A. Belchior

Future development of biologically relevant dosimetry

Proton and ion beams are radiotherapy modalities of increasing importance and interest. Because of the different biological dose response of these radiations as compared with high-energy photon beams, the current approach of treatment prescription is based on the product of the absorbed dose to water and a biological weighting factor, but this is found to be insufficient for providing a generic method to quantify the biological outcome of radiation. It is therefore suggested to define new dosimetric quantities that allow a transparent separation of the physical processes from the biological ones. Given the complexity of the initiation and occurrence of biological processes on various time and length scales, and given that neither microdosimetry nor nanodosimetry on their own can fully describe the biological effects as a function of the distribution of energy deposition or ionization, a multiscale approach is needed to lay the foundation for the aforementioned new physical quantities relating track structure to relative biological effectiveness in proton and ion beam therapy. This article reviews the state-of-the-art microdosimetry, nanodosimetry, track structure simulations, quantification of reactive species, reference radiobiological data, cross-section data and multiscale models of biological response in the context of realizing the new quantities. It also introduces the European metrology project, Biologically Weighted Quantities in Radiotherapy, which aims to investigate the feasibility of establishing a multiscale model as the basis of the new quantities. A tentative generic expression of how the weighting of physical quantities at different length scales could be carried out is presented.

Dose mapping of a 60Co irradiation facility using PENELOPE and MCNPX and its validation by chemical dosimetry

The Monte Carlo simulation programs PENELOPE and MCNPX have been used for simulating the dose rate distribution in a (60)Co gamma irradiator. The simulated isodose curves obtained for each simulation code were validated comparing them to the dose measurements performed with a Fricke solution, which is a standard dosemeter widely used in radiation processing for calibration purposes. The agreement between the simulated values and the measurements indicates the effectiveness of both codes in performing the dose-mapping simulation for gamma irradiators.

Novel (188)Re multi-functional bone-seeking compounds: Synthesis, biological and radiotoxic effects in metastatic breast cancer cells.

INTRODUCTION Radiolabeled bisphosphonates (BPs) have been used for bone imaging and delivery of β(-) emitting radionuclides for bone pain palliation. As a β(-) emitter, (188)Re has been considered particularly promising for bone metastases therapy. Aimed at finding innovative bone-seeking agents for systemic radiotherapy of bone metastases, we describe herein novel organometallic compounds of the type fac-[(188)Re(CO)3(k(3)-L)], (L=BP-containing chelator), their in vitro and in vivo stability, and their cellular damage in MDAMB231 cells, a metastatic breast cancer cell line. METHODS After synthesis and characterization of the novel organometallic compounds of the type fac-[(188)Re(CO)3(k(3)-L)] their radiochemical purity and in vitro stability was assessed by HPLC. In vivo stability and pharmacokinetic profile were evaluated in mice and the radiocytotoxic activity and DNA damage were assessed by MTT assay and by the cytokinesis-block micronucleus (CBMN) assay, respectively. RESULTS Among all complexes, (188)Re3 was obtained with high radiochemical purity (>95%) and high specific activity and presented high in vitro and in vivo stability. Biodistribution studies of (188)Re3 in Balb/c mice showed fast blood clearance, high bone uptake (16.1 ± 3.3% IA/g organ, 1h p.i.) and high bone-to-blood and bone-to-muscle radioactivity ratios, indicating that it is able to deliver radiation to bone in a very selective way. The radiocytotoxic effect elicited by (188)Re3 in the MDAMB231 cells was dependent on its concentration, and was higher than that induced by identical concentrations of [(188)ReO4](-). Additionally, (188)Re3 elicited morphological changes in the cells and induced DNA damage by the increased number of MN observed. CONCLUSION Altogether, our results demonstrate that (188)Re3 could be considered an attractive candidate for further preclinical evaluation for systemic radionuclide therapy of bone metastases considering its ability to deliver radiation to bone in a very selective way and to induce radiation damage.

