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The aim of this study was to build a simulation framework to evaluate the number of DNA double-strand breaks (DSBs) induced by in vitro targeted radionuclide therapy (TRT). This work represents the first step toward exploring underlying biologic mechanisms and the influence of physical and chemical parameters to enable a better response prediction in patients. We used this tool to characterize early DSB induction by 177Lu-DOTATATE, a commonly used TRT for neuroendocrine tumors. Methods: A multiscale approach was implemented to simulate the number of DSBs produced over 4 h by the cumulated decays of 177Lu distributed according to the somatostatin receptor binding. The approach involves 2 sequential simulations performed with Geant4/Geant4-DNA. The radioactive source is sampled according to uptake experiments on the distribution of activities within the medium and the planar cellular cluster, assuming instant and permanent internalization. A phase space is scored around the nucleus of the central cell. Then, the phase space is used to generate particles entering the nucleus containing a multiscale description of the DNA in order to score the number of DSBs per particle source. The final DSB computations are compared with experimental data, measured by immunofluorescent detection of p53-binding protein 1 foci. Results: The probability of electrons reaching the nucleus was significantly influenced by the shape of the cell compartment, causing a large variance in the induction pattern of DSBs. A significant difference was found in the DSBs induced by activity distributions in cell and medium, as is explained by the specific energy (z) distributions. The average number of simulated DSBs was 14 DSBs per cell (range, 7-24 DSBs per cell), compared with 13 DSBs per cell (range, 2-30 DSBs per cell) experimentally determined. We found a linear correlation between the mean absorbed dose to the nucleus and the number of DSBs per cell: 0.014 DSBs per cell mGy-1 for internalization in the Golgi apparatus and 0.017 DSBs per cell mGy-1 for internalization in the cytoplasm. Conclusion: This simulation tool can lead to a more reliable absorbed-dose-to-DNA correlation and help in prediction of biologic response.

International audience; Research dealing with a better understanding of the origin and mechanisms behind deleterious effects of radiation therapies on healthy tissues is a crucial topic in radiation protection. These side effects can result from damage to the nuclear DN molecule of healthy cells exposed to ionizing radiation during treatment. In order to better describe the molecular mechanisms underlying these undesirable effects, our laboratory is developing a simulation chain to calculate early radiation-induced damage to DNA for different beam qualities. Studies at this scale require a nanodosimetric description of energy deposits, enabled by the Geant4-DNA Monte-Carlo toolkit, coupled to DNA geometrical models with molecular precision. The current version of this simulation chain allows a realistic modeling of the topology of DNA damage (number of DSBs, complexity, spatial distribution) at the cellular scale. Up to now the geometries of the cell genome used in the simulation take into account chromatin compaction by implementing 52% euchromatin and 48% heterochromatin, distributed randomly along the genome which makes it possible to account for experimental observations.To improve the quality of these DNA damage results, a more realistic nuclear geometric model is presented. It is based on the isochoric biological model carrying out a mapping of the genome, by segmenting it into portions of 1 Mbp in our application. Each of these segments is then classified into one of five isochoric families (L1, L2, H1, H2, H3) based on the ratio of CG base pairs it contains and related to different degrees of chromatin fiber compaction. To evaluate the influence of these new geometric models on the topology of radiation-induced DNA damage, simulations are performed for perpendicular irradiations of protons (4.28 keV.μm-1, 19.51 keV.μm-1, 43.24 keV.μm-1).The DNA damage results obtained for a cell using the new geometric isochoric model are compared with those obtained previously with a random distribution of chromatin compaction respecting the same overall heterochromatin / euchromatin ratio. This comparison indicates that the organization of chromatin fiber compaction in accordance with the isochore theory of our new model has an impact via different mechanisms on the number and type of calculated damages.

International audience; Abstract: High‐Z metallic nanoparticles (NPs) are new players in the therapeutic arsenal againstcancer, especially radioresistant cells. Indeed, the presence of these NPs inside malignant cells isbelieved to enhance the effect of ionizing radiation by locally increasing the dose deposition. In thiscontext, the potential of platinum nanoparticles (PtNPs) as radiosensitizers was investigated in twobreast cancer cell lines, T47D and MDA‐MB‐231, showing a different radiation sensitivity. PtNPswere internalized in the two cell lines and localized in lysosomes and multivesicular bodies. Anal‐yses of cell responses in terms of clonogenicity, survival, mortality, cell‐cycle distribution, oxidativestress, and DNA double‐strand breaks did not reveal any significant enhancement effect when cellswere pre‐exposed to PtNPs before being irradiated, as compared to radiation alone. This result isdifferent from that reported in a previous study performed, under the same conditions, on cervicalcancer HeLa cells. This shows that the efficacy of radio‐enhancement is strongly cell‐type‐depend‐ent. Simulation of the early stage ionization processes, taking into account the irradiation character‐istics and realistic physical parameters in the biological sample, indicated that PtNPs could weaklyincrease the dose deposition (by 3%) in the immediate vicinity of the nanoparticles. Some featuresthat are potentially responsible for the biological effect could not be taken into account in the simu‐lation. Thus, chemical and biological effects could explain this discrepancy. Forinstance, we showedthat, in these breast cancer cell lines, PtNPs exhibited ambivalent redox properties, with an antiox‐idant potential which could counteract the radio‐enhancement effect. This work shows that the ef‐ficacy of PtNPs for enhancing radiation effects is strongly cell‐dependent and that no effect is ob‐served in the case of the breast cancer cell lines T47D and MDA‐MB‐231. Thus, more extensive ex‐periments using other relevant biological models are needed in order to evaluate such combinedstrategies, since several clinical trials have already demonstrated the success of combiningnanoagents with radiotherapy in the treatment of a range of tumor types

International audience; The Geant4-DNA extension of the Geant4 Monte Carlo simulation toolkit for particle-matter interactions has beendeveloped to provide the scientific community with the first open source platform for simulating the early andlate biological effects of ionizing radiation on DNA. In particular, it offers the possibility to simulate radiolysis ofliquid water. This article presents the approaches used.; L’extension Geant4-DNA de l’outil de simulation Monte Carlo des interactions particules-matière Geant4 a étédéveloppée pour fournir à la communauté scientifique la première plateforme ouverte pour simuler les effetsbiologiques précoces et tardifs des radiations ionisantes sur l’ADN. Elle offre notamment la possibilité de simulerla radiolyse de l’eau liquide. Cet article présente les approches utilisées.

