Cellular Dosimetry Research Articles

The Emission of Internal Conversion Electrons Rather Than Auger Electrons Increased the Nucleus-Absorbed Dose for 161Tb Compared with 177Lu with a Higher Dose Response for [161Tb]Tb-DOTA-LM3 Than for [161Tb]Tb-DOTATATE.

Preclinical data have shown that 161Tb-labeled peptides targeting the somatostatin receptor are therapeutically more effective for peptide receptor radionuclide therapy than are their 177Lu-labeled counterparts. To further substantiate this enhanced therapeutic effect, we performed cellular dosimetry to quantify the absorbed dose to the cell nucleus and compared dose-response curves to evaluate differences in relative biological effectiveness invitro. Methods: CA20948 cell survival was assessed after treatment with [161Tb]Tb- and [177Lu]Lu-DOTATATE (agonist) and with [161Tb]Tb- and [177Lu]Lu-DOTA-LM3 (antagonist) via a clonogenic assay. Cell binding, internalization, and dissociation assays were performed up to 7 d to acquire time-integrated activity coefficients. Separate S values for each type of particle emission (Auger/internal conversion [IC] electrons and β- particles) were computed via Monte Carlo simulations, while considering spheric cells. Once the absorbed dose to the cell nucleus was calculated, survival curves were fitted to the appropriate linear or linear-quadratic model and corresponding relative biological effectiveness was evaluated. Results: Although the radiopeptide uptake was independent of the radionuclide, [161Tb]Tb-DOTATATE and [161Tb]Tb-DOTA-LM3 delivered a 3.6 and 3.8 times higher dose to the nucleus, respectively, than their 177Lu-labeled counterparts on saturated receptor binding. This increased nucleus-absorbed dose was mainly due to the additional emission of IC and not Auger electrons by 161Tb. When activity concentrations were considered, both [161Tb]Tb-DOTATATE and [161Tb]Tb-DOTA-LM3 showed a lower survival fraction than did labeling with 177Lu. When the absorbed dose to the nucleus was considered, no significant difference could be observed between the dose-response curves for [161Tb]Tb- and [177Lu]Lu-DOTATATE. [161Tb]Tb-DOTA-LM3 showed a linear-quadratic dose response, whereas [161Tb]Tb-DOTATATE showed only a linear dose response within the observed dose range, suggesting additional cell membrane damage by Auger electrons. Conclusion: The IC, rather than Auger, electrons emitted by 161Tb resulted in a higher absorbed dose to the cell nucleus and lower clonogenic survival for [161Tb]Tb-DOTATATE and [161Tb]Tb-DOTA-LM3 than for the 177Lu-labeled analogs. In contrast, [161Tb]Tb-DOTATATE showed no higher dose response than [177Lu]Lu-DOTATATE, whereas for [161Tb]Tb-DOTA-LM3 an additional quadratic response was observed. Because of this quadratic response, potentially caused by cell membrane damage, [161Tb]Tb-DOTA-LM3 is a more effective radiopeptide than [161Tb]Tb-DOTATATE for labeling with 161Tb.

Dispersion and Dosimetric Challenges of Hydrophobic Carbon-Based Nanoparticles in In Vitro Cellular Studies.

Methodologies across the dispersion preparation, characterization, and cellular dosimetry of hydrophilic nanoparticles (NPs) have been developed and used extensively in the field of nanotoxicology. However, hydrophobic NPs pose a challenge for dispersion in aqueous culture media using conventional methods that include sonication followed by mixing in the culture medium of interest and cellular dosimetry. In this study, a robust methodology for the preparation of stable dispersions of hydrophobic NPs for cellular studies is developed by introducing continuous energy over time via stirring in the culture medium followed by dispersion characterization and cellular dosimetry. The stirring energy and the presence of proteins in the culture medium result in the formation of a protein corona around the NPs, stabilizing their dispersion, which can be used for in vitro cellular studies. The identification of the optimal stirring time is crucial for achieving dispersion and stability. This is assessed through a comprehensive stability testing protocol employing dynamic light scattering to evaluate the particle size distribution stability and polydispersity. Additionally, the effective density of the NPs is obtained for the stable NP dispersions using the volumetric centrifugation method, while cellular dosimetry calculations are done using available cellular computational modeling, mirroring approaches used for hydrophilic NPs. The robustness of the proposed dispersion approach is showcased using a highly hydrophobic NP model (black carbon NPs) and two culture media, RPMI medium and SABM, that are widely used in cellular studies. The proposed approach for the dispersion of hydrophobic NPs results in stable dispersions in both culture media used here. The NP effective density of 1.03-1.07 g/cm3 measured here for black carbon NPs is close to the culture media density, resulting in slow deposition on the cells over time. So, the present methodology for dispersion and dosimetry of hydrophobic NPs is essential for the design of dose-response studies and overcoming the challenges imposed by slow particle deposition.