Evaluation of Acridine Orange Derivatives as DNA-Targeted Radiopharmaceuticals for Auger Therapy: Influence of the Radionuclide and Distance to DNA

A new family of 99mTc(I)- tricarbonyl complexes and 125I-heteroaromatic compounds bearing an acridine orange (AO) DNA targeting unit was evaluated for Auger therapy. Characterization of the DNA interaction, performed with the non-radioactive Re and 127I congeners, confirmed that all compounds act as DNA intercalators. Both classes of compounds induce double strand breaks (DSB) in plasmid DNA but the extent of DNA damage is strongly dependent on the linker between the Auger emitter (99mTc or 125I) and the AO moiety. The in vitro evaluation was complemented with molecular docking studies and Monte Carlo simulations of the energy deposited at the nanometric scale, which corroborated the experimental data. Two of the tested compounds, 125I-C5 and 99mTc-C3, place the corresponding radionuclide at similar distances to DNA and produce comparable DSB yields in plasmid and cellular DNA. These results provide the first evidence that 99mTc can induce DNA damage with similar efficiency to that of 125I, when both are positioned at comparable distances to the double helix. Furthermore, the high nuclear retention of 99mTc-C3 in tumoral cells suggests that 99mTc-labelled AO derivatives are more promising for the design of Auger-emitting radiopharmaceuticals than the 125I-labelled congeners.

HUMAN EXPOSURE TO INDOOR RADON: A SURVEY IN THE REGION OF GUARDA, PORTUGAL

Radon ((222)Rn) is a radioactive gas, abundant in granitic areas, such as the city of Guarda at the northeast of Portugal. This gas is recognised as a carcinogenic agent, being appointed by the World Health Organization as the second leading cause of lung cancer after tobacco smoke. Therefore, the knowledge of radon concentrations inside the houses (where people stay longer) is important from the point of view of radiological protection. The main goal of this study was to assess the radon concentration in an area previously identified with a potentially high level of residential radon. The radon concentration was measured using CR-39 detectors, exposed for a period of 2 months in 185 dwellings in the Guarda region. The radon concentration in studied dwellings, ranged between 75 and 7640 Bq m(-3), with a geometric mean of 640 Bq m(-3) and an arithmetic mean of 1078 Bq m(-3). Based on a local winter-summer radon concentration variation model, these values would correspond to an annual average concentration of 860 Bq m(-3). Several factors contribute to this large dispersion, the main one being the exact location of housing construction in relation to the geochemical nature of the soil and others the predominant building material and ventilation. Based on the obtained results an average annual effective dose of 15 mSv y(-1) is estimated, well above the average previously estimated for Portugal.

Calibration of an alpha particle irradiator for in vitro cells irradiation

An alpha particle irradiator using 210Po radioactive sources was calibrated under specific conditions for cell irradiation. The energy spectrum of α-particles hitting the cell monolayer was measured by a surface barrier detector. The simulation of the results obtained with this detector was performed using the MCNPX Monte Carlo code with the same conditions used during the experimental measurements. Based on the experimental results the LET spectrum at the cell monolayer was extracted using the published Stopping Power and Range tables from the National Institute of Standards and Technology database. From this spectrum the average LET value was calculated to be (154 ± 9) keV/μm. In order to calculate the dose value at the cell monolayer, the alpha particle count rate hitting the cells was also estimated from the experimental measurements and compared with the activity of the source. Finally, the dose rate value at cell monolayer was calculated using the measured particle fluence.

Does the Number of Irradiated Cells Influence the Spatial Distribution of Bystander Effects

There is growing evidence that the radiation effects at low doses are not adequately described by a simple linear extrapolation from high doses, due, among others, to bystander effects. Though several studies have been published on this topic, the explanation of the mechanisms describing the bystander effects remains unclear. This study aims at understanding how the bystander signals are or can be propagated in the cell culture, namely if the number of irradiated cells influences the bystander response. An A549 cell line was exposed to several doses of α-particles, being the bystander response quantified in two non-irradiated areas. The radius of irradiated areas differs by a factor of 2, and the non-irradiated areas were optimally designed to have the same number of cells. Our results show evidence for bystander effects occurring in cells far away from the irradiated ones, meaning that bystander signals can easily spread throughout the cell culture. Additionally, our study highlights that the damage caused by radiation on the surrounding of irradiated areas could be different according to the number of irradiated cells, i.e., for the same dose value; the overall cellular damage could be different.

