Assessing cancer risks of low-dose radiation

Cancer risk from medical radiation procedures for coronary artery disease: a nationwide population-based cohort study

To assess the risk of cancer incidence after medical radiation exposure for coronary artery disease (CAD), a retrospective cohort study was conducted based on Taiwan's National Health Insurance Research Database (NHIRD). Patients with CAD were identified according to the International Classification of Diseases code, 9th Revision, Clinical Modification (ICD-9-CM), and their records of medical radiation procedures were collected from 1997 to 2010. A total of 18,697 subjects with radiation exposure from cardiac imaging or therapeutic procedures for CAD were enrolled, and 19,109 subjects receiving cardiac diagnostic procedures without radiation were adopted as the control group. The distributions of age and gender were similar between the two populations. Cancer risks were evaluated by age-adjusted incidence rate ratio (aIRR) and association with cumulative exposure were further evaluated with relative risks by Poisson regression analysis. A total of 954 and 885 subjects with various types of cancers in both cohorts after following up for over 10 years were found, with incidences of 409.8 and 388.0 per 100,000 person-years, respectively. The risk of breast cancer (aIRR=1.85, 95% confidence interval: 1.14-3.00) was significantly elevated in the exposed female subjects, but no significant cancer risk was found in the exposed males. In addition, cancer risks of the breast and lung were increased with the exposure level. The study suggests that radiation exposure from cardiac imaging or therapeutic procedures for CAD may be associated with the increased risk of breast and lung cancers in CAD patients.

Life-span carcinogenicity studies on Sprague-Dawley rats exposed to γ-radiation: design of the project and report on the tumor occurrence after post-natal radiation exposure (6 weeks of age) delivered in a single acute exposure

BACKGROUND

Experimental long-term carcinogenicity bioassays conducted on rats and mice proved that ionizing radiation can induce a variety of tumor types. However few studies have been conducted on rats.

METHODS

This report deals with the effects of γ-radiation in groups of 416-1,051 6-weeks old Sprague-Dawley rats exposed to 0, 0.1, 1, or 3 Gy of γ-radiation delivered in a single acute exposure. The experiment lasted for the animals' lifespan and all were necropsied and underwent full histopathological evaluation.

RESULTS

The results confirm the dose-related carcinogenic effects of γ-radiation for several organs and tissues. Moreover they indicate that exposure to 0.1 Gy induces a statistically significant increased incidence in Zymbal gland carcinomas and pancreas islet cell carcinomas in females.

CONCLUSIONS

Our data show that exposure to γ-radiation induces carcinogenic effects at all tested doses.

Estimated dietary intake of radionuclides and health risks for the citizens of Fukushima City, Tokyo, and Osaka after the 2011 nuclear accident

The radionuclides released from the Fukushima Daiichi nuclear power plant in 2011 pose a health risk. In this study, we estimated the 1st-year average doses resulting from the intake of iodine 131 (¹³¹I) and cesium 134 and 137 (¹³⁴Cs and ¹³⁷Cs) in drinking water and food ingested by citizens of Fukushima City (∼50 km from the nuclear power plant; outside the evacuation zone), Tokyo (∼230 km), and Osaka (∼580 km) after the accident. For citizens in Fukushima City, we considered two scenarios: Case 1, citizens consumed vegetables bought from markets; Case 2, citizens consumed vegetables grown locally (conservative scenario). The estimated effective doses of ¹³⁴Cs and ¹³⁷Cs agreed well with those estimated through market basket and food-duplicate surveys. The average thyroid equivalent doses due to ingestion of ¹³¹I for adults were 840 µSv (Case 1) and 2700 µSv (Case 2) in Fukushima City, 370 µSv in Tokyo, and 16 µSv in Osaka. The average effective doses due to ¹³⁴Cs and ¹³⁷Cs were 19, 120, 6.1, and 1.9 µSv, respectively. The doses estimated in this study were much lower than values reported by the World Health Organization and the United Nations Scientific Committee on the Effects of Atomic Radiation, whose assessments lacked validation and full consideration of regional trade in foods, highlighting the importance of including regional trade. The 95th percentile effective doses were 2–3 times the average values. Lifetime attributable risks (LARs) of thyroid cancers due to ingestion were 2.3–39×10⁻⁶ (Case 1) and 10–98×10⁻⁶ (Case 2) in Fukushima City, 0.95–14×10⁻⁶ in Tokyo, and 0.11–1.3×10⁻⁶ in Osaka. The contributions of LARs of thyroid cancers due to ingestion were 7.5%–12% of all exposure (Case 1) and 12%–30% (Case 2) in Fukushima City.

