Targeting mitochondria in cancer cells using gold nanoparticle-enhanced radiotherapy: a Monte Carlo study

Multiscale Monte Carlo simulations of gold nanoparticle dose-enhanced radiotherapy I: Cellular dose enhancement in microscopic models

BACKGROUND

The introduction of Gold NanoParticles (GNPs) in radiotherapy treatments necessitates considerations such as GNP size, location, and quantity, as well as patient geometry and beam quality. Physics considerations span length scales across many orders of magnitude (nanometer-to-centimeter), presenting challenges that often limit the scope of dosimetric studies to either micro- or macroscopic scales.

PURPOSE

To investigate GNP dose-enhanced radiation Therapy (GNPT) through Monte Carlo (MC) simulations that bridge micro-to-macroscopic scales. The work is presented in two parts, with Part I (this work) investigating accurate and efficient MC modeling at the single cell level to calculate nucleus and cytoplasm Dose Enhancement Factors (n,cDEFs), considering a broad parameter space including GNP concentration, GNP intracellular distribution, cell size, and incident photon energy. Part II then evaluates cell dose enhancement factors across macroscopic (tumor) length scales.

METHODS

Different methods of modeling gold within cells are compared, from a contiguous volume of either pure gold or gold-tissue mixture to discrete GNPs in a hexagonal close-packed lattice. MC simulations with EGSnrc are performed to calculate n,cDEF for a cell with radiusr cell = 7.35 $r_{\rm cell}=7.35$  µm and nucleusr nuc = 5 $r_{\rm nuc} = 5$  µm considering 10 to 370 keV incident photons, gold concentrations from 4 to 24 mgAu /gtissue , and three different GNP configurations within the cell: GNPs distributed around the surface of the nucleus (perinuclear) or GNPs packed into one (or four) endosome(s). Select simulations are extended to cells with different cell (and nucleus) sizes: 5 µm (2, 3, and 4 µm), 7.35 µm (4 and 6 µm), and 10 µm (7, 8, and 9 µm).

RESULTS

n,cDEFs are sensitive to the method of modeling gold in the cell, with differences of up to 17% observed; the hexagonal lattice of GNPs is chosen (as the most realistic model) for all subsequent simulations. Across cell/nucleus radii, source energies, and gold concentrations, both nDEF and cDEF are highest for GNPs in the perinuclear configuration, compared with GNPs in one (or four) endosome(s). Across all simulations of the (rcell , rnuc ) = (7.35, 5) µm cell, nDEFs and cDEFs range from unity to 6.83 and 3.87, respectively. Including different cell sizes, nDEFs and cDEFs as high as 21.5 and 5.5, respectively, are observed. Both nDEF and cDEF are maximized at photon energies above the K- or L-edges of gold by 10 to 20 keV.

CONCLUSIONS

Considering 5000 unique simulation scenarios, this work comprehensively investigates many physics trends on DEFs at the cellular level, including demonstrating that cellular DEFs are sensitive to gold modeling approach, intracellular GNP configuration, cell/nucleus size, gold concentration, and incident source energy. These data should prove especially useful in research as well as treatment planning, allowing one to optimize or estimate DEF using not only GNP uptake, but also account for average tumor cell size, incident photon energy, and intracellular configuration of GNPs. Part II will expand the investigation, taking the Part I cell model and applying it in cm-scale phantoms.

Gold-Nanoparticles-Enhanced Production of Reactive Oxygen Species in Cells at Spread-Out Bragg Peak under Proton Beam Radiation

This study investigates the radiobiological effects of gold nanoparticles (GNPs) as radiosensitizers for proton beam therapy (PBT). Specifically, we explore the enhanced production of reactive oxygen species (ROS) in GNP-loaded tumor cells irradiated by a 230 MeV proton beam in a spread-out Bragg peak (SOBP) zone obtained by a passive scattering system. Our findings indicate that the radiosensitization enhancement factor is 1.24 at 30% cell survival fraction, 8 days after 6 Gy proton beam irradiation. Since protons deposit the majority of their energy at the SOBP region and interact with GNPs to induce more ejected electrons from the high-Z GNPs, these ejected electrons then react with water molecules to produce excessive ROS that can damage cellular organelles. Laser scanning confocal microscopy reveals the excessive ROS induced inside the GNP-loaded cells immediately after proton irradiation. Furthermore, the damage to cytoskeletons and mitochondrial dysfunction in GNP-loaded cells caused by the induced ROS becomes significantly severe, 48 h after proton irradiation. Our biological evidence suggests that the cytotoxicity of GNP-enhanced ROS production has the potential to increase the tumoricidal efficacy of PBT.