DOSE AND TIME DEPENDENCE OF TARGETED AND UNTARGETED EFFECTS AFTER VERY LOW DOSES OF α-PARTICLE IRRADIATION OF HUMAN LUNG CANCER CELLS

Understanding the effects to human health resulting from exposure to low doses of ionizing radiation is a persisting challenge. No one questions the deleterious consequences for humans following exposure to high radiation doses; however, in the low dose range, the complex and to some extent unknown cellular responses raise important misgivings about the resulting protective or potentially detrimental effects. Bystander effects are involved in low dose exposures, being characterized by the appearance in unirradiated cells of a cellular damage associated with direct radiation exposure. The purpose of our work was to assess, by using clonogenic and micronuclei assays, the dose and time dependence of the bystander response after cells exposure to very low doses of α-particles and to evaluate its importance in the overall induced damage. The study includes an irradiated cells culture, a medium transfer culture with non-irradiated cells and a culture with irradiated cells after centrifugation. We observed a non-negligible contribution of the bystander effects in the overall cellular damage. Low-dose hyper-sensitivity was observed for medium transfer and irradiated cells after centrifugation cultures. Delayed and earlier cellular damage were similar in almost all experiments, suggesting an effectiveness of irradiated medium to induce a bystander response soon after irradiation.

Monte Carlo dose distribution calculation at nuclear level for Auger-emitting radionuclide energies

The distribution of radiopharmaceuticals in tumor cells represents a fundamental aspect for a successful molecular targeted radiotherapy. It was largely demonstrated at microscopic level that only a fraction of cells in tumoral tissues incorporate the radiolabel. In addition, the distribution of the radionuclides at sub-cellular level, namely inside each nucleus, should also be investigated for accurate dosimetry estimation. The most used method to perform cellular dosimetry is the MIRD one, where S-values are able to estimate cellular absorbed doses for several electron energies, nucleus diameters, and considering homogeneous source distributions. However the radionuclide distribution inside nuclei can be also highly non-homogeneous. The aim of this study is to show in what extent a non-accurate cellular dosimetry could lead to misinterpretations of surviving cell fraction vs dose relationship; in this context, a dosimetric case study with 99mTc is also presented. METHODS The state-of-art MCNP6 Monte Carlo simulation was used in order to model cell structures both in MIRD geometry (MG) and MIRD modified geometries (MMG), where also entire mitotic chromosome volumes were considered (each structure was modeled as liquid water material). In order to simulate a wide energy range of Auger emitting radionuclides, four mono energetic electron emissions were considered, namely 213eV, 6keV, 11keV and 20keV. A dosimetric calculation for 99mTc undergoing inhomogeneous nuclear internalization was also performed. RESULTS After a successful validation step between MIRD and our computed S-values for three Auger-emitting radionuclides (99mTc, 125I and 64Cu), absorbed dose results showed that the standard MG could differ from the MMG from one to three orders of magnitude. These results were also confirmed by considering the 99mTc spectrum emission (Auger and internal conversion electrons). Moreover, considering an inhomogeneous radionuclide distribution, the average electron energy that maximizes the absorbed dose was found to be different for MG and MMG. CONCLUSIONS The modeling of realistic radionuclide localization inside cells, including a inhomogeneous nuclear distribution, revealed that i) a strong bias in surviving cell fraction vs dose relationships (taking to different radiobiological models) can arise; ii) the alternative models might contribute to a more accurate prediction of the radiobiological effects inherent to more specific molecular targeted radiotherapy strategies.

Effect of the glandular composition on digital breast tomosynthesis image quality and dose optimisation.

In the image quality assessment for digital breast tomosynthesis (DBT), a breast phantom with an average percentage of 50 % glandular tissue is seldom used, which may not be representative of the breast tissue composition of the women undergoing such examination. This work aims at studying the effect of the glandular composition of the breast on the image quality taking into consideration different sizes of lesions. Monte Carlo simulations were performed using the state-of-the-art computer program PENELOPE to validate the image acquisition system of the DBT equipment as well as to calculate the mean glandular dose for each projection image and for different breast compositions. The integrated PENELOPE imaging tool (PenEasy) was used to calculate, in mammography, for each clinical detection task the X-ray energy that maximises the figure of merit. All the 2D cranial-caudal projections for DBT were simulated and then underwent the reconstruction process applying the Simultaneous Algebraic Reconstruction Technique. Finally, through signal-to-noise ratio analysis, the image quality in DBT was assessed.