PURPOSE: This work reports, in melanoma models, the theranostic potential of ICF15002 as a single fluorinated and iodinated melanin-targeting compound. METHODS: Studies were conducted in the murine syngeneic B16BL6 model and in the A375 and SK-MEL-3 human xenografts. ICF15002 was radiolabeled with fluorine-18 for positron emission tomography (PET) imaging and biodistribution, with iodine-125 for metabolism study, and iodine-131 for targeted radionuclide therapy (TRT). TRT efficacy was assessed by tumor volume measurement, with mechanistics and dosimetry parameters being determined in the B16BL6 model. Intracellular localization of ICF15002 was characterized by secondary ion mass spectrometry (SIMS). RESULTS: PET imaging with [18F]ICF15002 evidenced tumoral uptake of 14.33 ± 2.11%ID/g and 4.87 ± 0.93%ID/g in pigmented B16BL6 and SK-MEL-3 models, respectively, at 1 hour post inoculation. No accumulation was observed in the unpigmented A375 melanoma. SIMS demonstrated colocalization of ICF15002 signal with melanin polymers in melanosomes of the B16BL6 tumors. TRT with two doses of 20 MBq [131I]ICF15002 delivered an absorbed dose of 102.3 Gy to B16BL6 tumors, leading to a significant tumor growth inhibition [doubling time (DT) of 2.9 ± 0.5 days in treated vs 1.8 ± 0.3 in controls] and a prolonged median survival (27 days vs 21 in controls). P53S15 phosphorylation and P21 induction were associated with a G2/M blockage, suggesting mitotic catastrophe. In the human SK-MEL-3 model, three doses of 25 MBq led also to a DT increase (26.5 ± 7.8 days vs 11.0 ± 3.8 in controls) and improved median survival (111 days vs 74 in controls). CONCLUSION: Results demonstrate that ICF15002 fulfills suitable properties for bimodal imaging/TRT management of patients with pigmented melanoma.

The objective of this work was to study the differences in terms of early biological effects that might exist between different X-rays energies by using a mechanistic approach. To this end, radiobiological experiments exposing cell monolayers to three X-ray energies were performed in order to assess the yields of early DNA damage, in particular of double-strand breaks (DSBs). The simulation of these irradiations was set in order to understand the differences in the obtained experimental results. Hence, simulated results in terms of microdosimetric spectra and early DSB induction were analyzed and compared to the experimental data. Human umbilical vein endothelial cells (HUVECs) were irradiated with 40, 220 kVp, and 4 MV X-rays. The Geant4 Monte Carlo simulation toolkit and its extension Geant4-DNA were used for the simulations. Microdosimetric calculations aiming to determine possible differences in the variability of the energy absorbed by the irradiated cell population for those photon spectra were performed on 10,000 endothelial cell nuclei representing a cell monolayer. Nanodosimetric simulations were also carried out using a computation chain that allowed the simulation of physical, physico-chemical, and chemical stages on a single realistic endothelial cell nucleus model including both heterochromatin and euchromatin. DNA damage was scored in terms of yields of prompt DSBs per Gray (Gy) and per giga (109) base pair (Gbp) and DSB complexity was derived in order to be compared to experimental data expressed as numbers of histone variant H2AX (&gamma, H2AX) foci per cell. The calculated microdosimetric spread in the irradiated cell population was similar when comparing between 40 and 220 kVp X-rays and higher when comparing with 4 MV X-rays. Simulated yields of induced DSB/Gy/Gbp were found to be equivalent to those for 40 and 220 kVp but larger than those for 4 MV, resulting in a relative biological effectiveness (RBE) of 1.3. Additionally, DSB complexity was similar between the considered photon spectra. Simulated results were in good agreement with experimental data obtained by IRSN (Institut de radioprotection et de s&ucirc, ret&eacute, nucl&eacute, aire) radiobiologists. Despite differences in photon energy, few differences were observed when comparing between 40 and 220 kVp X-rays in microdosimetric and nanodosimetric calculations. Nevertheless, variations were observed when comparing between 40/220 kVp and 4 MV X-rays. Thanks to the simulation results, these variations were able to be explained by the differences in the production of secondary electrons with energies below 10 keV.

Understanding the fundamental mechanisms involved in the induction of biological damage by ionizing radiation remains a major challenge of todays radiobiology research. The Monte Carlo simulation of physical, physicochemical and chemical processes involved may provide a powerful tool for the simulation of early damage induction. The Geant4-DNA extension of the general purpose Monte Carlo Geant4 simulation toolkit aims to provide the scientific community with an open source access platform for the mechanistic simulation of such early damage. This paper presents the most recent review of the Geant4-DNA extension, as available to Geant4 users since June 2015 (release 10.2 Beta). In particular, the review includes the description of new physical models for the description of electron elastic and inelastic interactions in liquid water, as well as new examples dedicated to the simulation of physicochemical and chemical stages of water radiolysis. Several implementations of geometrical models of biological targets are presented as well, and the list of Geant4-DNA examples is described.