on microdosimetric-kinetic model

OBJECTIVES

We examine the contrast agent Lipiodol effect on the relative biological effectiveness (RBE) values for flattening filter free (FFF) and flattening filter (FF) beams of 6 MV-Xray (6 MVX) and 10 MVX.

METHODS

Lipiodol was placed at 5 cm depth in water. According to the microdosimetric kinetic model, the RBE values for killing the human liver hepatocellular cells were calculated from dose and lineal energy (yd(y)) from Monte Carlo simulations. RBE_200kVX and RBE_Co were defined as the ratios of dose using reference radiation (200 kVX, Co-ɤ) to the dose of test radiation (FFF and FF beams for 6 MV and 10 MV) to produce the same biological effects. The dose enhancement RBE (RBE_DE) was defined as the ratios of a dose without Lipiodol to with Lipiodol using to produce the same biological effects. The dose needed to achieve 10% (D_10%) and 1% cell survival (D_1%) was evaluated by cell surviving fraction (SF) formula.

RESULTS

The deviation of mean y‾_D values with and without Lipiodol were 3.9-4.8% for 6 MVX and 3.5-3.6% for 10 MVX. The RBE_200kVX and RBE_Co with Lipiodol were larger than that without Lipiodol. The RBE_DE was larger for FFF beam than for FF beam. The deviation of RBE_DE for FFF and FF beams of 6 MVX was larger than that of 10 MVX.

CONCLUSION

The presence of Lipiodol seemed to locally increase the absorbed dose and to also cause an enhancement of the relative biological effectiveness.

Gold Nanoparticles in Radiotherapy and Recent Progress in Nanobrachytherapy

Over the last few decades, gold nanoparticles (GNPs) have emerged as "radiosensitizers" in oncology. Radiosensitizers are additives that can enhance the effects of radiation on biological tissues treated with radiotherapy. The interaction of photons with GNPs leads to the emission of low-energy and short-range secondary electrons, which in turn increase the dose deposited in tissues. In this context, GNPs are the subject of intensive theoretical and experimental studies aiming at optimizing the parameters leading to greater dose enhancement and highest therapeutic effect. This review describes the main mechanisms occurring between photons and GNPs that lead to dose enhancement. The outcome of theoretical simulations of the interactions between GNPs and photons is presented. Finally, the findings of the most recent in vivo studies about interactions between GNPs and photon sources (e.g., external beams, brachytherapy sources, and molecules labeled with radioisotopes) are described. The advantages and challenges inherent to each of these approaches are discussed. Future directions, providing new guidelines for the successful translation of GNPs into clinical applications, are also highlighted.

Intratumoral Injection of Low-Energy Photon-Emitting Gold Nanoparticles: A Microdosimetric Monte Carlo-Based Model

Gold nanoparticles (Au NPs) distributed in the vicinity of low-dose rate (LDR) brachytherapy seeds could multiply their efficacy thanks to the secondary emissions induced by the photoelectric effect. Injections of radioactive LDR gold nanoparticles (LDR Au NPs), instead of conventional millimeter-size radioactive seeds surrounded by Au NPs, could further enhance the dose by distributing the radioactivity more precisely and homogeneously in tumors. However, the potential of LDR Au NPs as an emerging strategy to treat cancer is strongly dependent on the macroscopic diffusion of the NPs in tumors, as well as on their microscopic internalization within the cells. Understanding the relationship between interstitial and intracellular distribution of NPs, and the outcomes of dose deposition in the cancer tissue is essential for considering future applications of radioactive Au NPs in oncology. Here, LDR Au NPs (¹⁰³Pd:Pd@Au-PEG NPs) were injected in prostate cancer tumors. The particles were visualized at time-points by computed tomography imaging ( in vivo), transmission electron microscopy ( ex vivo), and optical microscopy ( ex vivo). These data were used in a Monte Carlo-based dosimetric model to reveal the dose deposition produced by LDR Au NPs both at tumoral and cellular scales. ¹⁰³Pd:Pd@Au-PEG NPs injected in tumors produce a strong dose enhancement at the intracellular level. However, energy deposition is mainly confined around vesicles filled with NPs, and not necessarily close to the nuclei. This suggests that indirect damage caused by the production of reactive oxygen species might be the leading therapeutic mechanism of tumor growth control, over direct damage to the DNA.