Low dose irradiation profoundly affects transcriptome and microRNAme in rat mammary gland tissues

Ionizing radiation has been successfully used in medical tests and treatment therapies for a variety of medical conditions. However, patients and health-care workers are greatly concerned about overexposure to medical ionizing radiation and possible cancer induction due to frequent mammographies and/or CT scans. Diagnostic imaging involves the use of low doses of ionizing radiation, and its potential carcinogenic role creates a cancer risk concern for exposed individuals. In this study, the effects of X-ray exposure of different doses on the gene expression patterns and the micro-RNA expression patterns in normal breast tissue were investigated in rats. Our results revealed the activation of immune response pathways upon low dose of radiation exposure. These included natural killer mediated cytotoxicity pathways, antigen processing and presentation pathways, chemokine signaling pathways, and T- and B-cell receptor signaling pathways. Both high and low doses of radiation led to miRNA expression alterations. Increased expression of miR-34a may be linked to cell cycle arrest and apoptosis. Up-regulation of miR-34a was correlated with down-regulation of its target E2F3 and up-regulation of p53. This data suggests that ionizing radiation at specific high and low doses leads to cell cycle arrest and a possible initiation of apoptosis.

Surgical management of giant secondary malignant fibrous histiocytoma following radiotherapy for nasopharyngeal carcinoma: A case report and literature review

Malignant fibrous histiocytoma (MFH) is rare in the chest wall, particularly in patients who have undergone radiotherapy for primary nasopharyngeal cancer. In the present study, a case of MFH of the upper chest wall that appeared four years after initial radiotherapy for squamous cell carcinoma of the nasopharynx is reported. Furthermore, two-step surgical management was successfully performed consisting of i) tumor-reductive excision and ii) limb salvage surgery, including wide resection of the tumor mass, defect reconstruction of the chest wall using left latissimus dorsi myocutaneous flap and dermatoplasty of the flap-supplied region. The progress of the clinical characteristics, the reasons for radiation-induced carcinogenesis, the treatment options and the prognostic factors of MFH are also reviewed. Finally, the importance of prevention and follow-up of this malignancy are highlighted and specific advice is offered.

Cancer risk at low doses of ionizing radiation: artificial neural networks inference from atomic bomb survivors

Cancer risk at low doses of ionizing radiation remains poorly defined because of ambiguity in the quantitative link to doses below 0.2 Sv in atomic bomb survivors in Hiroshima and Nagasaki arising from limitations in the statistical power and information available on overall radiation dose. To deal with these difficulties, a novel nonparametric statistics based on the 'integrate-and-fire' algorithm of artificial neural networks was developed and tested in cancer databases established by the Radiation Effects Research Foundation. The analysis revealed unique features at low doses that could not be accounted for by nominal exposure dose, including (i) the presence of a threshold that varied with organ, gender and age at exposure, and (ii) a small but significant bumping increase in cancer risk at low doses in Nagasaki that probably reflects internal exposure to (239)Pu. The threshold was distinct from the canonical definition of zero effect in that it was manifested as negative excess relative risk, or suppression of background cancer rates. Such a unique tissue response at low doses of radiation exposure has been implicated in the context of the molecular basis of radiation-environment interplay in favor of recently emerging experimental evidence on DNA double-strand break repair pathway choice and its epigenetic memory by histone marking.

Low-dose radiation exposure induces a HIF-1-mediated adaptive and protective metabolic response

Because of insufficient understanding of the molecular effects of low levels of radiation exposure, there is a great uncertainty regarding its health risks. We report here that treatment of normal human cells with low-dose radiation induces a metabolic shift from oxidative phosphorylation to aerobic glycolysis resulting in increased radiation resistance. This metabolic change is highlighted by upregulation of genes encoding glucose transporters and enzymes of glycolysis and the oxidative pentose phosphate pathway, concomitant with downregulation of mitochondrial genes, with corresponding changes in metabolic flux through these pathways. Mechanistically, the metabolic reprogramming depends on HIF1α, which is induced specifically by low-dose irradiation linking the metabolic pathway with cellular radiation dose response. Increased glucose flux and radiation resistance from low-dose irradiation are also observed systemically in mice. This highly sensitive metabolic response to low-dose radiation has important implications in understanding and assessing the health risks of radiation exposure.

Increased intracranial pressure in mini-pigs exposed to simulated solar particle event radiation

Changes in intracranial pressure (ICP) during space flight have stimulated an area of research in space medicine. It is widely speculated that elevations in ICP contribute to structural and functional ocular changes, including deterioration in vision, which is also observed during space flight. The aim of this study was to investigate changes in OP occurring as a result of ionizing radiation exposure (at doses and dose-rates relevant to solar particle event radiation). We used a large animal model, the Yucatan mini-pig, and were able to obtain measurements over a 90 day period. This is the first investigation to show long term recordings of ICP in a large animal model without an invasive craniotomy procedure. Further, this is the first investigation reporting increased ICP after radiation exposure.

