Joint research towards a better radiation protection—highlights of the Fifth MELODI Workshop

MELODI, acronym for Multidisciplinary European Low Dose Initiative, is a European Platform dedicated to low-dose radiation risk research (www.melodi-online.eu). MELODI was founded in 2010 as a registered association with 15 members. Today, MELODI includes 30 members from 16 European countries. The purpose of MELODI is (i) to propose research and training priorities for Europe in its fields of competence, (ii) to seek the views of stakeholders on the priorities for research, keep them informed on progress made, and contribute to the dissemination of knowledge, and (iii) to interface with international partners (like WHO, IAEA, ICRP, UNSCEAR). With this in mind, the Strategic Research Agenda (SRA) of MELODI is developed. The SRA is regularly updated based on the outcomes of the yearly MELODI workshops, and is available for consultation and comment on the MELODI website. To ensure an open and transparent discussion and development of the SRA, MELODI solicits contributions from a large number of institutions and stakeholders.

The Fifth International MELODI Workshop was organised by SCK•CEN, the Belgian Nuclear Research Centre, from 7 October through 10 October 2013 in Brussels (www.melodi2013.org). The workshop offered a unique opportunity to the 221 participants originating from 22 countries worldwide and representing various universities, research institutes and organisations, regulatory bodies, service providers, or national and international stakeholder organisations, to update their knowledge and discuss low-dose radiation research issues, as well as to be involved in the MELODI low-dose research platform. In addition, the MELODI 2013 workshop was reaching out to the broader radiation protection community, rather than only the low-dose community, with contributions from the fields of radioecology, emergency and recovery preparedness, and dosimetry. A total of 118 oral presentations were given and 44 posters were presented. Abstracts, and oral or poster presentations are available at the workshop website (www.melodi2013.org/en/Presentations).

During this particular MELODI 2013 workshop, plenary sessions focused on topics in which significant breakthrough progress had been made over the past few years, whilst three thematic sessions running in parallel were devoted to more specific research topics related to radiobiology, dosimetry, epidemiology, radiotherapy (RT), radioecology, emergency planning and other fields of low-dose risk research. The parallel sessions were rounded off by discussions which served to keep the SRA up-to-date and to further implement it into the various low-dose research groups throughout Europe. MELODI also aims to recognise talented and promising young scientists (under 35 years old) who have already contributed greatly to low-dose research. Therefore, for the second time, an annual MELODI award was granted during the workshop. In their MELODI award presentation, awardees Anna Acheva (Radiation and Nuclear Safety Authority, STUK, Finland) and Luca Mariotti (University of Pavia, Italy) presented their work about '3D skin and lung epithelial models for radiation biology studies', and 'Systems radiation biology to model non-linear effects', respectively.

This paper summarises the major scientific findings presented during the MELODI Workshop 2013. Here, we refer to the workshop speakers as well as to the selected recent publications highlighting the topics under discussion during the workshop.

The MELODI workshop 2013 in Brussels was the first reaching out to the broader Radiation Protection Community rather than only to the low-dose community. In this session, the four radiation protection associations, MELODI (www.melodi-online.eu), EURADOS (www.eurados.org), NERIS (www.eu-neris.net) and ALLIANCE (www.er-alliance.org) presented their latest versions of their respective SRA. The aim of the presentations was to foster integration and find interfaces for further collaboration between the research planned within the four organisations.

Radiation protection covers the fields of (1) radioecology (within ALLIANCE), (2) emergency and recovery preparedness (within NERIS), (3) research on the effect of low-dose ionising radiation (from environmental, accidental and medical origin) to humans (within MELODI) and (4) dosimetry (within EURADOS). The expertise needed in these fields rely on a broad range of scientific disciplines. Some of these disciplines are needed in two or more of the radiation protection fields. For example, genetics is a discipline needed in radioecology and low-dose research, or, meteorology is a vital discipline in radioecology and emergency preparedness. An in-depth analysis reveals numerous interfaces between the different radiation protection fields. To increase synergy, it is important to initiate joint efforts to map the respective expertise, the complementarities and the common challenges. MELODI, EURADOS, NERIS and ALLIANCE all acknowledged and appreciated the workshop initiative to bring these fields together and present/discuss their research areas in order to foster further collaboration/integration.

2.1. Further developments in June 2014

The UNSCEAR Fukushima report was summarised by Jean-René Jourdain (French Institute for Radiological Protection and Nuclear Safety, IRSN, France) [1]. After the Fukushima accident of 11 March 2011, Japan, other member states of the United Nations, and international organisations made available to the UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) extensive data for its assessment, regarding the radiation levels and deposition of radioactive materials in every prefecture in Japan as well as the radionuclide concentrations in foodstuffs, and public and workers exposures. Many of these data were provided by official Japanese governmental agencies and were published in peer-reviewed scientific journals. Furthermore, 25 other UN Member States provided information on request. Additional relevant data were made available by other international organisations. Several non-governmental organisations also made data available that were considered in the assessment. The quality of these data (data sets, reports, journal articles and so on) was evaluated in terms of their usefulness for the assessment.

Limited uncertainty/sensitivity studies have been conducted, as appropriate, to underpin the UNSCEAR's qualitative statements of its confidence in its conclusions. In addition UNSCEAR deployed five working groups to perform quantitative assessments of doses received to the public and emergency workers, and made a judgment on their reliability. Initially, the publication of the UNSCEAR's report was expected on 8 October 2013. However, due to the time required to take into account late comments from some UN delegations, the report was not released at the time of the Workshop (published April 2014 and available on www.unscear.org). The report discusses the course of the accident, the release of radioactive material into the environment, dose assessment, health implications, and radiation exposures and effects on nonhuman biota.

Richard Wakeford (University of Manchester, UK) commented the WHO (World Health Organisation) Fukushima reports [2, 3] (both reports are freely available on www.who.org). Following the releases of radioactive materials into the environment from the Fukushima Daiichi Nuclear Power Station in March 2011, WHO initiated a study of the possible health risks resulting from the releases. The study has two parts: an estimate of the radiation doses received during the first year after the accident, and the health risks arising from these doses.