The dichotomous nature of dose enhancement by gold nanoparticle aggregates in radiotherapy

AIM

In nanoparticle-aided radiotherapy, the computational paradigm has been that inside the cell, nanoparticles are distributed sparsely and solitarily. However, experiments reveal significant cluster formation, which affects radiosensitization and must be considered in clinical treatment planning. We characterize the impact of gold nanoparticle agglomeration on the predicted radiation dose enhancement as function of size, geometry, morphology and incident beam energy.

MATERIALS & METHODS

Next-generation coupled electron-photon deterministic computations were performed using subnanometric unstructured spatial mesh.

RESULTS

Unlike single nanoparticles, agglomerates develop two types of dose enhancement, smooth peripheral distributions and isolated hotspots, which depend on the cluster size and geometry in opposite ways.

CONCLUSION

The peripheral dose enhancement may have less importance than the hotspots, which can have greater contribution to cell kill via radical creation. Hence, aggregate formation may be beneficial in nanoparticle-aided radiotherapy.

Nanoscale dose deposition in cell structures under X-ray irradiation treatment assisted with nanoparticles: An analytical approach to the relative biological effectiveness

In this study, an analytical model for the assessment of the modification of cell culture survival under ionizing radiation assisted with nanoparticles (NPs) is presented. The model starts from the radial dose deposition around a single NP, which is used to describe the dose deposition in a cell structure with embedded NPs and, in turn, to evaluate the number of lesions formed by ionizing radiation. The model is applied to the calculation of relative biological effectiveness values for cells exposed to 0.5mg/g of uniformly dispersed NPs with a radius of 10nm made of Fe, I, Gd, Hf, Pt and Au and irradiated with X-rays of energies 20keV higher than the element K-shell binding energy.

Targeted dose enhancement in radiotherapy for breast cancer using gold nanoparticles, part 2: A treatment planning study

PURPOSE

In recent years, there has been growing interest in the use of gold nanoparticles (GNPs) combined with radiotherapy to improve tumor control. However, the complex interplay between GNP uptake and dose distribution in realistic clinical treatment are still somewhat unknown.

METHODS

The effects of different concentrations of 2 nm diameter GNP, ranging from 0 to 5×105 nanoparticles per tumoral cell, were theoretically investigated. A parametrization of the GNP distribution outside the target was carried out using a Gaussian standard deviation σ, from a zero value, relative to a selective concentration of GNPs inside the tumor volume alone, to 50mm, when GNPs are spatially distributed also in the healthy tissues surrounding the tumor. Treatment simulations of five patients with breast cancer were performed with 6 and 15 MV photons assuming a partial breast irradiation. A closed analytical reformulation of the Local Effect Model coupled with the estimation of local dose deposited around a GNP was validated using an in vitro study for MDA-MB-231 tumoral cells. The expected treatment outcome was quantified in terms of tumor control probability (TCP) and normal tissue complication probability (NTCP) as a function of the spatially varying gold uptake.

RESULTS

Breast cancer treatment planning simulations show improved treatment outcomes when GNPs are selectively concentrated in the tumor volume (i.e., σ = 0 mm). In particular, the TCP increases up to 18% for 5×105 nanoparticles per cell in the tumor region depending on the treatment schedules, whereas an improvement of the therapeutic index is observed only for concentrations of about 105 GNPs per tumoral cell and limited spatial distribution in the normal tissue.

CONCLUSIONS

The model provides a useful framework to estimate the nanoparticle-driven radiosensitivity in breast cancer treatment irradiation, accounting for the complex interplay between dose and GNP uptake distributions.