Embryos of the zebrafish Danio rerio in studies of non-targeted effects of ionizing radiation

The use of embryos of the zebrafish Danio rerio as an in vivo tumor model for studying non-targeted effects of ionizing radiation was reviewed. The zebrafish embryo is an animal model, which enables convenient studies on non-targeted effects of both high-linear-energy-transfer (LET) and low-LET radiation by making use of both broad-beam and microbeam radiation. Zebrafish is also a convenient embryo model for studying radiobiological effects of ionizing radiation on tumors. The embryonic origin of tumors has been gaining ground in the past decades, and efforts to fight cancer from the perspective of developmental biology are underway. Evidence for the involvement of radiation-induced genomic instability (RIGI) and the radiation-induced bystander effect (RIBE) in zebrafish embryos were subsequently given. The results of RIGI were obtained for the irradiation of all two-cell stage cells, as well as 1.5 hpf zebrafish embryos by microbeam protons and broad-beam alpha particles, respectively. In contrast, the RIBE was observed through the radioadaptive response (RAR), which was developed against a subsequent challenging dose that was applied at 10 hpf when <0.2% and <0.3% of the cells of 5 hpf zebrafish embryos were exposed to a priming dose, which was provided by microbeam protons and broad-beam alpha particles, respectively. Finally, a perspective on the field, the need for future studies and the significance of such studies were discussed.

Radiation-induced bystander effect: early process and rapid assessment

Radiation-induced bystander effect (RIBE) is a biological process that has received attention over the past two decades. RIBE refers to a plethora of biological effects in non-irradiated cells, including induction of genetic damages, gene expression, cell transformation, proliferation and cell death, which are initiated by receiving bystander signals released from irradiated cells. RIBE brings potential hazards to normal tissues in radiotherapy, and imparts a higher risk from low-dose radiation than we previously thought. Detection with proteins related to DNA damage and repair, cell cycle control, proliferation, etc. have enabled rapid assessment of RIBE in a number of research systems such as cultured cells, three-dimensional tissue models and animal models. Accumulated experimental data have suggested that RIBE may be initiated rapidly within a time frame as short as several minutes after radiation. These have led to the requirement of techniques capable of rapidly assessing RIBE itself as well as assessing the early processes involved.

Biological considerations when comparing proton therapy with photon therapy

Owing to the limited availability of data on the outcome of proton therapy, treatments are generally optimized based on broadly available data on photon-based treatments. However, the microscopic pattern of energy deposition of protons differs from that of photons, leading to a different biological effect. Consequently, proton therapy needs a correction factor (relative biological effectiveness) to relate proton doses to photon doses, and currently, a generic value is used. Moreover, the macroscopic distribution of dose in proton therapy differs compared with photon treatments. Although this may offer new opportunities to reduce dose to normal tissues, it raises the question whether data obtained from photon-based treatments offer sufficient information on dose-volume effects to optimally use unique features of protons. In addition, there are potential differences in late effects due to low doses of secondary radiation outside the volume irradiated by the primary beam. This article discusses the controversies associated with these 3 issues when comparing proton and photon therapy.

Side-by-side comparison of DNA damage induced by low-energy electrons and high-energy photons with solid TpTpT trinucleotide

The genotoxic effects of high-energy ionizing radiation have been largely attributed to the ionization of H2O leading to hydroxyl radicals and the ionization of DNA leading mostly to damage through base radical cations. However, the contribution of low-energy electrons (LEEs; ≤ 10 eV), which involves subionization events, has been considered to be less important than that of hydroxyl radicals and base radical cations. Here, we compare the ability of LEEs and high-energy X-ray photons to induce DNA damage using dried thin films of TpTpT trinucleotide as a simple and representative model for DNA damage. The main radiation-induced damage of TpTpT as measured by high-performance liquid chromatography (HPLC) with UV detection and HPLC coupled to tandem mass spectrometry analyses included thymine release (-Thy), strand breaks (pT, Tp, pTpT, TpTp, and TpT), and the formation of base modifications [5,6-dihydrothymine (5,6-dhT), 5-hydroxymethyluracil (5-hmU), and 5-formyluracil (5-fU)]. The global profile of products was very similar for both types of radiation indicating converging pathways of formation. The percent damage of thymine release, fragmentation, and base modification was 20, 19, and 61 for high-energy X-rays, respectively, compared to 35, 13, and 51 for LEEs (10 eV). Base release was significantly lower for X-rays. In both cases, phosphodiester bond cleavage gave mononucleotides (pT and Tp) and dinucleotides (pTpT and TpTp) containing a terminal phosphate as the major fragments. For base modifications, the ratio of reductive (5,6-dhT) to oxidative products (5-hmU plus 5-fU) was 0.9 for high-energy X-rays compared to 1.7 for LEEs. These results indicate that LEEs give a similar profile of products compared to ionizing radiation.

Magnetic Resonance Imaging Compared to Conventional Radiographs for Detection of Chronic Structural Changes in Sacroiliac Joints in Axial Spondyloarthritis

Objective.

