A Monte Carlo study of I-125 prostate brachytherapy with gold nanoparticles: dose enhancement with simultaneous rectal dose sparing via radiation shielding

Radial dependence of ionization clustering around a gold nanoparticle irradiated by X-rays under charged particle equilibrium

Objective.This work explores the enhancement of ionization clustering and its radial dependence around a gold nanoparticle (NP), indicative of the induction of DNA lesions, a potential trigger for cell-death.Approach.Monte Carlo track structure simulations were performed to determine (a) the spectral fluence of incident photons and electrons in water around a gold NP under charged particle equilibrium conditions and (b) the density of ionization clusters produced on average as well as conditional on the occurrence of at least one interaction in the NP using Associated Volume Clustering. Absorbed dose was determined for comparison with a recent benchmark intercomparison. Reported quantities are normalized to primary fluence, allowing to establish a connection to macroscopic dosimetric quantities.Main results.The modification of the electron spectral fluence by the gold NP is minor and mainly occurs at low energies. The net fluence of electrons emitted from the NP is dominated by electrons resulting from photon interactions. Similar to the known dose enhancement, increased ionization clustering is limited to a distance from the NP surface of up to200nm. The number of clusters per energy imparted is increased at distances of up to150nm, and accordingly the enhancement in clustering notably surpasses that of dose enhancement. Smaller NPs cause noticeable peaks in the conditional frequency of clusters between50nm-100nmfrom the NP surface.Significance.This work shows that low energy electrons emitted by NPs lead to an increase of ionization clustering in their vicinity exceeding that of energy imparted. While the electron component of the radiation field plays an important role in determining the background contribution to ionization clustering and energy imparted, the dosimetric effects of NPs are governed by the interplay of secondary electron production by photon interaction and their ability to leave the NP.

Multiscale Monte Carlo simulations of gold nanoparticle dose-enhanced radiotherapy II. Cellular dose enhancement within macroscopic tumor models

BACKGROUND

Gold NanoParticle (GNP) dose-enhanced radiation therapy (GNPT) requires consideration of physics across macro- to microscopic length scales, however, this presents computational challenges that have limited previous investigations.

PURPOSE

To develop and apply multiscale Monte Carlo (MC) simulations to assess variations in nucleus and cytoplasm dose enhancement factors (n,cDEFs) over tumor-scale volumes.

METHODS

The intrinsic variation of n,cDEFs (due to fluctuations in local gold concentration and cell/nucleus size variation) are estimated via MC modeling of varied cellular GNP uptake and cell/nucleus sizes. Then, the Heterogeneous MultiScale (HetMS) model is implemented in MC simulations by combining detailed models of populations of cells containing GNPs within simplified macroscopic tissue models to evaluate n,cDEFs. Simulations of tumors with spatially uniform gold concentrations (5, 10, or 20 mgAu /gtissue ) and spatially varying gold concentrations eluted from a point are performed to determine n,cDEFs as a function of distance from the source for 10 to 370 keV photons. All simulations are performed for three different intracellular GNP configurations: GNPs distributed on the surface of the nucleus (perinuclear) and GNPs packed into one or four endosome(s).

RESULTS

Intrinsic variations in n,cDEFs can be substantial, for example, if GNP uptake and cell/nucleus radii are varied by 20%, variations of up to 52% in nDEF and 25% in cDEF are observed compared to the nominal values for uniform cell/nucleus size and GNP concentration. In HetMS models of macroscopic tumors, subunity n,cDEFs (i.e., dose decreases) can occur for low energies and high gold concentrations due to attenuation of primary photons through the gold-filled volumes, for example, n,cDEF<1 is observed 3 mm from a 20 keV source for the four endosome configuration. In HetMS simulations of tumors with spatially uniform gold concentrations, n,cDEFs decrease with depth into the tumor as photons are attenuated, with relative differences between GNP models remaining approximately constant with depth in the tumor. Similar initial n,cDEF decreases with radius are seen in the tumors with spatially varying gold concentrations, but the n,cDEFs for all of the GNP configurations converge to a single value for each energy as gold concentration reaches zero.

CONCLUSIONS

The HetMS framework has been implemented for multiscale MC simulations of GNPT to compute n,cDEFs over tumor-scale volumes, with results demonstrating that cellular doses are highly sensitive to cell/nucleus size, GNP intracellular distribution, gold concentration, and cell position in tumor. This work demonstrates the importance of proper choice of computational model when simulating GNPT scenarios and the need to account for intrinsic variations in n,cDEFs due to variations in cell/nucleus size and gold concentration.