225Ac]Ac- and [111In]In-DOTA-trastuzumab theranostic pair: cellular dosimetry and cytotoxicity in vitro and tumour and normal tissue uptake in vivo in NRG mice with HER2-positive human breast cancer xenografts

BackgroundTrastuzumab (Herceptin) has improved the outcome for patients with HER2-positive breast cancer (BC) but brain metastases (BM) remain a challenge due to poor uptake of trastuzumab into the brain. Radioimmunotherapy (RIT) with trastuzumab labeled with α-particle emitting, 225Ac may overcome this challenge by increasing the cytotoxic potency on HER2-positive BC cells. Our first aim was to synthesize and characterize [111In]In-DOTA-trastuzumab and [225Ac]Ac-DOTA-trastuzumab as a theranostic pair for imaging and RIT of HER2-positive BC, respectively. A second aim was to estimate the cellular dosimetry of [225Ac]Ac-DOTA-trastuzumab and determine its cytotoxicity in vitro on HER2-positive BC cells. A third aim was to study the tumour and normal tissue uptake of [225Ac]Ac-DOTA-trastuzumab using [111In]In-DOTA-trastuzumab as a radiotracer in vivo in NRG mice with s.c. 164/8-1B/H2N.luc+ human BC tumours that metastasize to the brain.ResultsTrastuzumab was conjugated to 12.7 ± 1.2 DOTA chelators and labeled with 111In or 225Ac. [111In]In-DOTA-trastuzumab exhibited high affinity specific binding to HER2-positive SK-BR-3 human BC cells (KD = 1.2 ± 0.3 × 10–8 mol/L). Treatment with [225Ac]Ac-DOTA-trastuzumab decreased the surviving fraction (SF) of SK-BR-3 cells dependent on the specific activity (SA) with SF < 0.001 at SA = 0.74 kBq/µg. No surviving colonies were noted at SA = 1.10 kBq/µg or 1.665 kBq/µg. Multiple DNA double-strand breaks (DSBs) were detected in SK-BR-3 cells exposed to [225Ac]Ac-DOTA-trastuzumab by γ-H2AX immunofluorescence microscopy. The time-integrated activity of [111In]In-DOTA-trastuzumab in SK-BR-3 cells was measured and used to estimate the absorbed doses from [225Ac]Ac-DOTA-trastuzumab by Monte Carlo N-Particle simulation for correlation with the SF. The dose required to decrease the SF of SK-BR-3 cells to 0.10 (D10) was 1.10 Gy. Based on the D10 reported for γ-irradiation of SK-BR-3 cells, we estimate that the relative biological effectiveness of the α-particles emitted by 225Ac is 4.4. Biodistribution studies in NRG mice with s.c. 164/8-1B/H2N.luc+ human BC tumours at 48 h post-coinjection of [111In]In-DOTA-trastuzumab and [225Ac]Ac-DOTA-trastuzumab revealed HER2-specific tumour uptake (10.6 ± 0.6% ID/g) but spleen uptake was high (28.9 ± 7.4% ID/g). Tumours were well-visualized by SPECT/CT imaging using [111In]In-DOTA-trastuzumab.ConclusionWe conclude that [225Ac]Ac-DOTA-trastuzumab exhibited potent and HER2-specific cytotoxicity on SK-BR-3 cells in vitro and HER2-specific uptake in s.c. 164/8-1B/H2N.luc+ human BC tumours in NRG mice, and these tumours were imaged by SPECT/CT with [111In]In-DOTA-trastuzumab. These results are promising for combining [111In]In-DOTA-trastuzumab and [225Ac]Ac-DOTA-trastuzumab as a theranostic pair for imaging and RIT of HER2-positive BC.

A GATE simulation study for dosimetry in cancer cell and micrometastasis from the 225Ac decay chain