We investigated the performance of magnetic resonance imaging (MRI) compared to conventional radiographs for detection of chronic structural changes in the sacroiliac joints (SIJ) in patients with axial spondyloarthritis (SpA).

Methods.

We included 112 patients with definite axial SpA (68 with ankylosing spondylitis and 44 with nonradiographic axial SpA), for whom radiographs and MRI scans of the SIJ performed at the same time were available. Radiographs and MRI of the SIJ were scored for subchondral sclerosis (score 0–2), erosions (score 0–3), and joint space changes (score 0–5) in each SIJ. Readers provided an overall impression of the extent of damage according to the scoring system of the modified New York criteria.

Results.

In total, 224 SIJ from 112 patients were available for analysis. There was rather low agreement between MRI and radiographs concerning definite erosions of SIJ (κ = 0.11), moderate agreement for definite subchondral sclerosis (κ = 0.46) and definite joint space abnormalities (κ = 0.41), and almost perfect agreement for joint ankylosis (κ = 0.85). MRI demonstrated a good overall performance in detection of definite “chronic” sacroiliitis, with a sensitivity of 84% and a specificity of 61%. For sacroiliitis fulfilling the modified New York criteria, MRI had a sensitivity of 81% and a specificity of 64% using radiographs as the reference method.

Conclusion.

MRI demonstrated good overall performance for detection of chronic structural changes in the SIJ as compared to radiographs.

Fallout from the Chernobyl accident and overall cancer incidence in Finland

AIM

We studied whether incidence of all cancer sites combined was associated with the radiation exposure due to fallout from the Chernobyl accident in Finland. An emphasis was on the first decade after the accident to assess the suggested "promotion effect".

METHODS

The segment of Finnish population with a stable residence in the first post-Chernobyl year (2 million people) was studied. The analyses were based on a 250m × 250m grid squares covering all of Finland and all cancer cases except cancers of the breast, prostate and lung. Cancer incidence in four exposure areas (based on first-year dose due to external exposure <0.1 mSv, 0.1-1.3, 0.3-0.5, or ≥ 0.5 mSv) was compared before the Chernobyl accident (1981-1985) and after it (1988-2007) taking into account cancer incidence trends for a longer period prior to the accident (since 1966).

RESULTS

There were no systematic differences in the cancer incidence in relation to radiation exposure in any calendar period, or any subgroup by sex or age at accident.

CONCLUSION

The current large and comprehensive cohort analysis of the relatively low levels of the Chernobyl fallout in Finland did not observe a cancer promotion effect.

Unique phenotypes in the sperm of the earthworm Eudrilus eugeniae for assessing radiation hazards

The earthworm, Eudrilus eugeniae is a segmented worm. It has two pairs of testes whose cells are highly proliferative. It was found that the earthworm, which is irradiated with X-ray, shows the following phenotypic changes in its sperm: fragmented acrosome in the head, break in the tail, and the appearance of zigzag sperm tail. Sperm morphology can be used as a tool to study radiation hazards in local areas. These three phenotypes were not observed in the sperm of worms exposed to different concentration of toxic chemicals such as sodium arsenate, lead acetate, and mercuric chloride. In contrast, exposure of worms to ethidium bromide caused fragmented acrosome in the head of their sperm cells.

Charged particle therapy--optimization, challenges and future directions

The use of charged particle therapy to control tumours non-invasively offers advantages over conventional radiotherapy. Protons and heavy ions deposit energy far more selectively than X-rays, allowing a higher local control of the tumour, a lower probability of damage to healthy tissue, low risk of complications and the chance for a rapid recovery after therapy. Charged particles are also useful for treating tumours located in areas that surround tissues that are radiosensitive and in anatomical sites where surgical access is limited. Current trial outcomes indicate that accelerated ions can potentially replace surgery for radical cancer treatments, which might be beneficial as the success of surgical cancer treatments are largely dependent on the expertise and experience of the surgeon and the location of the tumour. However, to date, only a small number of controlled randomized clinical trials have made comparisons between particle therapy and X-rays. Therefore, although the potential advantages are clear and supported by data, the cost:benefit ratio remains controversial. Research in medical physics and radiobiology is focusing on reducing the costs and increasing the benefits of this treatment.

Rb1 haploinsufficiency promotes telomere attrition and radiation-induced genomic instability

Germline mutations of the retinoblastoma gene (RB1) predispose to both sporadic and radiation-induced osteosarcoma, tumors characterized by high levels of genomic instability, and activation of alternative lengthening of telomeres. Mice with haploinsufficiency of the Rb1 gene in the osteoblastic lineage reiterate the radiation susceptibility to osteosarcoma seen in patients with germline RB1 mutations. We show that the susceptibility is accompanied by an increase in genomic instability, resulting from Rb1-dependent telomere erosion. Radiation exposure did not accelerate the rate of telomere loss but amplified the genomic instability resulting from the dysfunctional telomeres. These findings suggest that telomere maintenance is a noncanonical caretaker function of the retinoblastoma protein, such that its deficiency in cancer may potentiate DNA damage-induced carcinogenesis by promoting formation of chromosomal aberrations, rather than simply by affecting cell-cycle control.