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.

Nanomedicines: intervention in inflammatory pathways of cancer

Inflammation is a complex defense process that maintains tissue homeostasis. However, this complex cascade, if lasts long, may contribute to pathogenesis of several diseases. Chronic inflammation has been exhaustively studied in the last few decades, for its contribution in development and progression of cancer. The intrinsic limitations of conventional anti-inflammatory and anti-cancer therapies triggered the development of nanomedicines for more effective and safer therapies. Targeting inflammation and tumor cells by nanoparticles, encapsulated with active therapeutic agents, offers a promising outcome with patient survival. Considerable technological success has been achieved in this field through exploitation of tumor microenvironment, and recognition of molecules overexpressed on endothelial cells or macrophages, through enhanced vascular permeability, or by rendering biomimetic approach to nanoparticles. This review focusses on the inflammatory pathways in progression of a tumor, and advancement in nanotechnologies targeting these pathways. We also aim to identify the gaps that hinder the successful clinical translation of nanotherapeutics with further clinical studies that will allow oncologist to precisely identify the patients who may be benefited from nanotherapy at time when promotion or progression of tumor initiates. It is postulated that the nanomedicines, in near future, will shift the paradigm of cancer treatment and improve patient survival.

Monte Carlo simulation of gold nanoparticles for X-ray enhancement application

BACKGROUND

Gold nanoparticles (Au NPs) are regarded as potential agents that enhance the radiosensitivity of tumor cells for theranostic applications. To elucidate the biological mechanisms of radiation dose enhancement effects of Au NPs as well as DNA damage attributable to the inclusion of Au NPs, Monte Carlo (MC) simulations have been deployed in a number of studies.

SCOPE OF REVIEW

This review paper concisely collates and reviews the information reported in the simulation research in terms of MC simulation of radiosensitization and dose enhancement effects caused by the inclusion of Au NPs in tumor cells, simulation mechanisms, benefits and limitations.

MAJOR CONCLUSIONS

In this review, we first explore the recent advances in MC simulation on Au NPs radiosensitization. The MC methods, physical dose enhancement and enhanced chemical and biological effects is discussed, followed by some results regarding the prediction of dose enhancement. We then review Multi-scale MC simulations of Au NP-induced DNA damages for X-ray irradiation. Moreover, we explain and look at Multi-scale MC simulations of Au NP-induced DNA damages for X-ray irradiation.

GENERAL SIGNIFICANCE

Using advanced chemical module-implemented MC simulations, there is a need to assess the radiation-induced chemical radicals that contribute to the dose-enhancing and biological effects of multiple Au NPs.

Gold nanoparticles meet medical radionuclides

Thanks to their unique optical and physicochemical properties, gold nanoparticles have gained increased interest as radiosensitizing, photothermal therapy and optical imaging agents to enhance the effectiveness of cancer detection and therapy. Furthermore, their ability to carry multiple medically relevant radionuclides broadens their use to nuclear medicine SPECT and PET imaging as well as targeted radionuclide therapy. In this review, we discuss the radiolabeling process of gold nanoparticles and their use in (multimodal) nuclear medicine imaging to better understand their specific distribution, uptake and retention in different in vivo cancer models. In addition, radiolabeled gold nanoparticles enable image-guided therapy is reviewed as well as the enhancement of targeted radionuclide therapy and nanobrachytherapy through an increased dose deposition and radiosensitization, as demonstrated by multiple Monte Carlo studies and experimental in vitro and in vivo studies.

Experimental determination of Gd dose enhancement and Gd dose sparing by 192Ir brachytherapy source with Gafchromic EBT3 dosimeter