For two highly exposed locations in the Fukushima Prefecture, effective doses during the first year after the accident were assessed to be in the range 10–50 mSv, of which organ dose to the thyroid accounted for 10–100 mSv (and for infants, the estimated thyroid dose was in the range 100–200 mSv). Doses elsewhere were less than this, generally much less. The WHO report points out that for a child aged 10 years living in most affected areas, 57% of the cumulative effective dose (mainly from ground deposition) and 79% of the cumulative thyroid dose (mainly from inhalation) were received during the first month after the accident.

On the basis of these dose estimates, the health consequences were evaluated. Both excess relative risks and excess absolute risks were calculated on the basis of recently developed risk models. For the population with highest assessed doses, the additional lifetime risk of cancer is about 1% (of which approximately 0.5% is due to thyroid cancer). The group with highest risk consists of females who were infants at the time of exposure. The given estimates are for the highest exposed areas, and subsequently the estimated excess risk is lower for other age groups and locations. For emergency workers, about one-third received thyroid doses that, for the youngest workers, could increase the risk of thyroid cancer by about 20%. The proportional increase of leukaemia for the youngest workers is estimated to be less than 1%.

Thyroid cancer is one of the major health concerns after the accident in the Fukushima Daiichi Nuclear Power Station. Ultrasonography surveys are being performed for persons residing in the Fukushima Prefecture at the time of the accident with an age of up to 18 years, as was explained by Peter Jacob (Helmholtz Zentrum München, HMGU, Germany). The prevalence of thyroid cancer was estimated to be 0.034% (95% confidence interval (CI): 0.009–0.085%). Compared to the incidence rate in Japan in 2007, the ultrasonography survey is predicted to increase baseline thyroid cancer incidence by a factor of 7.2 (95% CI: 3.7; 10.7). Under the condition of continued screening, thyroid cancer during the first 50 years after the accident is predicted to be detected for about 2% of the screened population. The prediction of radiation-related thyroid cancer in the screened population of the Fukushima prefecture has a large uncertainty with best estimates of the average risk of 0.1–0.3%, depending on average dose.

Johan Camps (Belgian Nuclear Research Centre, SCK•CEN, Belgium) provided details concerning the response of SCK•CEN to the Fukushima nuclear accident in the context of the protection of Belgian citizens. As a research organisation, SCK•CEN is a partner of the authorities related to emergency planning and response. The response was organised ad hoc because the Belgian nuclear emergency plan was not declared. The challenges in the response included (i) an early radiological impact assessment for advice for Belgians in Japan with little and uncertain information: this showed the importance of including uncertainties in assessments and combining atmospheric dispersion modelling efforts with monitoring data, (ii) mobile and laboratory measurements of people returning from Japan: first persons measured resulted in more qualitative information of the impact on the locations the persons stayed. Although only limited contaminations were found, this campaign showed to be very important for re-assuring the worried travellers, (iii) increased surveillance of the Belgian territory (air concentration, grass, milk): the clearly measureable but trivial exposures made communication about the effect of the accident on the Belgian territory a challenge, (iv) many questions related to imported goods: this illustrated the need for a better preparedness related to contaminated goods. In this context a NERIS (European Platform on preparedness for nuclear and radiological emergency response and recovery, www.eu-neris.net) working group and a work package in the FP7 PREPARE project were set up.

A study by cytogenetic analysis of 12 restoration workers in the aftermath of the Fukushima Daiichi Nuclear Power Station accident was presented by Yumiko Suto (National Institute of Radiological Sciences, NIRS, Japan). Two methods were used to examine the blood samples of the 12 workers. Dicentric chromosome assay (DCA) showed for none of the workers values exceeding the dose limit of 300 mGy (at 95% upper confidence limit), which is lower than the lower limit level of medical triage for acute radiation syndrome which is 1 Gy. These results confirm the fact that no acute radiation syndromes were observed among the workers examined and the obtained values are in good agreement with physically estimated doses by personal dosimeters. Fluorescence in situ hybridisation (FISH), more especially multiplex and three-colour FISH, was used for translocation analysis. The results suggest that the frequency of translocations is considered to be 1.5 times higher in the workers compared to an unexposed control group. Based on this experience the need for improved cytogenetic research strategies adopted for mass-casualty management was reconsidered.

Yutaka Hamaoka (Keio University, Japan) presented preliminary results of an analysis of radiation dose and occurrence of thyroid nodules using data from 14 cities and villages. More specifically a relationship was made between the number of thyroid nodules and the radiation dose. The radiation dose used was based on available data on radiation level within cities and villages. A conjecture 'If a nodule was caused by radiation, taking into account the slow growth of thyroid nodules, the number of smaller nodules would correlate with radiation dose rather than that of larger nodules' is supported by thyroid dose estimates made by WHO and NIRS on Fukushima external doses. Although the sample size was limited, the robustness of the results was confirmed. The results might indicate an early warning for future incidence of thyroid cancer. Follow-up is necessary.

Transgenerational instability can be defined as a genome-wide phenomenon affecting the frequency of chromosome aberrations and gene mutations in the next-generation offspring. The thematic issues debated for transgenerational effects covered both paternal and maternal transmission of genomic instability in mice. The first experimental evidence of increased germ-line mutation rates in the first and second generation of unexposed offspring of neutron-irradiated male mice was obtained in the CBA/H mouse strain [4]. The effects of strain specificity and high and low linear energy transfer (LET) exposure on germ-line mutation rates in the first and second generation progeny of irradiated males were addressed by Yuri Dubrova (University of Leicester, UK). The data revealed elevated ESTR (Expanded Simple Tandem Repeat) mutation in both generations of all of the three inbred strains (CBA/H, C57BL/6, BALAB/c) tested and that either high or low LET irradiation resulted in an increase in germ-line mutation rates in both generations [5]. Other irradiation criteria can influence the paternal transmission of genomic instability in mice. In this regard, the dose and dose rate effects were discussed. The data presented showed that acute γ-radiations (50 and 100 cGy) of the male parents increased evenly the frequencies of ESTR mutations in the brain and in the germ line of their progeny. Unlike, high dose acute radiation, acute irradiation at lower doses (10–25 cGy) and low-dose-rate exposure to 100 cGy did not affect stability of the next-generation offspring. From these data, it appears that the manifestation of transgenerational instability is triggered by a threshold dose of acute paternal irradiation [6].