Increased mammogram-induced DNA damage in mammary epithelial cells aged in vitro

Concerned about the risks of mammography screening in the adult population, we analyzed the ability of human mammary epithelial cells to cope with mammogram-induced DNA damage. Our study shows that an X-ray dose of 20 mGy, which is the standard dose received by the breast surface per two-view mammogram X-ray exploration, induces increased frequencies of DNA double-strand breaks to in vitro aged–but not to young–human mammary epithelial cells. We provide evidence that aged epithelial breast cells are more radiosensitive than younger ones. Our studies point to an inefficient damage response of aged cells to low-dose radiation, this being due to both delayed and incomplete mobilization of repair proteins to DNA strand breaks. This inefficient damage response is translated into an important delay in double-strand break disappearance and consequent accumulation of unrepaired DNA breaks. The result of this is a significant increase in micronuclei frequency in the in vitro aged mammary epithelial cells exposed to doses equivalent to a single mammogram X-ray exploration. Since our experiments were carried out in primary epithelial cell cultures in which cells age at the same time as they undergo replication-dependent telomere shortening, we needed to determine the contribution of these two factors to their phenotype. In this paper, we report that the exogenous expression of human telomerase retrotranscriptase in late population doubling epithelial cells does not rescue its delayed repair phenotype. Therefore, retarded DNA break repair is a direct consequence of cellular aging itself, rather than a consequence of the presence of dysfunctional telomeres. Our findings of long-lasting double strand breaks and incomplete DNA break repair in the in vitro aged epithelial cells are in line with the increased carcinogenic risks of radiation exposures at older ages revealed by epidemiologic studies.

The shape of the radiation dose response for DNA double-strand break induction and repair

DNA double-strand breaks are among the most deleterious lesions induced by ionising radiation. A range of inter-connected cellular response mechanisms has evolved to enable their efficient repair and thus protect the cell from the harmful consequences of un- or mis-repaired breaks which may include early effects such as cell killing and associated acute toxicities and late effects such as cancer. A number of studies suggest that the induction and repair of double-strand breaks may not always occur linearly with ionising radiation dose. Here we have aimed to identify and discuss some of the biological and methodological factors that can potentially modify the shape of the dose response curve obtained for these endpoints using the most common assays for double-strand breaks, pulsed-field gel electrophoresis and microscopic scoring of radiation-induced foci.

The preference for error-free or error-prone postreplication repair in Saccharomyces cerevisiae exposed to low-dose methyl methanesulfonate is cell cycle dependent

Cells employ error-free or error-prone postreplication repair (PRR) processes to tolerate DNA damage. Here, we present a genome-wide screen for sensitivity to 0.001% methyl methanesulfonate (MMS). This relatively low dose is of particular interest because wild-type cells exhibit no discernible phenotypes in response to treatment, yet PRR mutants are unique among repair mutants in their exquisite sensitivity to 0.001% MMS; thus, low-dose MMS treatment provides a distinctive opportunity to study postreplication repair processes. We show that upon exposure to low-dose MMS, a PRR-defective rad18Δ mutant stalls into a lengthy G2 arrest associated with the accumulation of single-stranded DNA (ssDNA) gaps. Consistent with previous results following UV-induced damage, reactivation of Rad18, even after prolonged G2 arrest, restores viability and genome integrity. We further show that PRR pathway preference in 0.001% MMS depends on timing and context; cells preferentially employ the error-free pathway in S phase and do not require MEC1-dependent checkpoint activation for survival. However, when PRR is restricted to the G2 phase, cells utilize REV3-dependent translesion synthesis, which requires a MEC1-dependent delay and results in significant hypermutability.

Radiation treatment effects on the proteome of the tumour microenvironment

Exposure of tumourous tissue to ionizing radiation initiates a wound-healing response involving remodelling of the extracellular microenvironment. The initial reaction involves direct damage to the matrix proteins and the secretion and activation of proteolytic enzymes that lead to local destruction of the extracellular matrix. Subsequently the wounded area may undergo complete repair, may enter a prolonged period of heightened proteolysis, or may overproduce matrix proteins leading to fibrosis. The source of matrix degrading enzymatic activity may be the tumour cells and the tumour stroma. Additional complexity is provided by proteolytic activity released from tissue macrophages, mast cells and by invading inflammatory cells. The local production of growth factors, including VEGF and TGF-β play a key role in coordinating the response. It is anticipated that the application of modern proteomic technologies will reveal hitherto unrecognised levels of complexity in these processes. Hopefully this will lead to the development of new therapeutic strategies to prevent long-term health implications of radiation exposure.