This work evaluates experimentally the dose enhancement factor (DEF) and dose sparing factor (DSF) due to radiation self-shielding, produced by Gd infused in tumor phantom irradiated with brachytherapy HDR ¹⁹²Ir source by Gafchromic EBT3 dosimeter. The phantom was made of a set of solid water slabs (30 × 30 × 1.0) cm³ and three acrylic slabs of (30 × 30 × 0.5) cm³ machined to contain in the central axis acrylics vials of (1 × 1 × 5) cm³. The first and second acrylic vials were filled with an identical Gd solution of 0, 10 and 20 mg/ml, simulating Gd-doped and undoped tumor, and the third vial was filled in all the measurement only with water, representing an organ at risk. Additional solid water slabs were used to complete a phantom of (30 × 30 × 16) cm³. In the phantom center an acrylic slab was machined to introduce the 2.5 mm flexible guide tube of GammaMed plus iX equipment and positioning the ¹⁹²Ir source in the phantom central part. EBT3 fragments of (0.9 × 4) cm² were placed on the inner edge of the second and third vials to measure dose enhancement and dose sparing simultaneously. Phantom CT images were acquired for planning and to prescribe a dose of 6.0 Gy at 2.0 cm of the source, achieving an isodose curve of 44.5% at 3.0 cm (positions of the EBT3 films). Additionally, Monte Carlo simulation of the identical experimental setup was implemented to compare measurement values. The results showed the feasibility of measuring a DEF of 1.15 ± 0.05 in 20 mg/ml of Gd concentration consistent with the Monte Carlo DEF of 1.112 ± 0.005 for the same concentration. DEF value for concentration of 10 mg/ml would not be detected (1.00 ± 0.04) by an expected under measurement of the EBT3 films associated with the non-detection of photoelectrons and Auger electrons of very low energy that cannot reach the radiosensitive substrate.

Intercomparison of Monte Carlo calculated dose enhancement ratios for gold nanoparticles irradiated by X-rays: Assessing the uncertainty and correct methodology for extended beams

Results of a Monte Carlo code intercomparison exercise for simulations of the dose enhancement from a gold nanoparticle (GNP) irradiated by X-rays have been recently reported. To highlight potential differences between codes, the dose enhancement ratios (DERs) were shown for the narrow-beam geometry used in the simulations, which leads to values significantly higher than unity over distances in the order of several tens of micrometers from the GNP surface. As it has come to our attention that the figures in our paper have given rise to misinterpretation as showing 'the' DERs of GNPs under diagnostic X-ray irradiation, this article presents estimates of the DERs that would have been obtained with realistic radiation field extensions and presence of secondary particle equilibrium (SPE). These DER values are much smaller than those for a narrow-beam irradiation shown in our paper, and significant dose enhancement is only found within a few hundred nanometers around the GNP. The approach used to obtain these estimates required the development of a methodology to identify and, where possible, correct results from simulations whose implementation deviated from the initial exercise definition. Based on this methodology, literature on Monte Carlo simulated DERs has been critically assessed.

Gold nanoparticle detection and quantification in therapeutic MV beams via pair production

PURPOSE

We propose a new detection method of gold nanoparticles (AuNP) in therapeutic megavoltage (MV) x-ray beams by means of coincidence counting of annihilation photons following pair production in gold.

METHODS

The proposed MV x-ray induced positron emission (MVIPE) imaging technique is studied by radiation transport computations using MCNP6 (3D) and CEPXS/ONEDANT (1D) codes for two water phantoms: a 35 cm slab and a similarly sized cylinder, both having a 5 cm AuNP filled region in the center. MVIPE is compared to the standard x-ray fluorescence computed tomography (XFCT). MVIPE adopts MV x-ray sources (Co-60, 2 MV, 6 MV, 6 MV with closed MLC and 15 MV) and relies on the detection of 511 keV photon-pairs. XFCT uses kilovoltage sources (100 kVp, 120 kVp and 150 kVp) and imaging is characterized by analysis of k _α1,2 Au characteristic lines. Three levels of AuNP concentration were studied: 0.1%, 1% and 10% by weight.

RESULTS

Annihilation photons in the MVIPE technique originate both in the AuNP and in water along the x-ray beam path with significantly larger production in the AuNP-loaded region. MVIPE signal from AuNP is linearly increasing with AuNP concentration up to 10%wt, while XFCT signal reaches saturation due to self-absorption within AuNP. The production of annihilation photons is proportional to the MV source energy. MVIPE technique using a 15 MV pencil beam and 10 wt% AuNP detects about 4.5 × 10³ 511 keV-photons cm^-2 at 90° w/r to the incident beam per 10⁹ source photons cm^-2; 500 of these come from AuNP. In contrast, the XFCT technique using 150 kVp detects only about 100 k _α1-photons cm^-2 per 10⁹ source photons cm^-2.