BackgroundRadiopharmaceutical therapy (RPT) with alpha-emitting radionuclides has shown great promise in treating metastatic cancers. The successive emission of four alpha particles in the 225Ac decay chain leads to highly targeted and effective cancer cell death. Quantifying cellular dosimetry for 225Ac RPT is essential for predicting cell survival and therapeutic success. However, the leading assumption that all 225Ac progeny remain localized at their target sites likely overestimates the absorbed dose to cancer cells. To address limitations in existing semi-analytic approaches, this work evaluates S-values for 225Ac’s progeny radionuclides with GATE Monte Carlo simulations.MethodsThe cellular geometries considered were an individual cell (10 µm diameter with a nucleus of 8 µm diameter) and a cluster of cells (micrometastasis) with radionuclides localized in four subcellular regions: cell membrane, cytoplasm, nucleus, or whole cell. The absorbed dose to the cell nucleus was scored, and self- and cross-dose S-values were derived. We also evaluated the total absorbed dose with various degrees of radiopharmaceutical internalization and retention of the progeny radionuclides 221Fr (t1/2 = 4.80 m) and 213Bi (t1/2 = 45.6 m).ResultsFor the cumulative 225Ac decay chain, our self- and cross-dose nuclear S-values were both in good agreement with S-values published by MIRDcell, with per cent differences ranging from − 2.7 to − 8.7% for the various radionuclide source locations. Source location had greater effects on self-dose S-values than the intercellular cross-dose S-values. Cumulative 225Ac decay chain self-dose S-values increased from 0.167 to 0.364 GyBq−1 s−1 with radionuclide internalization from the cell surface into the cell. When progeny migration from the target site was modelled, the cumulative self-dose S-values to the cell nucleus decreased by up to 71% and 21% for 221Fr and 213Bi retention, respectively.ConclusionsOur GATE Monte Carlo simulations resulted in cellular S-values in agreement with existing MIRD S-values for the alpha-emitting radionuclides in the 225Ac decay chain. To obtain accurate absorbed dose estimates in 225Ac studies, accurate understanding of daughter migration is critical for optimized injected activities. Future work will investigate other novel preclinical alpha-emitting radionuclides to evaluate therapeutic potency and explore realistic cellular geometries corresponding to targeted cancer cell lines.

Large-scale in vitro microdosimetry via live cell microscopy imaging: implications for radiosensitivity and RBE evaluations in alpha-emitter radiopharmaceutical therapy

BackgroundAlpha-emitter radiopharmaceutical therapy (αRPT) has shown promising outcomes in metastatic disease. However, the short range of the alpha particles necessitates dosimetry on a near-cellular spatial scale. Current knowledge on cellular dosimetry is primarily based on in vitro experiments using cell monolayers. The goal of such experiments is to establish cell sensitivity to absorbed dose (AD). However, AD cannot be measured directly and needs to be modeled. Current models, often idealize cells as spheroids in a regular grid (geometric model), simplify binding kinetics and ignore the stochastic nature of radioactive decay. It is unclear what the impact of such simplifications is, but oversimplification results in inaccurate and non-generalizable results, which hampers the rigorous study of the underlying radiobiology.MethodsWe systematically mapped out 3D cell geometries, clustering behavior, agent binding, internalization, and subcellular trafficking kinetics for a large cohort of live cells under representative experimental conditions using confocal microscopy. This allowed for realistic Monte Carlo-based (micro)dosimetry. Experimentally established surviving fractions of the HER2 + breast cancer cell line treated with a 212Pb-labelled anti-HER2 conjugate or external beam radiotherapy, anchored a rigorous statistical approach to cell sensitivity and relative biological effectiveness (RBE) estimation. All outcomes were compared to a reference geometric model, which allowed us to determine which aspects are crucial model components for the proper study of the underlying radiobiology.ResultsIn total, 567 cells were measured up to 26 h post-incubation. Realistic cell clustering had a large (2x), and cell geometry a small (16.4% difference) impact on AD, compared to the geometric model. Microdosimetry revealed that more than half of the cells do not receive any dose for most of the tested conditions, greatly impacting cell sensitivity estimates. Including these stochastic effects in the model, resulted in significantly more accurate predictions of surviving fraction and RBE (permutation test; p < .01).ConclusionsThis comprehensive integration of the biological and physical aspects resulted in a more accurate method of cell survival modelling in αRPT experiments. Specifically, including realistic stochastic radiation effects and cell clustering behavior is crucial to obtaining generalizable radiobiological parameters.

Integration of In Vitro and In Vivo Models to Predict Cellular and Tissue Dosimetry of Nanomaterials Using Physiologically Based Pharmacokinetic Modeling.

Nanomaterials (NMs) have been increasingly used in a number of areas, including consumer products and nanomedicine. Target tissue dosimetry is important in the evaluation of safety, efficacy, and potential toxicity of NMs. Current evaluation of NM efficacy and safety involves the time-consuming collection of pharmacokinetic and toxicity data in animals and is usually completed one material at a time. This traditional approach no longer meets the demand of the explosive growth of NM-based products. There is an emerging need to develop methods that can help design safe and effective NMs in an efficient manner. In this review article, we critically evaluate existing studies on in vivo pharmacokinetic properties, in vitro cellular uptake and release and kinetic modeling, and whole-body physiologically based pharmacokinetic (PBPK) modeling studies of different NMs. Methods on how to simulate in vitro cellular uptake and release kinetics and how to extrapolate cellular and tissue dosimetry of NMs from in vitro to in vivo via PBPK modeling are discussed. We also share our perspectives on the current challenges and future directions of in vivo pharmacokinetic studies, in vitro cellular uptake and kinetic modeling, and whole-body PBPK modeling studies for NMs. Finally, we propose a nanomaterial in vitro to in vivo extrapolation via physiologically based pharmacokinetic modeling (Nano-IVIVE-PBPK) framework for high-throughput screening of target cellular and tissue dosimetry as well as potential toxicity of different NMs in order to meet the demand of efficient evaluation of the safety, efficacy, and potential toxicity of a rapidly increasing number of NM-based products.