Maternal transmission of genomic instability was discussed during this session after the presentation given by Paul Jacquet (SCK•CEN, Belgium). Transgenerational effects and congenital malformations were investigated after moderate x-irradiation (0.2 and 0.4 Gy) of two mouse strain (ICR and CF1) embryos during the pre-implantation stage (1-cell embryo). In both ICR and CF1 mouse strains, irradiation of female zygotes did not result in an increase in the frequency of malformations nor to the increase of chromosomal instability in the next-generation embryos. Overall, these results suggest that, at the moderate doses used, the very few developmental defects observed after X-irradiation of female zygotes of these two sensitive mouse strains should not be transmitted to the next generation [7].

Overall, transgenerational instability is attributed to the presence of a persistent subset of endogenous DNA lesions and to the epigenetic changes affecting the expression of a subset of genes, involved in rhythmic process and regulation of transcription. From the results of the mouse studies, it would appear that maternal or paternal irradiation with low doses and low-dose-rate irradiation do not destabilise the genome of the offspring while irradiation of the male parent with acute high doses does. Despite the latter finding in mice, experimental evidence for transgenerational instability in humans remains highly controversial especially when comparing the transgenerational effects in children from fathers exposed to post-Chernobyl radioactive contamination or from cancer RT survivors. Nevertheless, the risk for transgenerational instability in humans cannot be completely excluded as only a few generations have been observed. Moreover, lack of human evidence does not mean evidence of lack of effect as was stressed by Patrick Smeesters (Federal Agency for Nuclear Control, FANC, Belgium).

The session on mixed exposure started with a general overview talk of Nele Horemans (SCK•CEN, Belgium) defining what is meant by mixed exposure conditions and what approaches could be used to estimate possible interacting effects from different compounds. In our environment, mammals (including humans) are exposed to various types of ionising radiation and both persistent and non-persistent toxic chemicals. This area of research certainly deserves more interest to understand the combined effect of radiation and environmental toxicants; it would also inform us about the effect of confounding factors rendering the epidemiological data from radiation-exposed cohorts in some situations less conclusive for the radiation effect. Most scientists today acknowledge that in natural situations organisms are generally exposed to multiple stressors either simultaneously or at different times during their lifespan. In general, however, most experimental studies assess effects of stressors including low-dose radiation in controlled single contaminant conditions. Moreover, for some mixed exposure experiments results or possible interactions are sometimes misinterpreted [8]. Hence, a key question to assess the risk of low-dose gamma exposure is to know whether interacting effects might occur when humans or nonhuman biota are exposed to several stressors. The presentation focused on the different mathematical models that can be used for the prediction of combined effects based on the known individual effects, namely concentration addition (CA) and independent action (IA) [9]. After this more general overview, four specific presentations dealing with different aspects of exposure to radiation in a mixed contaminant set-up were presented.

RT can come with a secondary cost such as possible gonadal dysfunction or DNA damage in germ cells during spermatogenesis or oogenesis leading to meiosis malfunction, abortions or hereditary effects. A study on radiation-induced genome instability and possible induction of transgenerational effects was given by Aurora Ruiz-Herrera (Autonomous University of Barcelona, UAB, Spain). Foetal offspring of x-ray exposed female rats (5 Gy, acute dose) were evaluated. In addition the possible interaction between pre-treatment with x-rays on the action of the chemical mutagen aphidicolin was studied. Cytogenetic analysis showed a statistical increase in the frequency of chromosomal breaks and aberrant metaphases in the F1 foetal somatic cells from F0 exposed mothers to irradiation. This study concluded that the x-ray treatment resulted in transgenerational chromosomal instability. Moreover, this genome instability was enhanced by the secondary stressor aphidicolin as indicated by the increased induction of chromosomal damage.

Eeva Salminen (STUK, Finland) dealt with RT given together with other chemical agents. In her presentation, an overview was given of the advantages and disadvantages of a combination of RT and chemotherapy. In most cancer treatments, both therapies are combined resulting in lesser dose of both and a higher survival rate for the patient. Advantages and disadvantages of the timing of the two treatments (sequential, concomitant or intermittent) were discussed indicating, as could be expected, that concomitant application of chemo- and radiotherapy is the most toxic. In this situation additional growth factors are often applied to reduce the dose effects on non-cancer cells. Further research in this area will not only result in less toxic and more specific drugs for chemotherapy but also in fine-tuning of the dose and timing of the chemo- and radiotherapy. Finally, some attention was given to the possible beneficiary effects of the use of drugs that would reduce DNA damage in combination with the RT [10, 11].

The presentation given by Sonia Buratovic, on behalf of Per Eriksson (Uppsala University, Sweden), dealt with the possible changes in habituation and altered cognitive function in adult mice due to exposure to gamma radiation or nicotine or a combination of both during a critical period of neonatal brain development. Exposure to gamma radiation (0.2 Gy with a dose rate of 0.07 Gy min⁻¹) and/or nicotine (66 µg kg⁻¹) was given at one to three successive days starting at postnatal day 10. Subsequently, two-month old adult mice were scored for spontaneous behaviour in a novel home environment. Additionally, neurotoxic susceptibility to nicotine was tested by additional exposure of the adult animals to nicotine. Their data indicate that interactive effects between gamma irradiation and nicotine during prenatal brain development exist and can lead to enhanced behavioural aberrations and increased susceptibility to nicotine at adulthood. In addition, it was suggested that the cholinergic system is involved in the increased radiation susceptibility upon mixed gamma radiation and nicotine exposure during neonatal brain development. Together, these data provide evidence on the confounding effect of smoking in radiation-exposed epidemiological cohorts.