Second malignant neoplasms following radiotherapy

More than half of all cancer patients receive radiotherapy as a part of their treatment. With the increasing number of long-term cancer survivors, there is a growing concern about the risk of radiation induced second malignant neoplasm [SMN]. This risk appears to be highest for survivors of childhood cancers. The exact mechanism and dose-response relationship for radiation induced malignancy is not well understood, however, there have been growing efforts to develop strategies for the prevention and mitigation of radiation induced cancers. This review article focuses on the incidence, etiology, and risk factors for SMN in various organs after radiotherapy.

Assessment of the risk for developing a second malignancy from scattered and secondary radiation in radiation therapy

With the average age of radiation therapy patients decreasing and the advent of more complex treatment options comes the concern that the incidences of radiation-induced cancer might increase in the future. The carcinogenic effects of radiation are not well understood for the entire dose range experienced in radiation therapy. Longer epidemiologic studies are needed to improve current risk models and reduce uncertainties of current risk model parameters. On the other hand, risk estimations are needed today to judge the risks versus benefits of modern radiation therapy techniques. This paper describes the current state-of-the-art in risk modeling for radiation-induced malignancies in radiation therapy, distinguishing between two volumes: first, the organs within the main radiation field receiving low or intermediate doses (typically between 0.1 and 50 Gy); and second, the organs far away from the treatment volume receiving low doses mainly due to scattered and secondary radiation (typically below 0.1 Gy). The dosimetry as well as the risk model formalisms are outlined. Furthermore, example calculations and results are presented for intensity-modulated photon therapy versus proton therapy.

Exposure to diagnostic radiation and risk of breast cancer among carriers of BRCA1/2 mutations: retrospective cohort study (GENE-RAD-RISK)

OBJECTIVE

To estimate the risk of breast cancer associated with diagnostic radiation in carriers of BRCA1/2 mutations.

DESIGN

Retrospective cohort study (GENE-RAD-RISK).

SETTING

Three nationwide studies (GENEPSO, EMBRACE, HEBON) in France, United Kingdom, and the Netherlands,

PARTICIPANTS

1993 female carriers of BRCA1/2 mutations recruited in 2006-09.

MAIN OUTCOME MEASURE

Risk of breast cancer estimated with a weighted Cox proportional hazards model with a time dependent individually estimated cumulative breast dose, based on nominal estimates of organ dose and frequency of self reported diagnostic procedures. To correct for potential survival bias, the analysis excluded carriers who were diagnosed more than five years before completion of the study questionnaire.

RESULTS

In carriers of BRCA1/2 mutations any exposure to diagnostic radiation before the age of 30 was associated with an increased risk of breast cancer (hazard ratio 1.90, 95% confidence interval 1.20 to 3.00), with a dose-response pattern. The risks by quarter of estimated cumulative dose <0.0020 Gy, ≥ 0.0020-0.0065 Gy, ≥ 0.0066-0.0173 Gy, and ≥ 0.0174 Gy were 1.63 (0.96 to 2.77), 1.78 (0.88 to 3.58), 1.75 (0.72 to 4.25), and 3.84 (1.67 to 8.79), respectively. Analyses on the different types of diagnostic procedures showed a pattern of increasing risk with increasing number of radiographs before age 20 and before age 30 compared with no exposure. A history of mammography before age 30 was also associated with an increased risk of breast cancer (hazard ratio 1.43, 0.85 to 2.40). Sensitivity analysis showed that this finding was not caused by confounding by indication of family history.

CONCLUSION

In this large European study among carriers of BRCA1/2 mutations, exposure to diagnostic radiation before age 30 was associated with an increased risk of breast cancer at dose levels considerably lower than those at which increases have been found in other cohorts exposed to radiation. The results of this study support the use of non-ionising radiation imaging techniques (such as magnetic resonance imaging) as the main tool for surveillance in young women with BRCA1/2 mutations.

Accumulation of DNA damage in complex normal tissues after protracted low-dose radiation

The biological consequences of low levels of radiation exposure and their effects on human health are unclear. Ionizing radiation induces a variety of lesions of which DNA double-strand breaks (DSBs) are the most biologically significant, because unrepaired or misrepaired DSBs can lead to genomic instability and cell death. Using repair-proficient mice as an in vivo system we monitored the accumulation of DNA damage in normal tissues exposed to daily low-dose radiation of 100mGy or 10mGy. Radiation-induced foci in differentiated and tissue-specific stem cells were quantified by immunofluorescence microscopy after 2, 4, 6, 8, and 10 weeks of daily low-dose radiation and DNA lesions were characterized using transmission electron microscopy (TEM) combined with immunogold-labeling. In brain, long-living cortical neurons had a significant accumulation of foci with increasing cumulative doses. In intestine and skin, characterized by constant cell renewal of their epithelial lining, differentiated enterocytes and keratinocytes had either unchanged or only slightly increased foci levels during protracted low-dose radiation. Significantly, analysis of epidermal stem cells in skin revealed a constant increase of 53BP1 foci during the first weeks of low-dose radiation even with 10mGy, suggesting substantial accumulations of DSBs. However, TEM analysis suggests that these remaining 53BP1 foci, which are predominantly located in compact heterochromatin, do not co-localize with phosphorylated Ku70 or DNA-PKcs, core components of non-homologous end-joining. The biological relevance of these persistent 53BP1 foci, particularly their contribution to genomic instability by genetic and epigenetic alterations, has to be defined in future studies.