An In Vitro Dosimetry Tool for the Numerical Transport Modeling of Engineered Nanomaterials Powered by the Enalos RiskGONE Cloud Platform.

A freely available "in vitro dosimetry" web application is presented enabling users to predict the concentration of nanomaterials reaching the cell surface, and therefore available for attachment and internalization, from initial dispersion concentrations. The web application is based on the distorted grid (DG) model for the dispersion of engineered nanoparticles (NPs) in culture medium used for in vitro cellular experiments, in accordance with previously published protocols for cellular dosimetry determination. A series of in vitro experiments for six different NPs, with Ag and Au cores, are performed to demonstrate the convenience of the web application for calculation of exposure concentrations of NPs. Our results show that the exposure concentrations at the cell surface can be more than 30 times higher compared to the nominal or dispersed concentrations, depending on the NPs' properties and their behavior in the cell culture medium. Therefore, the importance of calculating the exposure concentration at the bottom of the cell culture wells used for in vitro arrays, i.e., the particle concentration at the cell surface, is clearly presented, and the tool introduced here allows users easy access to such calculations. Widespread application of this web tool will increase the reliability of subsequent toxicity data, allowing improved correlation of the real exposure concentration with the observed toxicity, enabling the hazard potentials of different NPs to be compared on a more robust basis.

A Monte Carlo study on the effects of a static uniform magnetic field on micro-scale dosimetry of Auger-emitters using Geant4-DNA

In this work, an effort has been made to improve cellular dosimetry of Auger radionuclides through investigating any possible effect of an applied static uniform magnetic field (up to 10 T) on the absorbed doses of ionizing radiation in the micron-sized spherical volumes of liquid water per unit cumulated activity (S-values). The Geant4-DNA extension of the Geant4 Monte Carlo toolkit was employed to simulate the radioactive decay of several theranostic Auger-emitters, including 99mTc, 111In, 123I, 125I, and 201Tl which, distributed within various compartments of a typical spherical cell model. The S-values were calculated in the target regions belonging to both the cell containing the electron source (self-dose) and the second similar cell adjacent to it (cross-dose), with and without the presence of the magnetic field. The results related to the self- and cross-dose cases in the zero magnetic fields were compared to the corresponding MIRD ones. Based on the obtained results for the examined radionuclides and cell dimensions, it can be concluded that the self-dose values are not affected by a static uniform magnetic field, but the cross-dose ones increase slightly (up to 19%) depending on the orientation and strength of the magnetic field.

A feasibility study of the therapeutic application of a mixture of 67/64 Cu radioisotopes produced by cyclotrons with proton irradiation.