As explained during her presentation, the objective of Ann-Karin Olsen's research (Norwegian Institute of Public Health, NIPH, Norway) is to investigate the genotoxic and reproductive effects of low-dose-rate gamma exposure in combination with a varying selenium (Se) level in mice. Mice deficient in the repair of oxidised DNA (8-oxoguanine DNA glycosylase, Ogg1) were used in addition to control mice. The Ogg1 model was used to mimic repair characteristics of human germ cells as this differs between humans and rodents in this respect [12]. Selenium intake which is sub-optimal in many regions of the world is essential for the optimal function of antioxidative selenoproteins enzymes (such as glutathione peroxidase), a number of enzymes known to play an important role in hormonal activation (like thioredoxin reductases) and in the functioning of sperm. Mice were exposed to an average gamma dose rate of 1.63 mGy h⁻¹ during 45 d resulting in a total dose of 1.71 Gy. Gamma exposure resulted in higher DNA damage levels (as evidenced by the Comet assay), induced clastogenic effects (such as increased micronuclei) and increased gene mutation rates (as measured by Pig-a mutation). However gamma-induced effects were irrespective of the Se status or the Ogg1-genotype showing that selenium does not seem to play a role in gamma-induced toxicity.

Traditionally radiation protection has focused on the protection of humans. The past decades however have been marked by a growing awareness that levels of radiation that are considered safe to humans may not always result in no harm to wildlife and/or ecosystems [13, 14]. A short introduction of this session was given by Tom Hinton (IRSN, France). It was said that for the development of a framework for deriving protection criteria for nonhuman biota there is an urging need for more environmental relevant data and for a better mechanistic understanding of radiation-induced effects. After the introduction four specialised talks from different research areas were given.

A critical view on the realism and scientific value of acute lab-based ecotoxicological tests for risk assessment as well as the setting and evaluation of radiation protection standards was given in the presentation of Nele Horemans (SCK•CEN, Belgium). An introduction was given on the approach. A species sensitivity distribution (SSD) led to a generic no-effect dose rate for wildlife of 10 µGy h⁻¹ that currently can be used in environmental radiation protection to screen out situations of no concern [15]. The basic information that is put in to this SSD, are effect concentrations or effect dose rates (ECx or EDRx) derived from individual dose–response curves. However, as the amount of effects for nonhuman biota is limited, it is not evident to make specific SSDs for specific exposure scenarios such as chronic exposure compared to acute exposure or separate lab from field experiments. Using the OECD (Organisation for Economic Cooperation and Development) guidelines for a growth inhibition test for Lemna minor [16], some of the drawbacks of the use of ecotoxicological tests were presented. As such it was shown that the derived ECx/EDRx values are highly dependent on the endpoint chosen (Lemna frond area, number of fronds, fresh weight, dry weight), the growth conditions (e.g. nutrient medium), as well as the experimental set-up (e.g. duration of the test). As an example the effective dose rate resulting in 50% growth inhibition (EDR50) of Lemna plants shifted from 900 to 270 mGy h⁻¹ when the plants were given an extra 7 d recovery after the exposure to gamma irradiation. This lower EDR50 value suggests a higher sensitivity and a lack of DNA damage repair in the plants during the recovery period. In conclusion, it was indicated that an increased data collection especially in environmental relevant situations is needed to lead to more scientific robust bench marks for radioprotection for nonhuman biota.

Christelle Adam-Guillermin (IRSN, France) presented the preliminary results of the project FREEBIRD (that stands for Fukushima Radiation Exposure and Effects in Bird populations) that aims at studying effects of radiation originating from the Fukushima accident on bird populations in the 100 km zone around the Daiichi nuclear power plant. As indicated by Adam-Guillermin the strength and innovative character of this project find their origin in the multidisciplinary nature of the approach in which behavioural ecology, toxicological responses on the physiology and the reproductive success of the birds are studied and integrated with a dosimetry as accurate as possible for wildlife. During the Workshop, the set-up and first sampling campaign of the project were introduced. Although only preliminary results were available at the time of the meeting, already high interspecies differences could be found. As such it was shown that internal exposure levels in frogs were much higher than in birds probably due to differences in living habit between these animals. Based on the preliminary dose estimation, including internal and external doses, it was indicated that frogs are exposed to dose rates above the safe threshold of 10 µGy h⁻¹ [15].

In order to investigate if modulation of radiation at background levels can modify the response of organisms to genotoxic agents there is a need to have experimental conditions with different radiation environments. As presented by Maria Antonella Taboccini (National Institute of Health, ISS, Italy) the underground 'Gran Sasso National' Laboratory (LGNS) of the Italian National institute of Nuclear Physics offers the possibility to have near-zero background radiation levels. As such in this facility the cosmic ray and neutron flux are up to six orders of magnitude lower than at the surface. Yeast cells, grown for 120 generations in this low background radiation environment and subsequently exposed to different genotoxic agents, had higher frequency of recombination in the LGNS compared to a reference culture [17]. Further studies have been carried out on higher eukaryotic cell cultures (V79 Chinese hamster lung fibroblasts, TK6 lymphoblasts and A11 mouse cell line) for long exposure conditions to reach comparable number of generations as the yeast cells. In general the experiments with the different eukaryotic lines confirmed that cells cultured in reduced environmental radiation conditions were less tolerant to radiation-induced DNA damage and less efficient in scavenging reactive oxygen species [18–20]. Future experiments will include in vivo studies.

7.1. Radiation exposure of the eye lens and cataract risk

7.2. Radiation-induced cognitive and cerebrovascular effects

7.3. Cardiovascular risks associated with radiation

With the ever increasing technological power, more and more high-throughput technologies are being applied in the field of radiation biology. Several of these methods were presented during the workshop. Regarding transcriptomics, mircoarrays seem to be still the preferred method for genome-wide gene expression screenings, although it is expected that also next-generation sequencing will soon enter the field. Grainne Manning (Public Health England, PHE, UK) investigated the dose–response curve of the gene expression response to low-dose radiation exposure (5–100 mGy) of human blood using predicted markers from the ATM/p53 pathway [35]. Gene expression was analysed by quantitative PCR, at different time points (2 h, 24 h) after radiation and showed that early changes were quite modest with only three genes FDXR, CDKN1A and BBC3 showing a linear response. After 24 h, nine out of 13 tested genes produced a linear response, although for most of these the individual variability in the response between donors was quite high. Some of the donors had a consistent higher or lower response than others at different doses, suggesting that gene responses may also be useful as markers for individual radiosensitivity.