A method for converting dose-to-medium to dose-to-tissue in Monte Carlo studies of gold nanoparticle-enhanced radiotherapy

Gold nanoparticles (GNPs) have shown potential in recent years as a means of therapeutic dose enhancement in radiation therapy. However, a major challenge in moving towards clinical implementation is the exact characterisation of the dose enhancement they provide. Monte Carlo studies attempt to explore this property, but they often face computational limitations when examining macroscopic scenarios. In this study, a method of converting dose from macroscopic simulations, where the medium is defined as a mixture containing both gold and tissue components, to a mean dose-to-tissue on a microscopic scale was established. Monte Carlo simulations were run for both explicitly-modeled GNPs in tissue and a homogeneous mixture of tissue and gold. A dose ratio was obtained for the conversion of dose scored in a mixture medium to dose-to-tissue in each case. Dose ratios varied from 0.69 to 1.04 for photon sources and 0.97 to 1.03 for electron sources. The dose ratio is highly dependent on the source energy as well as GNP diameter and concentration, though this effect is less pronounced for electron sources. By appropriately weighting the monoenergetic dose ratios obtained, the dose ratio for any arbitrary spectrum can be determined. This allows complex scenarios to be modeled accurately without explicitly simulating each individual GNP.

Nanoscale radiation transport and clinical beam modeling for gold nanoparticle dose enhanced radiotherapy (GNPT) using X-rays

We review radiation transport and clinical beam modelling for gold nanoparticle dose-enhanced radiotherapy using X-rays. We focus on the nanoscale radiation transport and its relation to macroscopic dosimetry for monoenergetic and clinical beams. Among other aspects, we discuss Monte Carlo and deterministic methods and their applications to predicting dose enhancement using various metrics.

Nanoparticle-aided Radiotherapy for Retinoblastoma and Choroidal Melanoma

This work investigates the dosimetric feasibility of employing gold nanoparticles (AuNPs) or carboplatin nano-particles (CNPs) to enhance radiotherapy (RT) treatment efficacy for ocular cancers: retinoblastoma (Rb) and choroidal melanoma (CM), during kV-energy internal and external beam radiotherapy. The results predict that substantial dose enhancement may be achieved by employing AuNPs or CNPs in conjunction with radiotherapy for ocular cancer using kV-energy photon beams. Brachytherapy sources yield higher dose enhancement than the external beam in kV energy range. However, the external beam has the advantage of being non-invasive.

Radiation dose enhancement of gadolinium-based AGuIX nanoparticles on HeLa cells

Radiation dose enhancement of high-Z nanoparticles is an active area of research in cancer therapeutics. When kV and MV energy photon beams interact with high-Z nanoparticles in a tumor, the release of secondary electrons can injure tumor cells, leading to a higher treatment efficacy than radiation alone. We present a study that characterizes the radiation dose enhancing effects of gadolinium-based AGuIX nanoparticles on HeLa cells. Our in vitro clonogenic survival assays showed an average dose enhancement of 1.54× for 220 kVp radiation and 1.15× for 6 MV radiation. The sensitivity enhancement ratio at 4 Gy (SER4Gy) was 1.54 for 220 kVp and 1.28 for 6 MV, indicating that these nanoparticles may be useful for clinical radiation therapy.

FROM THE CLINICAL EDITOR

This study characterized the radiation dose enhancing effects of gadolinium-based AGuIX nanoparticles on HeLa cells, showing clear effects at 220 kV as well as 6 MV, suggesting that after additional studies, these nanoparticles may be beneficial in human radiation therapy.

Optimal energy for cell radiosensitivity enhancement by gold nanoparticles using synchrotron-based monoenergetic photon beams