Gold Nanoparticles Enhancing Generation of ROS for Cs-137 Radiotherapy

Radiotherapy is an important modality for the treatment of cancer, e.g., X-ray, Cs-137 γ-ray (peak energy: 662 keV). An important therapy pathway of radiation is to generate the double strand breaks of DNA to prohibit the proliferation of cancer cells. In addition, the excessive amount of reactive oxygen species (ROS) is induced to damage the organelles, which can cause cellular apoptosis or necrosis. Gold nanoparticles (GNPs) have been proven potential as a radiosensitizer due to the high biocompatibility, the low cytotoxicity and the high-Z property (Z = 79) of gold. The latter property may allow GNPs to induce more secondary electrons for generating ROS in cells as irradiated by high-energy photons. In this paper, the radiobiological effects on A431 cells with uptake of 55-nm GNPs were studied to investigate the GNPs-enhanced production of ROS on these cells as irradiated by Cs-137 γ-ray. The fluorescence-labeling image of laser scanning confocal microscopy (LSCM) shows the excessive expression of ROS in these GNPs-uptake cells after irradiation. And then, the follow-up disruption of cytoskeletons and dysfunction of mitochondria caused by the induced ROS are observed. From the curves of cell survival fraction versus the radiation dose, the radiosensitization enhancement factor of GNPs is 1.29 at a survival fraction of 30%. This demonstrates that the tumoricidal efficacy of Cs-137 radiation can be significantly raised by GNPs. Because of facilitating the production of excessive ROS to damage tumor cells, GNPs are proven to be a prospective radiosensitizer for radiotherapy, particularly for the treatment of certain radioresistant tumor cells. Through this pathway, the tumoricidal efficacy of radiotherapy can be raised.

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.

Application of High-Z Gold Nanoparticles in Targeted Cancer Radiotherapy-Pharmacokinetic Modeling, Monte Carlo Simulation and Radiobiological Effect Modeling

High-Z gold nanoparticles (AuNPs) conjugated to a targeting antibody can help to improve tumor control in radiotherapy while simultaneously minimizing radiotoxicity to adjacent healthy tissue. This paper summarizes the main findings of a joint research program which applied AuNP-conjugates in preclinical modeling of radiotherapy at the Klinikum rechts der Isar, Technical University of Munich and Helmholtz Zentrum München. A pharmacokinetic model of superparamagnetic iron oxide nanoparticles was developed in preparation for a model simulating the uptake and distribution of AuNPs in mice. Multi-scale Monte Carlo simulations were performed on a single AuNP and multiple AuNPs in tumor cells at cellular and molecular levels to determine enhancements in the radiation dose and generation of chemical radicals in close proximity to AuNPs. A biologically based mathematical model was developed to predict the biological response of AuNPs in radiation enhancement. Although simulations of a single AuNP demonstrated a clear dose enhancement, simulations relating to the generation of chemical radicals and the induction of DNA strand breaks induced by multiple AuNPs showed only a minor dose enhancement. The differences in the simulated enhancements at molecular and cellular levels indicate that further investigations are necessary to better understand the impact of the physical, chemical, and biological parameters in preclinical experimental settings prior to a translation of these AuNPs models into targeted cancer radiotherapy.

Combined cell and nanoparticle models for TOPAS to study radiation dose enhancement in cell organelles

Dose enhancement by gold nanoparticles (AuNP) increases the biological effectiveness of radiation damage in biomolecules and tissue. To apply them effectively during cancer therapy their influence on the locally delivered dose has to be determined. Hereby, the AuNP locations strongly influence the energy deposit in the nucleus, mitochondria, membrane and the cytosol of the targeted cells. To estimate these effects, particle scattering simulations are applied. In general, different approaches for modeling the AuNP and their distribution within the cell are possible. In this work, two newly developed continuous and discrete-geometric models for simulations of AuNP in cells are presented. These models are applicable to simulations of internal emitters and external radiation sources. Most of the current studies on AuNP focus on external beam therapy. In contrast, we apply the presented models in Monte-Carlo particle scattering simulations to characterize the energy deposit in cell organelles by radioactive ¹⁹⁸AuNP. They emit beta and gamma rays and are therefore considered for applications with solid tumors. Differences in local dose enhancement between randomly distributed and nucleus targeted nanoparticles are compared. Hereby nucleus targeted nanoparticels showed a strong local dose enhancement in the radio sensitive nucleus. These results are the foundation for future experimental work which aims to obtain a mechanistic understanding of cell death induced by radioactive ¹⁹⁸Au.

Influence of gold nanoparticles embedded in water on nanodosimetry for keV photon irradiation

PURPOSE

For the past two decades, high-Z nanoparticles have been of high interest to improve the therapeutic outcomes of radiation therapy, especially for low-energy x-rays. Monte Carlo (MC) simulations have been used to evaluate the boost of dose deposition induced by Auger electrons near the nanoparticle surface, by calculating average energy deposition at the nanoscale. In this study, we propose to go beyond average quantities and quantify the stochastic nature of energy deposition at such a scale. We present results of probability density of the specific energy (restricted to ionization, excitation and electron attachment events) in cylindrical nanotargets of height and radius set at 10 nm. This quantity was evaluated for nanotargets located within 200 nm around 5-50 nm gold nanoparticles (GNPs), for 20-90 keV photon irradiation.