Continuing on the issue of gene expression biomarkers for low-dose exposure, Gaëtan Gruel (IRSN, France) commented on the results from Manning, which showed no significant modulation of gene expression below doses of 20–50 mGy. One of the problems at these doses is that only a small fraction of the cells will effectively produce a radiation-induced DNA DSB, so that any effect related to DSB repair becomes highly diluted among the entire cell population. He therefore irradiated (5–500 mGy) human blood and extracted CD4-positive cells to evaluate gene expression changes at different time points post-irradiation (150, 300, 450, 600 min) using microarrays [36]. He identified two main clusters of genes; the first cluster represented genes with a linear, dose-dependent expression profile. Most of these genes were known targets of p53 and some of them showed significant differences even at a dose of 10 mGy. The second cluster consisted of genes which were modulated at the same level at all doses. For these genes, no link to p53 could be found, but many of them seemed to play a role in mitochondrial function and were enriched in binding sites for transcription factors involved in mitochondrial function, biosynthesis and replication. So, high-throughput technologies may provide additional information about the molecular effects at very low doses, but must be validated in order to definitively conclude about the biological impact of very low dose exposure.

A third presentation about the transcriptional response to radiation was given by Rodolfo Negri (Sapienza Università di Roma, Italy). He first provided an overview of recent literature, showing that the transcriptional response to ionising radiation is strongly cell type-dependent. Going further on previous work [37], he showed results from a meta-analysis of 208 different radiation conditions (doses, time points, cell types, tissues, etc), from which a signature of 34 genes was identified which were modulated above 1.5-fold in at least 30% of the conditions. Of these, CDKN1A, involved in cell cycle arrest and apoptosis, was found to be the most general radiation-induced gene, whereas negatively modulated genes were enriched in mitotic cell cycle regulation. Some of these data can be found on a public database of radiation-responsive genes, containing data from 180 different experiments in human, mouse and rat [38]. Because of the cell- and tissue-specific differences in the radiation response, Negri proposed that future research should more focus on the tissue level to identify sensors, signalling molecules and effectors for the tissue radiation response. He further discussed the role of leptin, an adipose-derived hormone which is induced in the epidermis and adipocytes both after irradiation and during wound healing [39].

In an attempt to unravel the mechanisms induced after exposure to low doses of ionising radiation, Houssein El Saghire and colleagues (SCK•CEN, Belgium) also used transcriptomic analyses (microarrays) and different bioinformatics approaches. Whole blood samples collected from healthy donors, x-irradiated in vitro with low (0.05 Gy) or high (1 Gy) doses, revealed two distinct dose-dependent profiles. In contrast to high doses, they found that a low dose of 0.05 Gy showed higher statistical ranking of immune-related pathways that are mainly involved in the response to and/or secretion of growth factors, chemokines and cytokines. However, at 1 Gy the response was dominated by classical radiation response genes activated by the tumour suppressor TP53, and involved in apoptosis, DNA damage and repair pathways [40]. They confirmed similar low-dose specific responses in vivo in a cohort of prostate cancer patients undergoing intensity modulated radiotherapy (IMRT). These patients receive a high RT dose targeted at the level of the tumour but the rest of the normal tissues receive low doses [41].

8.1. Epigenetics in radiation biology

8.2. Radiogenomics

8.3. Systems biology

Part of the MELODI workshop focused on advanced RT techniques which are used for cancer treatment. During the last decades, innovative techniques have been introduced including IMRT (intensity modulated radiation therapy) with photons and particle therapy with protons or ions. Compared to conventional RT, these advanced techniques lead to a reduction in the dose delivered to the surrounding healthy tissue. However, compared to IMRT, particle beams have superior physical (better ballistic accuracy) and biological properties (especially for heavy ions) resulting in an even more accurate and efficient irradiation of the tumour, thereby sparing the surrounding healthy tissues and thus leading to a lower integral dose to the patient [53].

Short- and long-term side effects following RT are strongly related to the amount of dose deposited to the healthy tissue surrounding the tumour. In this context, the generation of secondary neutrons after IMRT photon or particle treatment is of particular concern [54]. This session focused on ongoing studies in this field.

As presented by Liliana Stolarczyk (Institute of Nuclear Physics, PAN, Poland), characterisation of the radiation field outside the planned target volume is the first step for estimating health risks. A comparison between the results from previous dosimetric studies on secondary radiation and their contribution to the absorbed dose and equivalent dose for different RT technologies with the results obtained by the EURADOS Working group 9 'Radiation Protection Dosimetry in Medicine' were presented and discussed. These show that both passive and active proton and ion therapies result in a lower effective secondary radiation dose compared to IMRT and conventional RT. These data can be useful for further estimation of RT induced health risks. However, one must be aware that estimating the health risks from neutrons is complicated and depends on the neutron dose and energy. Therefore, at this moment, it is very difficult to obtain these risk data where the neutron energy is confined to a narrow spectrum. Moreover, the occurrence of risk events in the low-dose range is extremely low resulting in poor statistics.

The EU FP7 ANDANTE project which evaluates the risk of secondary cancer development from neutrons, was presented by Andrea Ottolenghi (University of Pavia, Italy). This project involves a multidisciplinary approach including physics, stem cell radiobiology and epidemiology in order to further clarify secondary neutrons risks in RT. Progress on characterising the exposure beams, initial radiobiological experiments with stem cells, and the data collection for the epidemiological studies were reviewed.