Low-dose gamma-irradiation inhibits IL-6 secretion from human lung fibroblasts that promotes bronchial epithelial cell transformation by cigarette-smoke carcinogen

Despite decades of research in defining the health effects of low-dose (<100 mGy) ionizing photon radiation (LDR), the relationship between LDR and human cancer risk remains elusive. Because chemical carcinogens modify the tumor microenvironment, which is critical for cancer development, we investigated the role and mechanism of LDR in modulating the response of stromal cells to chemical carcinogen-induced lung cancer development. Secretion of proinflammatory cytokines such as interleukin-6 (IL-6), CXCL1 and CXCL5 from human lung fibroblasts was induced by cigarette-smoke carcinogen benzo[a]pyrene diol epoxide (BPDE), which was inhibited by a single dose of LDR. The activation of NF-κB, which is important for BPDE-induced IL-6 secretion, was also effectively suppressed by LDR. In addition, conditioned media from BPDE-treated fibroblasts activated STAT3 in the immortalized normal human bronchial epithelial cell line Beas-2B, which was blocked with an IL-6 neutralizing antibody. Conditioned medium from LDR-primed and BPDE-treated fibroblast showed diminished capacity in activating STAT3. Furthermore, IL-6 enhanced BPDE-induced Beas-2B cell transformation in vitro. These results suggest that LDR inhibits cigarette smoke-induced lung carcinogenesis by suppressing secretion of cytokines such as IL-6 from fibroblasts in lung tumor-prone microenvironment.

Estimation of thyroid doses and health risks resulting from the intake of radioactive iodine in foods and drinking water by the citizens of Tokyo after the Fukushima nuclear accident

The release of radioactive materials from the Fukushima nuclear power plant after the Great East Japan Earthquake on 11 March 2011 poses health risks. In this study, the intake of iodine 131 (I-131) in drinking water and foods (milk, dairy products, and vegetables) by citizens of Tokyo was estimated. The effects of countermeasures (restrictions on the distribution of foods and the distribution of bottled water for infants) on reducing intake were also evaluated. The average thyroid equivalent doses without countermeasures from 21 March 2011 were 0.42 mSv in adults, 1.49 mSv in children, and 2.08 mSv in infants. Those with countermeasures were 0.28, 0.97, and 1.14 mSv respectively, reductions of 33%, 35%, and 45%. Drinking water contributed more to intake by adults and children than foods. The intake of I-131 within the first 2 weeks was more than 80% of the estimated intake, owing to its short half-life, indicating that rapid countermeasures are important in reducing intake. The average risks of cancer incidence and mortality due to I-131 for infants were estimated to be 3×10(-5) and 0.2×10(-5), respectively, lower than the annual risks of traffic accidents, naturally occurring radioactive material (potassium 40), and environmental pollutants such as diesel exhaust particles.

Low-dose radiation exposure and carcinogenesis

Absorption of energy from ionizing radiation by the genetic material in the cell leads to damage to DNA, which in turn leads to cell death, chromosome aberrations and gene mutations. While early or deterministic effects result from organ and tissue damage caused by cell killing, latter two are considered to be involved in the initial events that lead to the development of cancer. Epidemiological studies have demonstrated the dose-response relationships for cancer induction and quantitative evaluations of cancer risk following exposure to moderate to high doses of low-linear energy transfer radiation. A linear, no-threshold model has been applied to assessment of the risks resulting from exposure to moderate and high doses of ionizing radiation; however, a statistically significant increase has hardly been described for radiation doses below 100 mSv. This review summarizes our current knowledge of the physical and biological features of low-dose radiation and discusses the possibilities of induction of cancer by low-dose radiation.