METHODS

This nanodosimetry study was based on the MC simulation MDM that allows tracking of electrons down to thermalization energy. We introduced a new quantity, namely the probability enhancement ratio (PER), by estimating the probability of imparting to nanotargets a restricted specific energy larger than a threshold z 0 (1, 10, and 20 kGy), normalized to the probability for pure water. The PER was calculated as a function of the distance between the nanotarget and the GNP surface. The threshold values were chosen in light of the biophysical model NanOx that predicts cell survival by calculating local lethal events based on the restricted specific energy and an effective local lethal function. z 0 then represents a threshold above which the nanotarget damages induce efficiently cell death.

RESULTS

Our calculations showed that the PER varied a lot with the GNP radius, the photon energy, z 0 and the distance of the GNP to the nanotarget. The highest PER was 95 when the nanotarget was located at 5 nm from the GNP surface, for a photon energy of 20 keV, a threshold of 20 kGy, and a GNP radius of 50 nm. This enhancement dramatically decreased with increasing GNP-nanotarget distances as it went below 1.5 for distances larger than 200 nm.

CONCLUSIONS

The PER seems better adapted than the mean dose deposition to describe the formation of biological damages. The significant increase of the PER within 200 nm around the GNP suggests that severe damages could occur for biological nanotargets located near the GNP. These calculations will be used as an input of the biophysical model NanOx to convert PER into estimation of radiation-induced cell death enhanced by GNPs.

The potential role of borophene as a radiosensitizer in boron neutron capture therapy (BNCT) and particle therapy (PT)

The potential role of borophene as a radiosensitizer in PT and BNCT was investigated. Our study focused on two aspects: (1) the synthesis and characterization of borophene nanomaterials; and (2) biocompatibility and dose enhancement. To overcome the limitation of vapor-based technology, we successfully deployed the liquid-phase exfoliation (LPE) method to produce borophene targeting for biomedical applications. Bringing together spatial distribution and dose deposition, the in vitro microdosimetry study was carried out in the presence of borophene. A quantitative study of the dose enhancement ratio (DER) was performed with Monte-Carlo simulation. The synthesized borophene showed good biocompatibility with less than 10% cell death at a concentration of up to 0.2 mg ml-1. The uptake of borophene within individual cells penetrated through cell membranes but outside the nucleus. For proton PT, no significant change in the DER is found. For carbon PT, the DER increases by about 5% as the concentration of 10B reaches 1 mg g-1. For BNCT, a DER of more than 2 can be obtained for a concentration as low as 100 μg g-1. This study lays a foundation for utilizing novel borophene-based nanomaterials as radiosensitizers as well as imaging probes in cancer treatment.

Roadmap for metal nanoparticles in radiation therapy: current status, translational challenges, and future directions

This roadmap outlines the potential roles of metallic nanoparticles (MNPs) in the field of radiation therapy. MNPs made up of a wide range of materials (from Titanium, Z = 22, to Bismuth, Z = 83) and a similarly wide spectrum of potential clinical applications, including diagnostic, therapeutic (radiation dose enhancers, hyperthermia inducers, drug delivery vehicles, vaccine adjuvants, photosensitizers, enhancers of immunotherapy) and theranostic (combining both diagnostic and therapeutic), are being fabricated and evaluated. This roadmap covers contributions from experts in these topics summarizing their view of the current status and challenges, as well as expected advancements in technology to address these challenges.

Monte Carlo simulation and analytical calculation methods to investigate the potential of nanoparticles for INTRABEAM® IORT machine

In the present study, Monte Carlo (MC) simulation and analytical calculation methods were used to investigate the potential of cancer treatment for the combination of IORT with nanoparticles (NPs). The Geant4 MC toolkit was used to simulate ZEISS INTRABEAM® IORT machine and its smallest applicator with 1.5 cm diameter. The dose enhancement effects (DEFs) were obtained for silver (Ag), gold (Au), bismuth (Bi), copper (Cu) and iron (Fe) spherical NPs considering different concentrations. In addition, analytical calculations were performed based on attenuation coefficient formula for sample NPs. Our MC results showed that the use of different NPs led to an increase in DEF up to 40%. Among different NPs, Au had the maximum DEF. In addition, analytical calculations revealed a significant increase, using NPs as well. Our study has suggested that the use of NPs in combination with IORT has the potential to enhance treatment outcomes.