Next to secondary cancer formation, metastasis is another potential long-term risk that can occur after RT. Annelies Suetens (SCK•CEN, Belgium) presented results about the difference in the impact of carbon ions and x-irradiation on the expression levels of motility-related genes in human prostate cancer (PC3) cells. These data indicated that in PC3 cells, expression levels of several motility genes (CCDC88A, ROCK1, FN1, MYH9) were much more downregulated after carbon ion exposure compared to x-irradiation [55]. Given the current lack of epidemiological data of long-term health risks after particle therapy, it was emphasised that basic radiobiological research is needed and can help to further understand underlying biological mechanisms in order to reduce radiation risk uncertainties in this field.

In the session radiopharmaceuticals and RT, the link between radiation protection and molecular radiotherapy (MRT), also called targeted radiotherapy (TRT) or nuclear medicine therapy (NMT), was emphasised. MRT covers therapies with for example iodine radionuclides, or radionuclides attached to carrier molecules like peptides. In addition, the link was made with decontamination techniques aiming at desorption of radionuclides such as plutonium and actinides ingested due to an accident. The technique is based on administering chelating agents such as DTPA (diethylene triamine pentaacetic acid) to remove the ingested radionuclides through natural excretion pathways. During the session it was shown that largely analogous expertise is needed to understand and improve the radioprotection aspects of both MRT and DTPA decontamination therapy. In figure 1 it is shown how biology and metrology lay on the basis of determining the dosimetry, which is a key factor from a radiation protection point of view in both MRT and contamination techniques.

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Vere Smyth (National Physical Laboratory, UK) focused during his presentation on dosimetry and metrology (http://projects.npl.co.uk/metromrt/). The absorbed dose, the dose rate and the dose distribution need to be determined accurately. Indeed, the same administered activity in case of MRT can result in a different absorbed dose with a factor of 100. Metrology should help to better assess which method(s) will deliver the most accurate doses. Moreover, the more accurate the dose, the less patients will be needed in clinical studies. However, it is important to note that differences due to individual effects can never be avoided. Concerning the determination of the MRT biokinetics, quantitative imaging plays a key role. A better quantification of the activity administered, the imaging, the biokinetics and detection of inhomogeneities are important to upgrade the protocols of MRT.

Bastian Breustedt (Karlsruhe Institute of Technology, Germany) elaborated more on the importance of biokinetic models in his presentation. The development of biokinetic models must consider which physiological processes are important for the specific situation and which tissues are of interest. In case of DTPA decontamination therapy, the activity ingested due to an accident is in general unknown. In addition, for unintended radionuclide uptake, the biokinetics are only available through reference models like the ones published in the International Commission on Radiological Protection (ICRP) 30 report [56–59] for the alimentary tract, ICRP 66 [60] for the respiratory tract, and ICRP 67 [61] on the systemic model. Administering DTPA disturbs the natural biokinetics of the radionuclides. Better in vitro and in vivo models are needed to improve decontamination biokinetics. The coupling of compartmental models and biokinetic modelling of DTPA therapy is described in [62].

Furthermore, the biokinetic models needed for radiopharmaceuticals are somewhat different from most other biokinetic models used in radiation protection, as the half-life of radionuclides for radiopharmaceuticals is short (in the range of hours–days), as discussed by Dietmar Nosske (Federal Office for Radiation Protection, BfS, Germany). In case of MRT, especially peptide receptor radionuclide therapy, making use of peptides as vector molecules to target malignant cells, it is important to minimise late kidney damage (as presented by Mark Konijnenberg, University Medical Center Rotterdam, Erasmus MC, The Netherlands) [63–66].

Systemic effects of MRT, finally, were discussed in a presentation from Eva Forssell Aronsson (University of Gothenburg, Sweden). Transcriptional effects of normal tissues were studied for At-211, an alpha emitter with similar chemical behaviour as iodine. The exposure to At-211 results both in effects in the thyroid [67] as well as in a systemic response through transcriptional gene regulation [68].

Researchers have tended to study single cell types arranged as monolayer cultures for many decades. To study radiation effects this however leads to a limited representation of the real situation of affected cells within a body [69, 70]. Indeed, single cell types arranged as monolayer cultures lack the microenvironment, the typical cell shape, and lack of polarisation means and differentiation. In order to overcome these shortcomings, in vivo studies could be envisaged. However, this is very costly, and therefore the idea to study 3D cultures (both homo- and heterotypic) can offer many advantages. 3D cell culture can incorporate both the spatial and differentiated function of the tissue in vivo. 3D in vitro models allow to study cell-to-cell and cell–extracellular matrix interactions, as well as the influence of the microenvironment on cellular differentiation, proliferation, apoptosis and gene expression.

Anna Acheva (STUK, Finland) presented two examples of 3D cultures that may be of great interest to study radiation effects: a skin and a bronchial epithelium equivalent. The skin equivalent is useful to study the effects on skin after radiation therapy. It could be used to characterise the DNA damage induction levels and repair capacity [71], to investigate the role of the apoptotic and differentiation processes, and to study the pro-inflammatory signalling pathways involved in the development of acute skin reactions. It could also be a starting point to find possible ways to modify the signal propagation by targeting key molecules in the signalling process. One of the most important conclusions from this talk is that there are substantial differences observed between the 2D and 3D cultures. The bronchial epithelium equivalent finds its application in studying molecular mechanisms of radiation-induced lung carcinogenesis, and in studying the role of the tissue microenvironment herein. This is part of the EPIRADBIO FP7 project. The focus areas are to study the epithelial–mesenchymal transition, the signalling pathways and the interactions between epithelial and stromal cells.

In addition there were talks devoted to the use of tissues and stem cells, instead of cell cultures, to assess radiation effects. For these presentations we refer to section 8.

In the session 'New developments in dosimetry', the state of the art in medical dosimetry and new developments were presented. A particular emphasis was put on RT and computed tomography, two fields with major breakthroughs and innovations during the past years.

