Microdosimetry and nanodosimetry for internal emitters

The European Radiation Dosimetry Group-a 40 year success story

The European Radiation Dosimetry Group (EURADOS) was founded in 1982. Since then, the group has continuously developed and is currently a network of 80 institutions and more than 600 individual scientists across Europe, including exchange with the scientific community outside of Europe. EURADOS supports research and development of dosimetry and harmonising dosimetric practices. This paper describes the major milestones in the history of the organization. It starts from the very beginning when the idea was born and describes periods during which the role and strategy of the network had to be defined, elaborated and refined. Finally, it ends to date where EURADOS appears as an independent self-sustainable association, which is a reliable partner for various international organisations in radiation research and radiation protection. Major activities of EURADOS are highlighted such as (1) establishment and coordination of Working Groups, (2) regular organization of dosimetric intercomparisons for quality assurance of dosimetry procedures, (3) development and organization of education and training events, and (4) contributions towards the development of strategic and integrated radiation research in Europe.

Estimation of relative biological effectiveness of 225Ac compared to 177Lu during [225Ac]Ac-PSMA and [177Lu]Lu-PSMA radiopharmaceutical therapy using TOPAS/TOPAS-nBio/MEDRAS

AIM

Over recent years, [225Ac]Ac-PSMA and [177Lu]Lu-PSMA radiopharmaceutical therapy have evolved as a promising treatment option for advanced prostate cancer. Especially for alpha particle emitter treatments, there is still a need for improving dosimetry, which requires accurate values of relative biological effectiveness (RBE). To achieve that, consideration of DNA damages in the cell nucleus and knowledge of the energy deposition in the location of the DNA at the nanometer scale are required. Monte Carlo particle track structure simulations provide access to interactions at this level. The aim of this study was to estimate the RBE of 225Ac compared to 177Lu. The initial damage distribution after radionuclide decay and the residual damage after DNA repair were considered.

METHODS

This study employed the TOol for PArtcile Simulation (TOPAS) based on the Geant4 simulation toolkit. Simulation of the nuclear DNA and damage scoring were performed using the TOPAS-nBio extension of TOPAS. DNA repair was modeled utilizing the Python-based program MEDRAS (Mechanistic DNA Repair and Survival). Five different cell geometries of equal volume and two radionuclide internalization assumptions as well as two cell arrangement scenarios were investigated. The radionuclide activity (number of source points) was adopted based on SPECT images of patients undergoing the above-mentioned therapies.

RESULTS

Based on the simulated dose-effect curves, the RBE of 225Ac compared to 177Lu was determined in a wide range of absorbed doses to the nucleus. In the case of spherical geometry, 3D cell arrangement and full radionuclide internalization, the RBE based on the initial damage had a constant value of approximately 2.14. Accounting for damage repair resulted in RBE values ranging between 9.38 and 1.46 for 225Ac absorbed doses to the nucleus between 0 and 50 Gy, respectively.

CONCLUSION

In this work, the consideration of DNA repair of the damage from [225Ac]Ac-PSMA and [177Lu]Lu-PSMA revealed a dose dependency of the RBE. Hence, this work suggested that DNA repair is an important aspect to understand response to different radiation qualities.

Microdosimetric investigation of the radiation quality of low-medium energy electrons using Geant4-DNA

The increasing clinical use of low-energy photon and electron sources (below few tens of keV) has raised concerns on the adequacy of the existing approximation of an energy-independent radiobiological effectiveness. In this work, the variation of the quality factor (Q) and relative biological effectiveness (RBE) of electrons over the low-medium energy range (0.1 keV-1 MeV) is examined using several microdosimetry-based Monte Carlo methodologies with input data obtained from Geant4-DNA track-structure simulations. The sensitivity of the results to the different methodologies, Geant4-DNA physics models, and target sizes is examined. Calculations of Q and RBE are based on the ICRU Report 40 recommendations, the Kellerer-Hahn approximation, the site version of the theory of dual radiation action (TDRA), the microdosimetric kinetic model (MKM) of cell survival, and the calculated yield of DNA double strand breaks (DSB). The stochastic energy deposition spectra needed as input in the above approaches have been calculated for nanometer spherical volumes using the different electron physics models of Geant4-DNA. Results are normalized at 100 keV electrons which is here considered the reference radiation. It is shown that in the energy range ~50 keV-1 MeV, the calculated Q and RBE are approximately unity (to within 1-2%) irrespective of the methodology, Geant4-DNA physics model, and target size. At lower energies, Q and RBE become energy-dependent reaching a maximum value of ~1.5-2.5 between ~200 and 700 eV. The detailed variation of Q and RBE at low energies depends mostly upon the adopted methodology and target size, and less so upon the Geant4-DNA physics model. Overall, the DSB yield predicts the highest RBE values (with RBE_max≈2.5) whereas the MKM the lowest RBE values (with RBE_max≈1.5). The ICRU Report 40, Kellerer-Hahn, and TDRA methods are in excellent agreement (to within 1-2%) over the whole energy range predicting a Q_max≈2. In conclusion, the approximation Q=RBE=1 was found to be valid only above ~50 keV whereas at lower energies both Q and RBE become strongly energy-dependent. It is envisioned that the present work will contribute towards establishing robust methodologies to determine theoretically the energy-dependence of radiation quality of individual electrons which may then be used in subsequent calculations involving practical electron and photon radiation sources.