The effects of a transverse magnetic field on the dose enhancement of nanoparticles in a proton beam: a Monte Carlo simulation

High-Z nanoparticles (NP) as radio-sensitization agents provide the feasibility of dose localization within the tumor in radiotherapy. Dose enhancement of NPs in the presence of a magnetic field (MF) could be challenged when magnetic resonance imaging (MRI) systems are used as an image-guided system. The MF can influence dose enhancement of NPs at their interfaces and surrounding medium and affect their dose deposition behavior. In the TOPAS Monte Carlo code, gold nanoparticle (GNP) and superparamagnetic iron oxide nanoparticle (SPION) were irradiated using 70 and 150 MeV proton beams, in presence of transverse MF strengths with 0, 1, 3, and 7 T. The changes in the liberated secondary electrons from NPs and their dose enhancement ratio (DER), magnetic dose enhancement ratio (MDER), and angular dose distribution in 10 nm shell thicknesses up to 500 nanometers from their centers were measured. The central plane of NPs was considered as a scorer. Its thickness was 2 nm and divided into 6-degree sectors with 10 nm radial length. The dose deposition in this voxelated scorer was calculated. The values of the deposited doses around NPs decrease rapidly while the DERs resulted from the secondary electrons are increased. MDERs are changed within [Formula: see text] and [Formula: see text] for 20 and 50 nm radius NPs, respectively. The variation in the angular dose distribution around a singular NP was not considerable when different MF strengths were applied. The dose values in the voxelated central plane show very similar results for the same NPs types in the different MF strengths. The typically used MF in the MRI systems would not considerably affect the energy deposition behavior of the secondary electrons produced in the interaction of proton beam with NPs, at least in the near vicinity of NPs. The DERs of NPs in a water medium resulted from emerged secondary electrons, experience a low degree of perturbation in the presence of an MF. The results of this study show that the NPs as dose enhancement agents can also be used in an MF without pronounced modification in their efficacy.

Auger electrons for cancer therapy - a review

Background

Auger electrons (AEs) are very low energy electrons that are emitted by radionuclides that decay by electron capture (e.g. ¹¹¹In, ⁶⁷Ga, ^99mTc, ^195mPt, ¹²⁵I and ¹²³I). This energy is deposited over nanometre-micrometre distances, resulting in high linear energy transfer (LET) that is potent for causing lethal damage in cancer cells. Thus, AE-emitting radiotherapeutic agents have great potential for treatment of cancer. In this review, we describe the radiobiological properties of AEs, their radiation dosimetry, radiolabelling methods, and preclinical and clinical studies that have been performed to investigate AEs for cancer treatment.

Results

AEs are most lethal to cancer cells when emitted near the cell nucleus and especially when incorporated into DNA (e.g. ¹²⁵I-IUdR). AEs cause DNA damage both directly and indirectly via water radiolysis. AEs can also kill targeted cancer cells by damaging the cell membrane, and kill non-targeted cells through a cross-dose or bystander effect. The radiation dosimetry of AEs considers both organ doses and cellular doses. The Medical Internal Radiation Dose (MIRD) schema may be applied. Radiolabelling methods for complexing AE-emitters to biomolecules (antibodies and peptides) and nanoparticles include radioiodination (¹²⁵I and ¹²³I) or radiometal chelation (¹¹¹In, ⁶⁷Ga, ^99mTc). Cancer cells exposed in vitro to AE-emitting radiotherapeutic agents exhibit decreased clonogenic survival correlated at least in part with unrepaired DNA double-strand breaks (DSBs) detected by immunofluorescence for γH2AX, and chromosomal aberrations. Preclinical studies of AE-emitting radiotherapeutic agents have shown strong tumour growth inhibition in vivo in tumour xenograft mouse models. Minimal normal tissue toxicity was found due to the restricted toxicity of AEs mostly on tumour cells targeted by the radiotherapeutic agents. Clinical studies of AEs for cancer treatment have been limited but some encouraging results were obtained in early studies using ¹¹¹In-DTPA-octreotide and ¹²⁵I-IUdR, in which tumour remissions were achieved in several patients at administered amounts that caused low normal tissue toxicity, as well as promising improvements in the survival of glioblastoma patients with ¹²⁵I-mAb 425, with minimal normal tissue toxicity.

Conclusions

Proof-of-principle for AE radiotherapy of cancer has been shown preclinically, and clinically in a limited number of studies. The recent introduction of many biologically-targeted therapies for cancer creates new opportunities to design novel AE-emitting agents for cancer treatment. Pierre Auger did not conceive of the application of AEs for targeted cancer treatment, but this is a tremendously exciting future that we and many other scientists in this field envision.

In vivo gadolinium nanoparticle quantification with SPECT/CT

Background

Gadolinium nanoparticles (Gd-NP) combined with radiotherapy are investigated for radiation dose enhancement in radiotherapy treatment. Indeed, NPs concentrated in a tumor could enhance its radiosensitization. The noninvasive quantification of the NP concentration is a crucial task for radiotherapy treatment planning and post-treatment monitoring as it will determine the absorbed dose. In this work, we evaluate the achievable accuracy of in vivo SPECT-based Gd-NP organ concentration on rats.

Methods

Gd-NPs were labeled with ¹¹¹In radionuclide. SPECT images have been acquired on phantom and rats, with various Gd-NP injections. Images have been calibrated and corrected for attenuation, scatter, and partial volume effect. Image-based estimations were compared to both inductively coupled plasma mass spectrometer (ICP-MS) for Gd concentration and ex vivo organ activity measured by gamma counter.