The protection quantities of equivalent dose in an organ or tissue and effective dose are generally not measurable; while operational dose quantities giving a conservative estimate of the protection quantities are measurable. The control of dose limits, given in terms of protection quantities, needs to be performed by measurements of the operational quantities. However, the system of operational quantities is only well established for a limited range of particle energies, but shows deficiencies and limitations for higher energies. Different options to overcome those insufficiencies, from maintaining the current situation to the redefinition of the operational quantities and relationships with the protection quantities, were presented by David Bartlett (former Public Health England, PHE, UK) on behalf of the International Commission on Radiation Units and Measurements (ICRU) Report Committee 26 (Operational Radiation Protection Quantities for External Radiation).

In x-ray medical imaging, specific dosimetric quantities are to be used for different modalities. Annalisa Trianni (Medical Physics Department, Udine University Hospital, Italy) reviewed the most used ones in today's medical practice. Among those, one might cite the entrance surface air kerma and the kerma–area product in planar radiography, the cumulative air kerma at the international reference point in fluoroscopy, and the computed tomography dose index and dose length product in computed tomography. These quantities are standardised parameters to evaluate the output of radiological equipment; they are useful tools to compare different modalities, equipment and procedures, but they are no estimation of the patient dose. Conversion coefficients and simulation codes are available in the literature to estimate patient exposure based on specific dosimetric quantities and patient's characteristics.

Aside from the assessment of the examination exposure, the image quality is also an important parameter to take into account in x-ray medical imaging. Eeva Salminen (STUK, Finland) presented a clinical study which investigated the association between radiation exposure and image quality in CT [72]. The study concluded that the quality of imaging for diagnostic purpose was reflected in increased radiation exposure; the highest effective and organ doses were related to examinations with image quality exceeding the need for diagnostic purpose. A careful choice of the examination parameters is needed to avoid unnecessary high exposure of patients, the exposure being further increased for specific needs.

Cellular effects and risk of alpha and Auger emitters depend on the distribution of radiation energy at the cellular and molecular levels; organ and tissue doses used alone cannot describe deposition of radiation energy in micro/nanometre ranges. Micro/nanodosimetry is therefore an important tool in low-dose research. Inhomogeneous dose distribution at cellular level correlates better to biological effects at low doses. Weibo Li (HMGU, Germany) presented microdosimetric models of alpha emitters deposited in the central airways [73] and in the kidneys (not published); an example of Auger emitters (gold particles) in RT was also given [74].

In RT, organs situated out of the primary field might receive important doses, but are not modelled by the treatment planning system. However, the dose in specific volumes such as foetus, ovaries, testes and pacemaker is of interest for radiation protection; it could also be used in the framework of epidemiological studies on secondary cancer induction. The three main components of the out-of-field dose are the leakage radiations, the scattered photons from the collimation system and the scattered photons in the medium. Jérémi Vu Bezin (French Institute of Health and Medical Research, INSERM, France) presented a multi-source simulation code developed to model the scattered photons from the collimation system for different structures of linear accelerators [75]. The same framework could be used to model the other components of the out-of-field dose. Ultimately, a complete out-of-field dose estimation system could be integrated in the treatment planning system.

The workshop on risk communication and risk perception was organised in the context of the FP7 project OPERRA. The workshop was a further step towards the identification of the needs for future research related to perception and communication of low-dose radiation risks. The purpose of this workshop was thus to lay the foundation for a discussion between social, human and natural sciences. This should, at the later stage of the project, help identifying main issues and new topics of research related to perception and communication of risks related to low doses and medical uses of ionising radiation to be included by the European Commission in the radiation research area.

Despite 50 years of extensive research on risk perception and communication, ionising radiation has not yet played a major role in this field of social science. Previous research investigated ionising radiation risks more as a case study, rather than as a prerequisite for building an intellectual and theoretical capacity, for both scientists and the public at large.

Four interrelated challenges of risk perception and risk communication in the fields of low doses and field of medical use of ionising radiation were suggested to be discussed at the workshop in order to identify new research topics. First, the issue of technical information and the use of risk estimates; second, the issue of perception and communication related to uncertainty of scientific information; third, the goal of communication by experts and/or authorities (persuasion for acceptance versus information for informed decision-making); and finally, the role of new media and social networks (e.g. blogs, Facebook, LinkeIin, etc) in the interpretation of risks from low radiation doses.

Two invited keynote speakers opened the discussion: Britt-Marie Drottz Sjöberg, social psychologist (Norwegian University of Science and Technology, Norway) and Peter Michael Booth, communication practitioner (Hylton Environmental, UK). They pointed out that although widely applied in daily life, radiation is discussed rather narrowly in the society. ICRP clearly defines principles of radiological protection, but leaves the essential element of interaction and communication with society rather underdeveloped. Radiological protection is an extremely complex science and the decisions taken at international and/or state level (not to mention local or individual level) are framed by ambiguous value choices and fraught with problems of uncertainty. The keynote speeches presented a justification why radiation research community needs to invest more in the R&D of interaction and communication with society and to promote a trans-disciplinary approach linking natural science, social science and humanities.

After the opening presentations, the participants, 44 researchers from different fields, discussed about the views, attitudes and experiences in the risk perception and risk communication field. They expressed that risk communication and perception related to low doses are a challenge and need to be further investigated, improved and applied.

The following specific ideas were then suggested by the participants: (i) communication should be a dialogue where social sciences can be of help in order to develop knowhow and practices so that people can make their own choices or decisions, (ii) improved participatory practices: people would participate not only to be better informed but also to act more responsibly to find solutions to problems; the scientific community should also better communicate what the limits of science are: science can(not) resolve all questions (epistemology), (iii) fear is a primal emotion: it is not only necessary to study risk perception, but also why people are afraid and what the specific role of their social environment is, (iv) better evaluation of the ethical basis of risk communication: what kind of communication do we want? It was suggested that we should look about the values that drive us: trustworthiness, honesty, communication on an equal level, (v) cross-cultural studies of risk perception are needed in different countries or different sub-populations (e.g. specific regions, also outside Europe) to include societal and culture-specific aspects, (vi) an important point is the communication of scientific uncertainty. There is a strong need of specific studies with a focus on low-dose ionising radiation, (vi) social values in communication to stakeholders should be taken into account, i.e. it is inevitably necessary that (natural) scientists in a communication process need to take the social values of their partner into account.