CONCLUSIONS

In MVIPE, the number of annihilation photons produced for different MV-beam energies and AuNP concentrations is significantly greater than the k _α1 photons generated in XFCT. Coincidence counting in MVIPE allows to avoid collimation, which is a major limiting factor in XFCT. MVIPE challenges include the filtering of Compton scatter and annihilation photons originating in water.

An in silico study on the effect of host tissue at brachytherapy dose enhancement by gold nanoparticles

PURPOSE

Iridium-192 brachytherapy dose enhancement by gold nanoparticles was investigated in five different tumor tissues to observe the tissue-related differences as an effective environmental factor in the applications of nanoparticles as radio-enhancer agents.

METHODS AND MATERIALS

The brachytherapy high-dose-rate source of BEBIG Ir-192, a tumor volume with five different tissues including water, Plexiglas, soft tissue, adipose, and bone with and without a uniform distribution of gold nanoparticles were mimicked by MCNPX Monte Carlo simulation code using lattice feature. Dose enhancement factors in the tumor volume were measured separately regarding the types of tissue, and a previous study using GEometry ANd Tracking 4 simulation was used for result validation.

RESULTS

The results demonstrated that various types of tissue, as the host of gold nanoparticles, lead to different dose enhancement level, so that the bone and adipose have the lowest and the highest amount of dose enhancement factor with values 20.8% and 39.75%, respectively. The maximum difference of 4.8% was achieved from data benchmarking.

CONCLUSIONS

The results of this study indicate that the MCNPX code can be used as a valid tool for dose measurement in the presence of nanoparticles. Moreover, tissue types of tumor as an environmental feature, alongside with the nanoparticle's size and concentration as well as the conditions of radiotherapy, should be considered in the dose calculation.

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.

A detailed Monte Carlo evaluation of 192Ir dose enhancement for gold nanoparticles and comparison with experimentally measured dose enhancements

Gold nanoparticles (GNPs) have been studied extensively as promising radiation dose enhancing agents. In the current study, the dose enhancement effect of GNPs for Ir-192 HDR brachytherapy is studied using Monte Carlo N-Particle code, version 6.2 (MCNP6.2) and compared with experimental results obtained using Burlin cavity theory formalism. The Ir-192 source is verified using TG-43 parameters and dose enhancement factors (DEFs) from GNPs are simulated for three different mass percentages of gold in the GNP solution. These results are compared to DEFs previously reported experimentally by our group (Bassiri et al 2019 Med. Phys.) for a GNP-containing volume in an apparatus designed in-house to measure dose enhancement with GNPs for high dose rate (HDR) Ir-192 brachytherapy. An HDR Ir-192 Microselectron v2 r HDR brachytherapy source was modeled using MCNP6.2 using the TG-43 formalism in water. Anisotropy and radial dose function were verified against known values. An apparatus designed to measure dose enhancement to a 0.75 cm³ volume of GNPs from an Ir-192 brachytherapy seed with average energy of 0.38 MeV was built in-house and modeled using MCNP6.2. Burlin cavity correction factors were applied to experimental measurements. The macroscopic DEF was calculated for GNPs of size 30 nm at mass percentages of gold of 0.28%, 0.56% and 0.77%, using the repeating structures capability of MCNP6.2. DEF was calculated by dividing dose to the GNP solution by dose to water in the same volume. The radial dose function and anisotropy factor values at varying angles and distances were accurate when compared against known values. DEFs of 1.018 ± 0.003, 1.031 ± 0.003, and 1.041 ± 0.003 for GNP solutions containing mass percent of gold of 0.28%, 0.56% and 0.77%, respectively, were computed. These DEFs were within 2% of experimental values with Burlin cavity correction factors applied for all three mass percentages of gold.

Determining dose enhancement factors of high-Z nanoparticles from simulations where lateral secondary particle disequilibrium exists

Nanoparticles (NPs) containing high atomic number (high-Z) materials have been shown to enhance the radiobiological effectiveness of ionizing radiation. This effect is often attributed to an enhancement of the absorbed dose in the vicinity of the NPs, based on Monte Carlo simulations that show a significant local enhancement of the energy deposition on the microscopic scale. The results of such simulations may be significantly biased and lead to a severe overestimation of the dose enhancement if the condition of secondary particle equilibrium is not met in the simulation setup. This current work shows an approach to estimate a 'realistic' dose enhancement from the results of such biased simulations which is based on published photon interaction data and provides a way for correcting biased results.

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.