Purpose 64Cu and 67Cu radioisotopes have nuclear characteristics suitable for nuclear medicine applications. The production of 64Cu is already well established. However, the production of 67Cu in quantities suitable to conduct clinical trials is more challenging as it leads to the coproduction of other Cu isotopes, in particular 64Cu. The aim of this study is to investigate the possibility of using a CuCl2 solution with a mixture of 67/64Cu radioisotopes for therapeutic purposes, providing an alternative solution for the cyclotron production problem.MethodsCopper radioisotopes activities were calculated by considering proton beam irradiation of the following targets: (i) 70Zn in the energy range 70–45 MeV; (ii) 68Zn in the energy range 70–35 MeV; (iii) a combination of 70Zn (70–55 MeV) and 68Zn (55–35 MeV). The contribution of each copper radioisotope to the human‐absorbed dose was estimated with OLINDA/EXM software using the biokinetic model for CuCl2 published by ICRP 53. The total absorbed dose generated by the 67/64CuCl2 mixture, obtained through different production routes, was calculated at different times after the end of the bombardment (EOB). A simple spherical model was used to simulate tumors of different sizes containing uniformly distributed 67/64Cu mixture and to calculate the absorbed dose of self‐irradiation. The biological damage produced by 67Cu and 64Cu was also evaluated through cellular dosimetry and cell surviving fraction assessment using the MIRDcell code, considering two prostate cancer cell lines with different radiosensitivity.ResultsThe absorbed dose to healthy organs and the effective dose (ED) per unit of administered activity of 67CuCl2 are higher than those of 64CuCl2. Absorbed dose values per unit of administered activity of 67/64CuCl2 mixture increase with time after the EOB because the amount of 67Cu in the mixture increases. Survival data showed that the biological damage caused per each decay of 67Cu is greater than that of 64Cu, assuming that radionuclides remain accumulated in the cell cytoplasm. Sphere model calculations demonstrated that 64Cu administered activity must be about five times higher than that of 67Cu to obtain the same absorbed dose for tumor mass between 0.01 and 10 g and about 10 times higher for very small spheres. Consequently, the 64CuCl2‐absorbed dose to healthy organs will reach higher values than those of 67CuCl2. The supplemental activity of the 67/64CuCl2 mixture, required to get the same tumor‐absorbed dose produced by 67CuCl2, triggers a dose increment (DI) in healthy organs. The waiting time post‐EOB necessary to keep this DI below 10% (t 10%) depends on the irradiation methods employed for the production of the 67/64CuCl2 mixture.ConclusionsA mixture of cyclotron produced 67/64Cu radioisotopes proved to be an alternative solution for the therapeutic use of CuCl2 with minimal DI to healthy organs compared with pure 67Cu. Irradiation of a 70Zn+68Zn target in the 70–35 MeV proton energy range for 185 h appears to be the best option from among all the production routes investigated, as it gives the maximum amount of activity, the shortest t 10% (10 h), and less than 1% of 61Cu and 60Cu impurities.

Comparison between 177Lu-iPSMA and 225Ac-iPSMA dosimetry at a cellular level in an animal bone metastasis model

The recent use of prostate-specific membrane antigen as a biological target have improved the theragnostic approach to prostate and other types of cancer. Radiopharmaceuticals based on PSMA inhibitors radiolabeled with beta emitters as Lutetium-177 have demonstrated remarkable efficacy and safety, however, their clinical evaluation have also shown that therapeutic response of bone located metastases is poorer than that presented by soft tissue lesions. These observations conducted to the development and study at different levels of PSMA-targeting alpha-particle therapy exhibiting effective and promising antitumor activity. However, some aspects of the use of alpha emitters such as cellular dosimetry should be considered before applying them safely. The aim of the present work was to compare and calculate the absorbed dose of 177Lu-iPSMA and 225Ac-iPSMA using an animal bone metastasis model and experimental data obtained from cellular fractionation. The number of disintegrations and the dose factors for the theragnostic iPSMA pair, molecule that can be radiolabeled with 177Lu or 225Ac, were determined based on MIRD methodology, and used to calculate the absorbed dose to cell nucleus. A five times difference between 225Ac-iPSMA and 177Lu-iPSMA average dose rate to the tumor was calculated, being 2.3 ± 0.037 for the first and 0.5 ± 0.018 Gy for the second, both for each activity unit (MBq) administered.

Development of patient-specific 3D models from histopathological samples for applications in radiation therapy.

The biological effects of ionizing radiation depend on the tissue, tumor type, radiation quality, and patient-specific factors. Inter-patient variation in cell/nucleus size may influence patient-specific dose response. However, this variability in dose response is not well investigated due to lack of available cell/nucleus size data. The aim of this study was to develop methods to derive cell/nucleus size distributions from digital images of 2D histopathological samples and use them to build digital 3D models for use in cellular dosimetry. Nineteen of sixty hematoxylin and eosin stained lung adenocarcinoma samples investigated passed exclusion criterion to be analyzed in the study. A difference of gaussians blob detection algorithm was used to identify nucleus centers and quantify cell spacing. Hematoxylin content was measured to determine nucleus radius. Pouring simulations were conducted to generate one-hundred 3D models containing volumes of equivalent cell spacing and nuclei radius to those in histopathological samples. The nuclei radius distributions of non-tumoral and cancerous regions appearing in the same slide were significantly different (p<0.01) in all samples analyzed. The median nuclear-cytoplasmic ratio was 0.36 for non-tumoral cells and 0.50 for cancerous cells. The average cellular and nucleus packing densities in the 3D models generated were 65.9% (SD: 1.5%) and 13.3% (SD: 0.3%) respectively. Software to determine cell spacing and nuclei radius from histopathological samples was developed. 3D digital tissue models containing volumes with equivalent cell spacing, nucleus radius, and packing density to cancerous tissues were generated.

Geant4 Modeling of Cellular Dosimetry of 188Re: Comparison between Geant4 Predicted Surviving Fraction and Experimentally Surviving Fraction Determined by MTT Assay.