Radiobiological shot noise explains Three Mile Island biodosimetry indicating nearly 1,000 mSv exposures

The 1979 accident at the Three Mile Island nuclear power station in Pennsylvania released about 22 million Curies of xenon-133 into the environment. Because physical dosimetry indicated exposures to the nearby population of less than about 2 mSv, discernible impacts to the health of the exposed population are not generally believed to have resulted. However, there is contrary evidence, including especially the results of biodosimetry via cytogenetic analysis using the FISH method. This report examines the discrepancy between the results of physical dosimetry and biodosimetry, which among the small number of persons examined indicated exposures between 600 and 900 mSv. The paradox reveals a fundamental error in the health physics body of knowledge: the definition of the energy imparted to tissue, ε, fails to properly account for the temporal distribution of ionization products resulting from dilute contamination with an internally incorporated beta-emitting radionuclide. Application of a century-old result describing "shot noise" in an electronic system repairs the deficiency. The Xe-133 concentration in the tissue of those individuals exposed to the most intense portion of the radioactive plume released from the TMI facility is shown to have been on the order of 0.1 μCi/l, persisting for multiple hours. Shot noise reference doses in the range from 820 to 1,700 mSv follow, a result which is consistent with biodosimetric analysis. The finding should motivate a comprehensive re-evaluation of the conventional understanding of the 1979 accident at the Three Mile Island nuclear power station, especially regarding its impact upon the population of the surrounding area.

Monte Carlo investigation of electron specific energy distribution in a single cell model

Knowledge of microdosimetric quantities of certain radionuclides is important in radio immune cancer therapies. Specific energy distribution of radionuclides, which are bound to the cell, is the microdosimetric quantity essential in the process of radionuclide selection for patient tumour treatment. The aim of this paper is to establish an applicable method to determine microdosimetric quantities for various radionuclides. The established method is based on knowledge of microdosimetric quantities of monoenergetic electrons. In this paper these quantities are determined for the single-cell model for a range of electron energies up to [Formula: see text], using the Monte Carlo transport code PENELOPE. The results show that using monoenergetic specific energies, reconstruction of the specific energy of beta-emitting radionuclides can be successfully done with very high accuracy. Microdosimetric quantities share information about the physical processes involved and give insight about energy depositions, which is of use in the procedure of radionuclide selection for a given type of therapy.

Internal microdosimetry of alpha-emitting radionuclides

At the tissue level, energy deposition in cells is determined by the microdistribution of alpha-emitting radionuclides in relation to sensitive target cells. Furthermore, the highly localized energy deposition of alpha particle tracks and the limited range of alpha particles in tissue produce a highly inhomogeneous energy deposition in traversed cell nuclei. Thus, energy deposition in cell nuclei in a given tissue is characterized by the probability of alpha particle hits and, in the case of a hit, by the energy deposited there. In classical microdosimetry, the randomness of energy deposition in cellular sites is described by a stochastic quantity, the specific energy, which approximates the macroscopic dose for a sufficiently large number of energy deposition events. Typical examples of the alpha-emitting radionuclides in internal microdosimetry are radon progeny and plutonium in the lungs, plutonium and americium in bones, and radium in targeted radionuclide therapy. Several microdosimetric approaches have been proposed to relate specific energy distributions to radiobiological effects, such as hit-related concepts, LET and track length-based models, effect-specific interpretations of specific energy distributions, such as the dual radiation action theory or the hit-size effectiveness function, and finally track structure models. Since microdosimetry characterizes only the initial step of energy deposition, microdosimetric concepts are most successful in exposure situations where biological effects are dominated by energy deposition, but not by subsequently operating biological mechanisms. Indeed, the simulation of the combined action of physical and biological factors may eventually require the application of track structure models at the nanometer scale.

Shot Noise Explains the Petkau 22Na+ Result for Rupture of a Model Phospholipid Membrane

The action of free radicals is believed responsible for much or most biological injury resulting from exposure to ionizing radiation. These molecules in solution possess short lifetimes on the order of nanoseconds to microseconds. As a result, the dose-which measures the energy dissipated in tissue due to radiolysis-should not be considered a reliable indicator of the free radical concentration, nor of the chemical effects that follow from it. Rather, the chemical state of affected tissue is properly represented only by the dissipated power, which describes the distribution of energy with time. The present report demonstrates the validity of this assertion using data contained in a report describing a benchtop experiment published in 1972. The experiment used the visible rupture of a model phospholipid membrane as a means to quantify the degree of chemical insult caused by ionizing radiation. The experiment found that beta doses in the range from 1-10 rad were equivalent to x-ray rupture doses of 3,500 rad. This report demonstrates that the experimental results are convincingly explained by reference to the properly calculated time-averaged dissipated power due to beta decay. The theoretical explanation is derived by analogy to a well-understood result from electronic systems known as shot noise. If the result described in this report is demonstrated to extrapolate from the benchtop to living systems, then it is likely that exposure to beta radiation via internal incorporation is far more hazardous than commonly believed. The finding could be revolutionary in the field of health physics.

Radiation-induced genomic instability in breast carcinomas of the Swedish hemangioma cohort

Radiation-induced genomic instability (GI) is hypothesized to persist after exposure and ultimately promote carcinogenesis. Based on the absorbed dose to the breast, an increased risk of developing breast cancer was shown in the Swedish hemangioma cohort that was treated with radium-226 for skin hemangioma as infants. Here, we screened 31 primary breast carcinomas for genetic alterations using the OncoScan CNV Plus Assay to assess GI and chromothripsis-like patterns associated with the absorbed dose to the breast. Higher absorbed doses were associated with increased numbers of copy number alterations in the tumor genome and thus a more unstable genome. Hence, the observed dose-dependent GI in the tumor genome is a measurable manifestation of the long-term effects of irradiation. We developed a highly predictive Cox regression model for overall survival based on the interaction between absorbed dose and GI. The Swedish hemangioma cohort is a valuable cohort to investigate the biological relationship between absorbed dose and GI in irradiated humans. This work gives a biological basis for improved risk assessment to minimize carcinogenesis as a secondary disease after radiation therapy.