A comparison between track-structure, condensed-history Monte Carlo simulations and MIRD cellularS-values

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

BACKGROUND

Radiopharmaceutical 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.

METHODS

The 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 (t_1/2 = 4.80 m) and ²¹³Bi (t_1/2 = 45.6 m).

RESULTS

For 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.

CONCLUSIONS

Our 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.

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 (S_N←CM, S_N←Cy, S_N←NM, S_N←N, S_CM←CM, S_NM←NM). S_N←CM, S_N←Cy and S_N←N were also calculated with MIRDcell.

RESULTS

MIRDcell and MCNP6-calculated S_N←N were in excellent agreement, but a slight discrepancy on S_N←Cy and S_N←CM was observed. The ratios of S_CM←CM or S_NM←NM vs. S_N←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.

Towards the characterization of neutron carcinogenesis through direct action simulations of clustered DNA damage

Neutron exposure poses a unique radiation protection concern because neutrons have a large, energy-dependent relative biological effectiveness (RBE) for stochastic effects. Recent computational studies on the microdosimetric properties of neutron dose deposition have implicated clustered DNA damage as a likely contributor to this marked energy dependence. So far, publications have focused solely on neutron RBE for inducing clusters of DNA damage containing two or more DNA double strand breaks (DSBs). In this study, we have conducted a novel assessment of neutron RBE for inducing all types of clustered DNA damage that contain two or more lesions, stratified by whether the clusters contain DSBs (complex DSB clusters) or not (non-DSB clusters). This assessment was conducted for eighteen initial neutron energies between 1 eV and 10 MeV as well as a reference radiation of 250 keV x-rays. We also examined the energy dependence of cluster length and cluster complexity because these factors are believed to impact the DNA repair process. To carry out our investigation, we developed a user-friendly TOPAS-nBio application that includes a custom nuclear DNA model and a novel algorithm for recording clustered DNA damage. We found that neutron RBE for inducing complex DSB clusters exhibited similar energy dependence to the canonical neutron RBE for stochastic radiobiological effects, at multiple depths in human tissue. Qualitatively similar results were obtained for non-DSB clusters, although the quantitative agreement was lower. Additionally we identified a significant neutron energy dependence in the average length and complexity of clustered lesions. These results support the idea that many types of clustered DNA damage contribute to the energy dependence of neutron RBE for stochastic radiobiological effects and imply that the size and constituent lesions of individual clusters should be taken into account when modeling DNA repair. Our results were qualitatively consistent for (i) multiple radiation doses (including a low-dose 0.1 Gy irradiation), (ii) variations in the maximal lesion separation distance used to define a cluster, and (iii) two distinct collections of physics models used to govern particle transport. Our complete TOPAS-nBio application has been released under an open-source license to enable others to independently validate our work and to expand upon it.

Investigating the feasibility of TOPAS-nBio for Monte Carlo track structure simulations by adapting GEANT4-DNA examples application

Purpose.The purpose of this work is to investigate the feasibility of TOPAS-nBio for track structure simulations using tuple scoring and ROOT/Python-based post-processing.Materials and methods.There are several example applications implemented in GEANT4-DNA demonstrating track structure simulations. These examples are not implemented by default in TOPAS-nBio. In this study, the tuple scorer was used to re-simulate these examples. The simulations contained investigations of different physics lists, calculation of energy-dependent range, stopping power, mean free path andW-value. Additionally, further applications of the TOPAS-nBio tool were investigated, focusing on physical interactions and deposited energies of electrons with initial energies in the range of 10-60 eV, not covered in the recently published GEANT4-DNA simulations. Low-energetic electrons are currently of great interest in the radiobiology research community due to their high effectiveness towards the induction of biological damage.Results.The quantities calculated with TOPAS-nBio show a good agreement with the simulations of GEANT4-DNA with deviations of 5% at maximum. Thus, we have presented a feasible way to implement the example applications included in GEANT4-DNA in TOPAS-nBio. With the extended simulations, an insight could be given, which further tracking information can be gained with the track structure code and how cross sections and physics models influence a particle's fate.Conclusion.With our results, we could show the potentials of applying the tuple scorer in TOPAS-nBio Monte Carlo track structure simulations. Using this scorer, a large amount of information about the track structure can be accessed, which can be analyzed as preferred after the simulation.

The effect of the nucleus random location on the cellular S-values - Based on Geant4-DNA

INTRODUCTION

The nucleus is the most crucial target in cell micro-dosimetry. At cell division time, cells do not have concentric geometry synchronously. This issue will be more essential for the low-energy electron emitters. This study investigates the variety of mean absorbed dose (S-value) in the non-concentric cell-nucleus model and random nucleus location within the cell.

METHODS

The S-values were calculated by Geant4-DNA for the cell and nucleus with different radius (with the R_C/R_N ratio = 1.2, 2, 3) and the cell geometry contains nuclei with varying positions inside the cell. Two important components, cytoplasm to the nucleus (N←Cy) and the cell surface to the nucleus (N←Cs) are considered in this work for mono energetic electrons (10-100 keV). To eliminate the effect of the nucleus position (during cell division) on the S-value, the nucleus location in each run was randomly selected inside the cell to represent the cell in a floating state.

RESULTS

As the nucleus becomes closer to the cell membrane the differences are more noticeable especially for electrons with energy less than 20 keV as for R_N/R_C = 1.2, 2, and 3 about 18, 70, and 200%, respectively.

CONCLUSION

Due to the variable position of the nucleus in cell division, using a random place defined in Geant4, the calculations are getting closer to the reality while there is not such possibility for analytical method used by MIRD.

The Effect of Nucleus Size on the Cell Dose in Targeted Radionuclide Therapy - A Monte Carlo Study

Background:

Nowadays, the use of radiopharmaceuticals in medicine is unavoidable. Depending on the distribution of the radiopharmaceutical in the cells, the nucleus absorbed dose changes by the variations in their geometry size. Therefore, this study aims to investigate the S-value by the variation of nucleus size using Geant4 toolkit.

Methods:

Two spherical cells with a variety of nucleus size have been considered as the cancerous cell. Monoenergetic electrons ranging from 5 to 300 keV are distributed uniformly. The S-value for four target-source components (including Nucleus←Cytoplasm, Nucleus←Cell surface, Nucleus←Nucleus, and Nucleus←Nucleus surface) is computed and plotted. Then, the obtained data are compared with analytical Medical Internal Radiation Dose (MIRD) data.

Results:

In Nucleus←Cytoplasm compartment for electrons below 10 keV, obtained S-values show a slight decrease for the nucleus in the radii of around half of the cell radius and then S-values increase with the increase in the nucleus radii. In the S-value of Nucleus←Cell surface, for all electron energy levels, a slight decrease observed with the increase of nucleus radii. For Nucleus←Nucleus and Nucleus←Nucleus surface cases, with an increase in the size of the cell nucleus, a sharp reduction in the S-values is detected.

Conclusion:

It can be concluded that for the beta emitters with low-energy radiation (<40 keV), the S-value is heavily dependent on the nucleus size which may affect the treatment of small tumors. While for the beta emitters with higher-energy radiation (>100 keV), the size of the nucleus is not very noticeable in the induced S-value.

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.