Results

The accuracy for the Gd mass measurements in organ was within 10% for activity above 2 MBq or concentrations above ∼ 3–4 MBq/mL. The Gd mass calculation is based on In-Gd coefficient which defines the Gd detection limit. It was found to be in a range from 2 mg/MBq to 2 µg/MBq depending on the proportions of initial injection preparations. Measurement was also impaired by free Gd and ¹¹¹In formed during metabolic processes.

Conclusions

Even if SPECT image quantification remains challenging mostly due to partial volume effect, this study shows that it has potential for the Gd mass measurements in organ. The main limitation of the method is its indirectness, and a special care should be taken if the organ of interest could be influenced by different clearance rate of free Gd and ¹¹¹In formed by metabolic processes. We also discuss the practical aspects, potential, and limitations of Gd-NP in vivo image quantification with a SPECT.

AN ALGORITHM TO DETERMINE THE NANODOSIMETRIC IMPACT OF GOLD NANOPARTICLES ON CELL MODELS

High-Z nanomaterials, e.g. gold nanoparticles (GNPs), are being investigated worldwide for potential application in radiation imaging and therapy. Photon irradiation of cells containing GNP was shown to produce enhanced DNA damage which is believed to be related to the increased secondary electron (SE) yield and ionization density. In this work, an algorithm was developed for simulating the physical radiation damage inside the nucleus of a spherical cell model for the case of uniformly distributed GNPs within the cytoplasm. Previously calculated energy spectra of SE emerging from a single NP irradiated with different photon sources are used as input to obtain the SE energy spectrum at the surface of the cell nucleus. In a second step, the SE transport inside the cell nucleus is simulated with a track structure Monte Carlo code to obtain the spatial distribution of ionizations. The preliminary results presented here show that the developed algorithm allows for a fast calculation of the SE spectra at the cell nucleus surface, thus enabling a more realistic assessment of the ionization density inside the cell nucleus than that obtained by the simulation of a single GNP. Furthermore, the algorithm can be easily adapted to investigate both the effect of GNP clustering and the impact of GNP-GNP interactions on SE spectra.

Experimental measurements validate the use of the binary encounter approximation model to accurately compute proton induced dose and radiolysis enhancement from gold nanoparticles

In protontherapy, it has been suggested that nanoparticles of high-Z material like gold (GNP) could be used as radiosensitizers. The origin of this enhancement phenomenon for proton radiation is not yet well understood and additional mechanistic insights are required. Previous works have highlighted the good capabilities of TRAX to reproduce secondary electron emission from gold material. Therefore, TRAX cross sections obtained with the binary encounter approximation (BEA) model for proton ionization were implemented within Geant4 for gold material. Based on the TRAX cross sections, improved Geant4 simulations have been developed to investigate the energy deposition and radical species production around a spherical gold nanoparticle (5 and 10 nm in diameter) placed in a water volume during proton irradiation. Simulations were performed for incident 2 MeV proton. The dose enhancement factor and the radiolysis enhancement factor were quantified. Results obtained with the BEA model were compared with results obtained with condensed-history models. Experimental irradiation of 200 nm gold films were performed to validate the secondary electron emission reproduction capabilities of physical models used in Monte Carlo (MC) simulations. TRAX simulations reproduced the experimental backscattered electron energy spectrum from gold film with better agreement than Geant4. Results on gold film obtained with the BEA model enabled to estimate the electron emission from GNPs. Results obtained in our study tend to support that the use of the BEA discrete model leads to a significant increase of the dose in the near vicinity of GNPs (<20 nm), while condensed history models used in Geant4 seem to overestimate the dose and the number of chemical species for increasing distances from the GNP. Based on discrete BEA model results, no enhancement effect due to secondary electron emitted from the GNP is expected if the GNP is not in close proximity to key cellular functional elements (DNA, mitochondria…).

A gold nanoparticle system for the enhancement of radiotherapy and simultaneous monitoring of reactive-oxygen-species formation

Gold nanoparticles (AuNPs) are known to sensitize cancer cells to radiation therapy (RT) by increasing the deposition of ionizing energy in their immediate vicinity. However, this process of dose enhancement is challenging to monitor because it is heterogeneous at the sub-cellular scale. Furthermore, radiation damage is primarily mediated by reactive oxygen species (ROS) that are produced following water radiolysis. Here, radiation-responsive PEGylated gold nanoparticles (RPAuNPs) were synthesized for the enhanced generation and concurrent detection of ROS in cancer cells and tumors. PEGylated gold particles (20 nm diameter) were functionalized with dihydrorhodamine 123 (DHR-123), a known ROS sensor, to monitor ROS generation in their immediate vicinity. These NPs were able to effectively radiosensitize cells, as measured by increased cell apoptosis following RT. Furthermore, the fluorescence of these RPAuNPs was 7-fold higher after 6 Gy RT due to the local production of ROS near the surface of the NP. Finally, multispectral fluorescence imaging was used to monitor NP-induced ROS in vivo, following conformal RT, in a xenograft model of breast cancer. This theranostic NP system provides a novel approach for monitoring the nanoscale enhancement of RT by high-Z metal NPs.