Background: The importance of cellular dosimetry in both diagnostic and radiation therapy is becoming increasingly recognized.Objective: This study aims to compare surviving fractions, which were predicted using Geant4 and contained three types of cancer cell lines exposed to 188Re with the experimentally surviving fraction determined by MTT assay.Material and Methods: In this comparative study, Geant4 was used to simulate the transport of electrons emitted by 188Re from the cell surface, cytoplasm, nucleus or medium around the cells. The nucleus dose per decay (S-value) was computed for models of single cell and random monolayer cell. Geant4-computed survival fraction (SF) of cancer cells exposed to 188Re was compared with the experimental SF values of MTT assay.Results: For single cell model, Geant4 S-values of nucleus-to-nucleus were consistent with values reported by Goddu et al. (ratio of S-values by analytical techniques vs. Geant4 = 0.811–0.975). Geant4 S-values of cytoplasm and cell surface to nucleus were relatively comparable to the reported values (ratio =0.914–1.21). For monolayer model, the values of SCy→N and SCS→N, were greater compared to those for model of single cell (2%–25% and 4%–38% were larger than single cell, respectively). The Geant4 predicted SF for monolayer MCF7, HeLa and A549 cells was in agreement with the experimental data in 10μCi activity (relative error of 2.29%, 2.69% and 2.99%, respectively).Conclusion: Geant4 simulation with monolayer cell model showed the highest accuracy in predicting the SF of cancer cells exposed to homogeneous distribution of 188Re in the medium.

Cellular dosimetry of 197Hg, 197mHg and 111In: comparison of dose deposition and identification of the cell and nuclear membrane as important targets

Purpose To examine the reliability to model cellular S-values for the Auger electron (AE) emitters, 111In, 197Hg and 197mHg with MCNP6 and their relative dose deposition in subcellular targets. Methods A model cell was defined as four concentric spheres consisting of the nucleus (N), cytoplasm (Cy), cell and nuclear membranes (CM, NM) in which radionuclides distributed homogeneously. The transport of AE, conversion electrons and photons were simulated by MCNP6 to calculate cellular S values (SN←CM, SN←Cy, SN←NM, SN←N, SCM←CM, SNM←NM). SN←CM, SN←Cy and SN←N were also calculated with MIRDcell. Results MIRDcell and MCNP6-calculated SN←N were in excellent agreement, but a slight discrepancy on SN←Cy and SN←CM was observed. The ratios of SCM←CM or SNM←NM vs. SN←N were 9.7–51.0 or 10.5–37.4, 7.9–41.8 or 8.4–31.8 and 7.2–36.9 or 8.0–28.1 for 111In, 197Hg, 197mHg, respectively. The mean S(197Hg)/S(111In) and S(197mHg)/S(111In) were 2.5 ± 0.5 and 2.5 ± 0.6, respectively. Conclusions Cellular S-values were reliably calculated with MCNP6. 197Hg and 197mHg deposit two-fold more doses than 111In at the subcellular scale. All AE emitters deposit a higher self-dose in the CM and NM than in the N, which warrants studies on the effects of targeting the CM and NM by AE emitters.

Cellular S values in spindle-shaped cells: a dosimetry study on more realistic cell geometries using Geant4-DNA Monte Carlo simulation toolkit

Cellular dosimetry plays a crucial role in radiobiology and evaluation of the relative merits of radiopharmaceuticals used for targeted radionuclide therapy. The present study aims to investigate the effects of various cell geometries on dosimetric characteristics of several Auger emitters distributed in different subcellular compartments using Monte Carlo simulation. The Geant4-DNA extension of the Geant4 Monte Carlo simulation toolkit was employed to calculate the mean absorbed dose per unit cumulated activity (S value) for different subcellular distributions of several Auger electron-emitting theranostic radionuclides including 99mTc, 111In, 123I, 125I, and 201Tl. The simulations were carried out in various single-cell models of liquid water including spherical, ellipsoidal, spherical spindle, and ellipsoidal spindle cell models. The latter two models which are generalized from the first two models were inspired by the morphologies of spindle-shaped (fusiform) cells, and were developed to provide more realistic modeling of this common geometry observed in many healthy and cancerous cells. Evaluation of the S values calculated for the examined cell models reveals that the differences are small (less than 9%) for the cell ← cell, cell ← cell surface, and nucleus ← nucleus source-target combinations. However, moderate discrepancies are seen (up to 28%) when the nucleus is considered as the target, as well as the radioactivity is either internalized into the cytoplasm or bound to the cell membrane. The findings of the present work suggest that the assumption of spherical cell geometry may provide reasonably accurate estimates of the cellular/nuclear dose for the considered Auger emitters, even for spindle-shaped cells. Of course, this approximation should be used with caution for the nucleus ← cytoplasm and nucleus ← cell surface configurations, since the S-value sensitivity to the cell geometry is somewhat significant in these cases.