The discussion was also dedicated to the various understandings of the concepts related to risk perceptions, risks and hazards from the different groups: social scientists, humanities and natural scientists. The focus of the discussion was thereafter about the definitions of risk, hazard, danger or harm from the point of view of the radiation protection society.

In addition risk perception concepts as seen by the natural scientists from radiation protection area were discussed.

From the discussion during the workshop it appeared clearly that good communication about ionising radiation is a matter of well-aligned values, for instance 'How safe is safe enough?'. We can more easily understand each other and reach decisions when there is some shared agreement about what is important for different stakeholders. When we communicate with stakeholders and different research communities about ionising radiation risk, we have to be aware of our own underlying values and of those of others, for instance health, feeling of safety, tampering with nature, moral values etc. Do we analyse and choose these values? Can we easily identify how our values differ from those of other people? How can we better identify our ethical positions, and better shape our communication about risk? These questions were identified as a starting point for a next discussion towards identification of the needs for future research related to perception and communication of ionising radiation risks organised in the context of the FP7 project OPERRA and linked to the FP7 project EAGLE (Enhancing education, training and communication process for informed behaviours and decision-making related to ionising radiation risks).

With the 221 attendees, and the 118 oral and 44 poster presentations this was the largest workshop since the start of the initiative. The 2013 edition of the MELODI workshop was also the first one including sessions with contributions originating from the entire field of radiation protection. Indeed, next to low-dose research, there were presentations regarding radioecology, emergency and recovery preparedness, and dosimetry. The strategic research agendas (SRA) of MELODI, ALLIANCE, NERIS and EURADOS were presented, indicating the interfacing research areas and/or disciplines between the four different areas of radiation protection. It is believed that interactions and further integration between the radiation protection areas will result in better radiation protection research in general. All MELODI workshop participants could vote for the priorities of the upcoming EU FP7 OPERRA call, and the participants were encouraged to express their ideas to update the respective SRAs.

The audience was updated about the latest situation in Fukushima after the nuclear accident in Spring 2011 as a result of the earthquake and tsunami. Dose estimations and health risks, with a focus on thyroid cancer and leukaemia, were presented. There were highly informative sessions on radiation-induced transgenerational effects on animals and plants, radiation effects on the wildlife, internal emitters, and mixed toxicity between radiation and other substances. Furthermore, new radiotherapy strategies, including new radiopharmaceuticals for targeted treatment as well as the use of heavy ions in hadron therapy, were discussed. Heavy ions were also discussed in relation to space radiation. Non-cancer diseases, like the cognitive-, cardiovascular- and the eye lens-related disorders, were presented in the human health effects plenary session and in the session concerning the underlying mechanisms of radiation-induced diseases. The common theme in these sessions was that research should actively mimic the human situation of long-term disease presentation to better understand the related mechanisms. Furthermore, the audience was updated about the investigations regarding genetics and epigenetics in radiation biology, about the increasing use of high-throughput technologies like proteomic and microarray analysis, and next-generation sequencing, and about how to integrate data from different levels in order to provide a holistic view of the radiation effects in a systems biology approach. For future research in radiation biology, it was also emphasised that new in vitro models should be used, like 3D cultures (both homo- and heterotypic), to resemble more the in vivo situation. Next, new developments in dosimetry were presented. The dosimetry, as well as emergency preparedness experts, advised to involve the medical sector in the future Workshops. For all these research fields, constant improvement on collaboration on infrastructure and biobanks should be aimed for. Finally, risk communication and risk perception was discussed. A reference document about communicating risk for low-dose ionising radiation effects is currently under preparation.

In the summary session at the end of the Workshop several questions were raised that remain to be answered. (i) How can we better monitor long-term effects of radiation exposure from the onset of the disease, be it cancer or non-cancer disorders, to the first symptoms and diagnosis? (ii) How can epigenetics help us to understand late effects? (iii) How can we further enhance collaboration between epidemiology, radiobiology, dosimetry and radioecology? (iv) How can we translate low-dose research into improved regulations?

Scientists and chairs from different disciplines were invited to present their opinion and views concerning the above questions. Regarding the 100 mSv limit for low doses, quantifying the risk of the more commonly encountered low radiation exposures (especially from diagnostic and interventional medicine) remains difficult and subject to uncertainty. Long-term follow-up of low-dose exposed cohorts are needed and will certainly help in reducing this uncertainty. Therefore, a major challenge lies in providing a sound mechanistic understanding of low-dose radiation effects, taking into account the early events after exposure (stress response) as well as the late consequences (genetic, epigenetic and metabolic events) at the onset of cancer/non-cancer diseases. In that way we can insure a better integration between radiobiology and epidemiology. Dosimetry plays a major role in this process. By continuously improving dose calculation in humans and animal models, we can increase the confidence and reduce the uncertainty for risk estimate.

Finally, there were sessions devoted to education and training. These are important aspects to ensure knowledge transfer to the new generations. The way forward in organising training and education in radiation protection was discussed in terms of multidisciplinarity, since it is evolving to a more integrated landscape. Several types of training need to be elaborated, such as academic courses, but also vocational and life-long trainings.

European low-dose radiation exposure research continues to evolve. During this Fifth MELODI Workshop, the intricacies and progression of the knowledge in the field were reviewed and new findings were presented. The field is very dynamic and the next-generation researchers from the different areas of radiation protection has shown to be active and eager to contribute jointly towards a better radiation protection.

The authors would like to thank the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, IRSN, SCK•CEN, EURADOS, ALLIANCE, NERIS, and the Euratom Seventh Framework Programme (FP7) projects DoReMi (Grant Agreement No. 249689), STAR (Grant Agreement No. 269672), CEREBRAD (Grant Agreement No. 295552), PROCARDIO (Grant Agreement No. 295823), COMET (Grant Agreement No. 604974) and OPERRA (Grant Agreement No. 604984) for sponsoring the Workshop. All the speakers, poster presenters, and session moderators are warmly thanked for their valuable contribution to this workshop.