Geometrical structures for radiation biology research as implemented in the TOPAS-nBio toolkit

Computational simulations, such as Monte Carlo track structure simulations, offer a powerful tool for quantitatively investigating radiation interactions within cells. The modelling of the spatial distribution of energy deposition events as well as diffusion of chemical free radical species, within realistic biological geometries, can help provide a comprehensive understanding of the effects of radiation on cells. Track structure simulations, however, generally require advanced computing skills to implement. The TOPAS-nBio toolkit, an extension to TOPAS (TOol for PArticle Simulation), aims to provide users with a comprehensive framework for radiobiology simulations, without the need for advanced computing skills. This includes providing users with an extensive library of advanced, realistic, biological geometries ranging from the micrometer scale (e.g. cells and organelles) down to the nanometer scale (e.g. DNA molecules and proteins). Here we present the geometries available in TOPAS-nBio.

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.

Track-structure simulations of energy deposition patterns to mitochondria and damage to their DNA

PURPOSE

Mitochondria have been implicated in initiating and/or amplifying the biological effects of ionizing radiation not mediated via damage to nuclear DNA. To help elucidate the underlying mechanisms, energy deposition patterns to mitochondria and radiation damage to their DNA have been modelled.

METHODS

Track-structure simulations have been performed with PARTRAC biophysical tool for ⁶⁰Co γ-rays and 5 MeV α-particles. Energy deposition to the cell's mitochondria has been analyzed. A model of mitochondrial DNA reflecting experimental information on its structure has been developed and used to assess its radiation-induced damage.

RESULTS

Energy deposition to mitochondria is highly inhomogeneous, especially at low doses. Although a dose-dependent fraction of mitochondria sees no energy deposition at all, the hit ones receive rather high amounts of energy. Nevertheless, only little damage to mitochondrial DNA occurs, even at large doses.

CONCLUSION

Mitochondrial DNA does not represent a critical target for radiation effects. Likely, the key role of mitochondria in radiation-induced biological effects arises from the communication between mitochondria and/or with the nucleus. Through this signaling, initial modifications in a few heavily hit mitochondria seem to be amplified to a massive long-term effect manifested in the whole cell or even tissue.

Geant4-DNA track-structure simulations for gold nanoparticles: The importance of electron discrete models in nanometer volumes

PURPOSE

Gold nanoparticles (GNPs) are known to enhance the absorbed dose in their vicinity following photon-based irradiation. To investigate the therapeutic effectiveness of GNPs, previous Monte Carlo simulation studies have explored GNP dose enhancement using mostly condensed-history models. However, in general, such models are suitable for macroscopic volumes and for electron energies above a few hundred electron volts. We have recently developed, for the Geant4-DNA extension of the Geant4 Monte Carlo simulation toolkit, discrete physics models for electron transport in gold which include the description of the full atomic de-excitation cascade. These models allow event-by-event simulation of electron tracks in gold down to 10 eV. The present work describes how such specialized physics models impact simulation-based studies on GNP-radioenhancement in a context of x-ray radiotherapy.

METHODS

The new discrete physics models are compared to the Geant4 Penelope and Livermore condensed-history models, which are being widely used for simulation-based NP radioenhancement studies. An ad hoc Geant4 simulation application has been developed to calculate the absorbed dose in liquid water around a GNP and its radioenhancement, caused by secondary particles emitted from the GNP itself, when irradiated with a monoenergetic electron beam. The effect of the new physics models is also quantified in the calculation of secondary particle spectra, when originating in the GNP and when exiting from it.

RESULTS

The new physics models show similar backscattering coefficients with the existing Geant4 Livermore and Penelope models in large volumes for 100 keV incident electrons. However, in submicron sized volumes, only the discrete models describe the high backscattering that should still be present around GNPs at these length scales. Sizeable differences (mostly above a factor of 2) are also found in the radial distribution of absorbed dose and secondary particles between the new and the existing Geant4 models. The degree to which these differences are due to intrinsic limitations of the condensed-history models or to differences in the underling scattering cross sections requires further investigation.

CONCLUSIONS

Improved physics models for gold are necessary to better model the impact of GNPs in radiotherapy via Monte Carlo simulations. We implemented discrete electron transport models for gold in Geant4 that is applicable down to 10 eV including the modeling of the full de-excitation cascade. It is demonstrated that the new model has a significant positive impact on particle transport simulations in gold volumes with submicron dimensions compared to the existing Livermore and Penelope condensed-history models of Geant4.