Cellular dosimetry of [177Lu]Lu-DOTA-[Tyr3]octreotate radionuclide therapy: the impact of modeling assumptions on the correlation with in vitro cytotoxicity

BackgroundSurvival and linear-quadratic model fitting parameters implemented in treatment planning for targeted radionuclide therapy depend on accurate cellular dosimetry. Therefore, we have built a refined cellular dosimetry model for [177Lu]Lu-DOTA-[Tyr3]octreotate (177Lu-DOTATATE) in vitro experiments, accounting for specific cell morphologies and sub-cellular radioactivity distributions.MethodsTime activity curves were measured and modeled for medium, membrane-bound, and internalized activity fractions over 6 days. Clonogenic survival assays were performed at various added activities (0.1–2.5 MBq/ml). 3D microscopy images (stained for cytoplasm, nucleus, and Golgi) were used as reference for developing polygonal meshes (PM) in 3DsMax to accurately render the cellular and organelle geometry. Absorbed doses to the nucleus per decay (S values) were calculated for 3 cellular morphologies: spheres (MIRDcell), truncated cone-shaped constructive solid geometry (CSG within MCNP6.1), and realistic PM models, using Geant4-10.03. The geometrical set-up of the clonogenic survival assays was modeled, including dynamic changes in proliferation, proximity variations, and cell death. The absorbed dose to the nucleus by the radioactive source cell (self-dose) and surrounding source cells (cross-dose) was calculated applying the MIRD formalism. Finally, the correlation between absorbed dose and survival fraction was fitted using a linear dose-response curve (high α/β or fast sub-lethal damage repair half-life) for different assumptions, related to cellular shape and localization of the internalized fraction of activity.ResultsThe cross-dose, depending on cell proximity and colony formation, is a minor (15%) contributor to the total absorbed dose. Cellular volume (inverse exponential trend), shape modeling (up to 65%), and internalized source localization (up to + 149% comparing cytoplasm to Golgi) significantly influence the self-dose to nucleus. The absorbed dose delivered to the nucleus during a clonogenic survival assay is 3-fold higher with MIRDcell compared to the polygonal mesh structures. Our cellular dosimetry model indicates that 177Lu-DOTATATE treatment might be more effective than suggested by average spherical cell dosimetry, predicting a lower absorbed dose for the same cellular survival. Dose-rate effects and heterogeneous dose delivery might account for differences in dose-response compared to x-ray irradiation.ConclusionOur results demonstrate that modeling of cellular and organelle geometry is crucial to perform accurate in vitro dosimetry.

Development of a dosimetric model for in vitro labelled cells with β+ emitters in PET tracking studies

Positron emission tomography (PET) offers an effective method for tracking β+ emitters-labeled cells in vivo. However, in vitro high labelling activities used may cause cell damage or death. Our understanding of the impact of such procedure remains limited by the fact that the biological effects are usually linked to the activity per cell rather than the absorbed dose. To assess the dose delivered to the cells during the radiolabelling, a multi-cellular dosimetry computational tool was developed, allowing the study of two key parameters: the cell density and the labelling efficiency. Through a hybrid method based on Monte Carlo simulations (MCNP6 code) and an analytical approach implemented in Python, the mean absorbed dose received by a target cell was calculated for distributions with a very large number of cells—up to hundreds of millions. An advanced investigation of in vitro cell labelling with β-emitting radionuclides was carried out via (i) a systematic study of the effects of the labelling parameters on the cell absorbed dose for 18F, 64Cu and 68Ga, and (ii) a quantitative comparison between cellular and conventional dosimetry. The results provided a thorough analysis of how the dose (self, cross and extracellular medium dose contributions) varies with the initial labelling parameters selected and highlighted the conditions where the cellular dosimetry is required over the conventional dosimetry. The dosimetric model was finally applied to real conditions of 18F-FDG labelling on the basis of eight reported studies. The results showed that similar activity per cell can lead to significantly different absorbed dose and pointed out differences between cellular and conventional dosimetry up to a factor of 5.