Gold nanoparticles have been shown to enhance radiation doses delivered to biological targets due to the high absorption coefficient of gold atoms, stemming from their high atomic number (Z) and physical density. These properties significantly increase the likelihood of photoelectric effects and Compton scattering interactions. Gold nanoparticles are a novel radiosensitizing agent that can potentially be used to increase the effectiveness of current radiation therapy techniques and improve the diagnosis and treatment of cancer. However, the optimum radiosensitization effect of gold nanoparticles is strongly dependent on photon energy, which theoretically is predicted to occur in the kilovoltage range of energy. In this research, synchrotron-generated monoenergetic X-rays in the 30–100 keV range were used to investigate the energy dependence of radiosensitization by gold nanoparticles and also to determine the photon energy that produces optimum effects. This investigation was conducted using cells in culture to measure dose enhancement. Bovine aortic endothelial cells with and without gold nanoparticles were irradiated with X-rays at energies of 30, 40, 50, 60, 70, 81, and 100 keV. Trypan blue exclusion assays were performed after irradiation to determine cell viability. Cell radiosensitivity enhancement was indicated by the dose enhancement factor which was found to be maximum at 40 keV with a value of 3.47. The dose enhancement factor obtained at other energy levels followed the same direction as the theoretical calculations based on the ratio of the mass energy absorption coefficients of gold and water. This experimental evidence shows that the radiosensitization effect of gold nanoparticles varies with photon energy as predicted from theoretical calculations. However, prediction based on theoretical assumptions is sometimes difficult due to the complexity of biological systems, so further study at the cellular level is required to fully characterize the effects of gold nanoparticles with ionizing radiation.

Dependence of Monte Carlo microdosimetric computations on the simulation geometry of gold nanoparticles

Recently, interactions of x-rays with gold nanoparticles (GNPs) and the resulting dose enhancement have been studied using several Monte Carlo (MC) codes (Jones et al 2010 Med. Phys. 37 3809-16, Lechtman et al 2011 Phys. Med. Biol. 56 4631-47, McMahon et al 2011 Sci. Rep. 1 1-9, Leung et al 2011 Med. Phys. 38 624-31). These MC simulations were carried out in simplified geometries and provided encouraging preliminary data in support of GNP radiotherapy. As these studies showed, radiation transport computations of clinical beams to obtain dose enhancement from nanoparticles has several challenges, mostly arising from the requirement of high spatial resolution and from the approximations used at the interface between the macroscopic clinical beam transport and the nanoscopic electron transport originating in the nanoparticle or its vicinity. We investigate the impact of MC simulation geometry on the energy deposition due to the presence of GNPs, including the effects of particle clustering and morphology. Dose enhancement due to a single and multiple GNPs using various simulation geometries is computed using GEANT4 MC radiation transport code. Various approximations in the geometry and in the phase space transition from macro- to micro-beams incident on GNPs are analyzed. Simulations using GEANT4 are compared to a deterministic code CEPXS/ONEDANT for microscopic (nm-µm) geometry. Dependence on the following microscopic (µ) geometry parameters is investigated: µ-source-to-GNP distance (µSAD), µ-beam size (µS), and GNP size (µC). Because a micro-beam represents clinical beam properties at the microscopic scale, the effect of using different types of micro-beams is also investigated. In particular, a micro-beam with the phase space of a clinical beam versus a plane-parallel beam with an equivalent photon spectrum is characterized. Furthermore, the spatial anisotropy of energy deposition around a nanoparticle is analyzed. Finally, dependence of dose enhancement on the number of GNPs in a finite cluster of nanoparticles is determined. Simulations were performed for 100 nm GNPs irradiated in water phantom by various monoenergetic (11 keV-1 MeV) and spectral (50 kVp) sources. The dose enhancement ratio (DER) is very sensitive to the specific simulation geometry (µSAD, µS, µC parameters) and µ-source type. For a single GNP the spatial distribution of DER is found to be nearly isotropic with limited magnitude and relatively short range (∼100-200 nm for DER significantly greater than 1). For a cluster of GNPs both the magnitude and range are found much greater (∼1-2 µm). The relation between DER for a cluster of GNPs and a single GNP is strongly nonlinear. Relatively strong dependence of DER on the simulation micro-geometry cautions future studies and the interpretation of existing MC results obtained in different simulations geometries. The nonlinear relation between DER for a single and multiple GNPs suggests that parameters such as the number of adjacent nanoparticles per cell and the distances between the GNPs and the cellular target may be important in assessing the biological effectiveness associated with GNP.