Study on dependence of dose enhancement on cluster morphology of gold nanoparticles in radiation therapy using a body-centred cubic model

Gold nanoparticles (GNPs) injected in a body for dose enhancement in radiation therapy are known to form clusters. We investigated the dependence of dose enhancement on the GNP morphology using Monte-Carlo simulations and compared the model predictions with experimental data. The cluster morphology was approximated as a body-centred cubic (BCC) structure by placing GNPs at the 8 corners and the centre of a cube with an edge length of 0.22-1.03 µm in a 4  ×  4  ×  4 µm³ water-filled phantom. We computed the dose enhancement ratio (DER) for 50 and 260 kVp photons as a function of the distance from the cube centre for 12 different cube sizes. A 10 nm-wide concentric shell shaped detector was placed up to 100 nm away from a GNP at the cube centre. For model validation, simulations based on BCC and nanoparticle random distribution (NRD) models were performed using parameters that corresponded to the experimental conditions, which measured increases in the relative biological effect due to GNPs. We employed the linear quadratic model to compute cell surviving fraction (SF) and sensitizer enhancement ratio (SER). The DER is inversely proportional to the distance to the GNPs. The largest DERs were 1.97 and 1.80 for 50 kVp and 260 kVp photons, respectively. The SF predicted by the BCC model agreed with the experimental value within 10%, up to a 5 Gy dose, while the NRD model showed a deviation larger than 10%. The SERs were 1.21  ±  0.13, 1.16  ±  0.11, and 1.08  ±  0.11 according to the experiment, BCC, and NRD models, respectively. We most accurately predicted the GNP radiosensitization effect using the BCC approximation and suggest that the BCC model is effective for use in nanoparticle dosimetry.

Heterogeneous multiscale Monte Carlo simulations for gold nanoparticle radiosensitization

PURPOSE

To introduce the heterogeneous multiscale (HetMS) model for Monte Carlo simulations of gold nanoparticle dose-enhanced radiation therapy (GNPT), a model characterized by its varying levels of detail on different length scales within a single phantom; to apply the HetMS model in two different scenarios relevant for GNPT and to compare computed results with others published.

METHODS

The HetMS model is implemented using an extended version of the EGSnrc user-code egs_chamber; the extended code is tested and verified via comparisons with recently published data from independent GNP simulations. Two distinct scenarios for the HetMS model are then considered: (a) monoenergetic photon beams (20 keV to 1 MeV) incident on a cylinder (1 cm radius, 3 cm length); (b) isotropic point source (brachytherapy source spectra) at the center of a 2.5 cm radius sphere with gold nanoparticles (GNPs) diffusing outwards from the center. Dose enhancement factors (DEFs) are compared for different source energies, depths in phantom, gold concentrations, GNP sizes, and modeling assumptions, as well as with independently published values. Simulation efficiencies are investigated.

RESULTS

The HetMS MC simulations account for the competing effects of photon fluence perturbation (due to gold in the scatter media) coupled with enhanced local energy deposition (due to modeling discrete GNPs within subvolumes). DEFs are most sensitive to these effects for the lower source energies, varying with distance from the source; DEFs below unity (i.e., dose decreases, not enhancements) can occur at energies relevant for brachytherapy. For example, in the cylinder scenario, the 20 keV photon source has a DEF of 3.1 near the phantom's surface, decreasing to less than unity by 0.7 cm depth (for 20 mg/g). Compared to discrete modeling of GNPs throughout the gold-containing (treatment) volume, efficiencies are enhanced by up to a factor of 122 with the HetMS approach. For the spherical phantom, DEFs vary with time for diffusion, radionuclide, and radius; DEFs differ considerably from those computed using a widely applied analytic approach.

CONCLUSIONS

By combining geometric models of varying complexity on different length scales within a single simulation, the HetMS model can effectively account for both macroscopic and microscopic effects which must both be considered for accurate computation of energy deposition and DEFs for GNPT. Efficiency gains with the HetMS approach enable diverse calculations which would otherwise be prohibitively long. The HetMS model may be extended to diverse scenarios relevant for GNPT, providing further avenues for research and development.

Optimization of photon beam energies in gold nanoparticle enhanced arc radiation therapy using Monte Carlo methods

As a recent area of development in radiation therapy, gold nanoparticle (GNP) enhanced radiation therapy has shown potential to increase tumour dose while maintaining acceptable levels of healthy tissue toxicity. In this study, the effect of varying photon beam energy in GNP enhanced arc radiation therapy (GEART) is quantified through the introduction of a dose scoring metric, and GEART is compared to a conventional radiotherapy treatment. The PENELOPE Monte Carlo code was used to model several simple phantoms consisting of a spherical tumour containing GNPs (concentration: 15 mg Au g^-1 tumour, 0.8 mg Au g^-1 normal tissue) in a cylinder of tissue. Several monoenergetic photon beams, with energies ranging from 20 keV to 6 MeV, as well as 100, 200, and 300 kVp spectral beams, were used to irradiate the tumour in a 360° arc treatment. A dose metric was then used to compare tumour and tissue doses from GEART treatments to a similar treatment from a 6 MV spectrum. This was also performed on a simulated brain tumour using patient computed tomography data. GEART treatments showed potential over the 6 MV treatment for many of the simulated geometries, delivering up to 88% higher mean dose to the tumour for a constant tissue dose, with the effect greatest near a source energy of 50 keV. This effect is also seen with the inclusion of bone in a brain treatment, with a 14% increase in mean tumour dose over 6 MV, while still maintaining acceptable levels of dose to the bone and brain.