Determination of silver nanoparticle dose in vitro

An important issue for interpreting in vitro nanomaterial testing is quantifying the dose delivered to target cells. Considerations include the concentration added to the culture, the proportion of the applied dose that interacts with the target cells, and the amount that is eventually absorbed by the target cells. Rapid and efficient techniques are needed to determine delivered doses. Previously, we demonstrated that TiO2 and silver nanoparticles (AgNP) were absorbed by cells in a dose dependent manner between 1 μg/ml and 30 μg/ml and were detected in cells by light scatter using a flow cytometer. Here, we compare four potential indices of the dose of AgNP to cells, including: inductively coupled plasma - mass spectrometry (ICP-MS); flow cytometry side scatter (SSC); and amount of silver deposited to the cell layer as estimated with both an integrated Volumetric Centrifugation Method - In Vitro Sedimentation, Diffusion and Dosimetry Model (VCM-ISDD) and a Distorted Grid (DG) model. A retinal pigment epithelial cell line was exposed to 20 nm or 75 nm citrate-coated AgNP for 24 h. The relationships between particle sizes and internalized doses varied according to the dose metric. Twenty-four hours after exposure, the cell layer contained a greater mass of silver when treated with 75 nm AgNP than with 20 nm AgNP. When the dose was expressed as the number of particles or as the total surface area of absorbed particles, however, the reverse was true; the dose to the cells was higher after exposure to 20 than 75 nm AgNP. Flow cytometry SSC increased with dose for both sizes of AgNP, and was correlated with Ag in cells measured by ICP-MS. The rate of SSC increase was greater for 75 than for 20 nm AgNP, suggesting it could be used as an indicator of cellular dose after accounting for particle size and composition. Silver was detected by ICP-MS in re-suspended supernates of the isolated cell layer suggested that not all the silver deposited to the cell layer was absorbed by the cells. Both the VCM-ISDD and DG models estimated the proportion of Ag deposited to the cellular layer, which in both cases was greater than the amount of silver in the cells measured by ICP-MS. Modeled deposition more closely compared to the total Ag deposition by ICP-MS, i.e. mass of silver in the cells plus the resuspended, unabsorbed Ag from the cell layer. ICP-MS indicated the mass of silver in cells from AgNP treatment, but not whether the Ag was in the form of particles or dissolved ions. Deposition models predicted the amount of AgNP deposited to the cell layer, but not cellular uptake. Flow cytometry SSC was correlated to cellular uptake of particle-form AgNP and could be calibrated against ICP-MS to indicate mass of cellular uptake. Therefore, a combination of approaches may be required to accurately understand cellular dosimetry of in vitro nanotoxicology experiments. In summary, cellular dosimetry is an important consideration for nanotoxicology experiments, and not necessarily related to the applied dose.

Computational assessment of the cellular dosimetry and microdosimetry of the gadolinium electrons released during neutron capture therapy

One of the challenges in radiobiology is the determination of the dose of Auger electrons in cellular-molecular dimensions and evaluation of their relative biological effectiveness (RBE). In this study, we try to assess the cellular dosimetry and microdosimetry of the internal conversion (IC) and Auger electrons released during Gadolinium-157 neutron capture reaction (157GdNCR) using the low-energy physics models included in Geant4 toolkit and MCNPX code. The results show that the MCNPX code does not have reliable results for the cellular dosimetry of low-energy electrons compared to the validated low-energy Geant4 physics models. Although the use of the Geant4-DNA models is a necessity in microdosimetric calculations, due to the time-consuming run of the Geant4-DNA models in the cellular dosimetry calculations, the use of Geant4-Penelope and Geant4-Livermore models can lead to the quick and reliable results. The results show that, with appropriate selection of physical parameters, the results of small-scale microdosimetry calculated using the CH models in Geant4 approach the results of Geant4-DNA. With regard to the cellular dosimetry results, the amount of the absorbed dose in the nucleus of a cell is strongly dependent on the distribution of Gd in the different compartments of the cell. The cross-dose in the nucleus from the Gd electrons released in adjacent cells is comparable to the self-dose of the nucleus to nucleus and noteworthy to destroy the quiescent cancer cells that do not accumulate Gd. The microdosimetry results show that, unlike the absorbed dose in DNA, RBE is not much dependent on the Gd distribution relative to the DNA within the molecular dimensions. If Gd is distributed at the center of the DNA, the absorbed dose of Gd electrons in the DNA (246 kGy) with a quality-factor-equivalent RBE of 7.93 is sufficiently large for the double-strand breaks (DSBs) of the DNA.

Determining what really counts: modeling and measuring nanoparticle number concentrations

This paper describes a comprehensive investigation of particle number concentrations including a multi-method comparison, theoretical modeling, and cellular dosimetry.

I205] Novel developments in ion beam dosimetry

Dosimetry for ion beams has to account for a number aspects differing from photon and electron beam dose measurements and has not yet reached the same level of accuracy. Also, procedures for quality assurance of complex beam deliveries can be very time-consuming. After touching on the specifics of ion beams, this presentation will give an overview on recent achievements in reference dosimetry, especially by calorimetric measurements. After that it will highlight new developments for non-reference situations such as time-efficient ionization chamber arrays. In a last part, the potential role of track detectors for cellular dosimetry will be discussed.