Evaluation of the microscopic dose enhancement for nanoparticle-enhanced Auger therapy

The aim of this study is to investigate the dosimetric characteristics of nanoparticle-enhanced Auger therapy. Monte Carlo (MC) simulations were performed to assess electron energy spectra and dose enhancement distributions around a nanoparticle. In the simulations, two types of nanoparticle structures were considered: nanoshell and nanosphere, both of which were assumed to be made of one of five elements (Fe, Ag, Gd, Au, and Pt) in various sizes (2-100 nm). Auger-electron emitting radionuclides (I-125, In-111, and Tc-99m) were simulated within a nanoshell or on the surface of a nanosphere. For the most promising combination of Au and I-125, the maximum dose enhancement was up to 1.3 and 3.6 for the nanoshell and the nanosphere, respectively. The dose enhancement regions were restricted within 20-100 nm and 0-30 nm distances from the surface of Au nanoshell and nanosphere, respectively. The dose enhancement distributions varied with sizes of nanoparticles, nano-elements, and radionuclides and thus should be carefully taken into account for biological modeling. If the nanoparticles are accumulated in close proximity to the biological target, this new type of treatment can deliver an enhanced microscopic dose to the target (e.g. DNA). Therefore, we conclude that Auger therapy combined with nanoparticles could have the potential to provide a better therapeutic effect than conventional Auger therapy alone.

Technical Note: spektr 3.0-A computational tool for x-ray spectrum modeling and analysis

PURPOSE

A computational toolkit (spektr 3.0) has been developed to calculate x-ray spectra based on the tungsten anode spectral model using interpolating cubic splines (TASMICS) algorithm, updating previous work based on the tungsten anode spectral model using interpolating polynomials (TASMIP) spectral model. The toolkit includes a matlab (The Mathworks, Natick, MA) function library and improved user interface (UI) along with an optimization algorithm to match calculated beam quality with measurements.

METHODS

The spektr code generates x-ray spectra (photons/mm(2)/mAs at 100 cm from the source) using TASMICS as default (with TASMIP as an option) in 1 keV energy bins over beam energies 20-150 kV, extensible to 640 kV using the TASMICS spectra. An optimization tool was implemented to compute the added filtration (Al and W) that provides a best match between calculated and measured x-ray tube output (mGy/mAs or mR/mAs) for individual x-ray tubes that may differ from that assumed in TASMICS or TASMIP and to account for factors such as anode angle.

RESULTS

The median percent difference in photon counts for a TASMICS and TASMIP spectrum was 4.15% for tube potentials in the range 30-140 kV with the largest percentage difference arising in the low and high energy bins due to measurement errors in the empirically based TASMIP model and inaccurate polynomial fitting. The optimization tool reported a close agreement between measured and calculated spectra with a Pearson coefficient of 0.98.

CONCLUSIONS

The computational toolkit, spektr, has been updated to version 3.0, validated against measurements and existing models, and made available as open source code. Video tutorials for the spektr function library, UI, and optimization tool are available.

Dose enhancement effects to the nucleus and mitochondria from gold nanoparticles in the cytosol

Gold nanoparticles (GNPs) have shown potential as dose enhancers for radiation therapy. Since damage to the genome affects the viability of a cell, it is generally assumed that GNPs have to localise within the cell nucleus. In practice, however, GNPs tend to localise in the cytoplasm yet still appear to have a dose enhancing effect on the cell. Whether this effect can be attributed to stress-induced biological mechanisms or to physical damage to extra-nuclear cellular targets is still unclear. There is however growing evidence to suggest that the cellular response to radiation can also be influenced by indirect processes induced when the nucleus is not directly targeted by radiation. The mitochondrion in particular may be an effective extra-nuclear radiation target given its many important functional roles in the cell. To more accurately predict the physical effect of radiation within different cell organelles, we measured the full chemical composition of a whole human lymphocytic JURKAT cell as well as two separate organelles; the cell nucleus and the mitochondrion. The experimental measurements found that all three biological materials had similar ionisation energies  ∼70 eV, substantially lower than that of liquid water  ∼78 eV. Monte Carlo simulations for 10-50 keV incident photons showed higher energy deposition and ionisation numbers in the cell and organelle materials compared to liquid water. Adding a 1% mass fraction of gold to each material increased the energy deposition by a factor of  ∼1.8 when averaged over all incident photon energies. Simulations of a realistic compartmentalised cell show that the presence of gold in the cytosol increases the energy deposition in the mitochondrial volume more than within the nuclear volume. We find this is due to sub-micron delocalisation of energy by photoelectrons, making the mitochondria a potentially viable indirect radiation target for GNPs that localise to the cytosol.

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.