Microvessel density is high in clear-cell renal cell carcinomas of Ukrainian patients exposed to chronic persistent low-dose ionizing radiation after the Chernobyl accident

During the 25-year period subsequent to the Chernobyl accident, the morbidity of malignant renal tumors in Ukraine has increased from 4.7 to 10.7 per 100,000 of the total population. Recent studies of our group have shown that increases in morbidity, aggressiveness, and proliferative activity of renal cell carcinomas (RCCs), especially clear-cell renal cell carcinoma (CCRCC), in Ukrainian patients continuously inhabiting the radio-contaminated areas could be explained by specific molecular changes influenced by the so-called "chronic persistent low-dose ionizing radiation" (CPLDIR) exposure. This study aimed to examine the role of angiogenesis in CCRCC carcinogenesis associated with CPLDIR in patients living more than 20 years in cesium 137 ((137)Cs) contaminated areas after the Chernobyl accident in Ukraine. Paraffin-embedded specimens of 106 CCRCs were studied: Control cases were 18 tumors from Spanish patients (group 1), 25 tumors from Ukrainian patients from so-called clean areas without known radio-contamination (group 2), and 63 tumors from Ukrainian patients from radio-contaminated areas (group 3). For intratumoral microvessel density (MVD) determination, anti-CD31 antibody was used. A computerized image analysis program was used to quantitatively calculate the vascular density. Seventy-three percent of group 3 and 72 % of group 2 CCRCCs displayed the highest MVD. A striking increase in MVD was seen in group 3 CCRCCs, in comparison with groups 1 and 2 (p < 0.001). The majority of the hot spot vessels in group 3 was poorly differentiated. Moreover, MVD values for total vessels as well as for capillaries and tumor grade were strongly correlated. When we compared only tumor-node-metastasis tumor stages I and II, the differences remained statistically significant (p < 0.1). The ratio of the average total vessels and capillaries in the Ukrainian groups combined was 1.65:1 in comparison to the Spanish group. Our results provide evidence that CPLDIR exposure increases MVD (particularly capillary) in CCRCCs and is associated with a higher histological grade.

No threshold for the induction of chromosomal damage at clinically relevant low doses of X rays

The recent steep increase in population dose from radiation-based medical diagnostics, such as computed tomography (CT) scans, requires insight into human health risks, especially in terms of cancer development. Since the induction of genetic damage is considered a prominent cause underlying the carcinogenic potential of ionizing radiation, we quantified the induction of micronuclei and loss of heterozygosity events in human cells after exposure to clinically relevant low doses of X rays. A linear dose-response relationship for induction of micronuclei was observed in human fibroblasts with significantly increased frequencies at doses as low as 20 mGy. Strikingly, cells exposed during S-phase displayed the highest induction, whereas non S-phase cells showed no significant induction below 100 mGy. Similarly, the induction of loss of heterozygosity in human lymphoblastoid cells quantified at HLA loci, was linear with dose and reached significance at 50 mGy. Together the findings favor a linear-no-threshold model for genetic damage induced by acute exposure to ionizing radiation. We speculate that the higher radiosensitivity of S-phase cells might relate to the excessive cancer risk observed in highly proliferative tissues in radiation exposed organisms.

In vivo interactions between ionizing radiation, inflammation and chemical carcinogens identified by increased DNA damage responses

Exposure to ionizing radiation or a variety of chemical agents is known to increase the risk of developing malignancy and many tumors have been linked to inflammatory processes. In most studies, the potentially harmful effects of ionizing radiation or other agents are considered in isolation, mainly due to the large number of experiments required to assess the effects of mixed exposures with different doses and different schedules, and the length of time and expense of studies using disease as the measure of outcome. Here, we have used short-term DNA damage responses to identify interactive effects of mixed exposures. The data demonstrate that exposure to ionizing radiation on two separate occasions ten days apart leads to an increase in the percentage of cells with a sub-G(0) DNA content compared to cells exposed only once, and this is a greater than additive effect. Short-term measurements of p53 stabilization, induction of p21/Cdkn1a and of apoptosis also identify these interactive effects. We also demonstrate similar interactive effects of radiation with the mutagenic chemical methyl-nitrosourea and with a nonspecific pro-inflammatory agent, lipopolysaccharide. The magnitude of the interactive effects is greater in cells taken from mice first exposed as juveniles compared to adults. These data indicate that short-term measurements of DNA damage and response to damage are useful for the identification of interactions between ionizing radiation and other agents.

Cardiac imaging: does radiation matter?

The use of ionizing radiation in cardiovascular imaging has generated considerable discussion. Radiation should not be considered in isolation, but rather in the context of a careful examination of the benefits, risks, and costs of cardiovascular imaging. Such consideration requires an understanding of some fundamental aspects of the biology, physics, epidemiology, and terminology germane to radiation, as well as principles of radiological protection. This paper offers a concise, contemporary perspective on these areas by addressing pertinent questions relating to radiation and its application to cardiac imaging.

Radiation exposure from imaging tests: is there an increased cancer risk?

The increasing exposure to low-dose radiation from diagnostic testing has prompted renewed interest in evaluating its carcinogenic risk, but quantifying health risk from low-dose radiation exposure remains controversial. The current approach is to adopt the linear non-threshold model, which is commonly applied to high-dose exposure, and apply it to assess risk from low-dose exposure. However, existing data are conflicting and limited to epidemiological studies and/or in vitro analyses. In this article, we will discuss the potential cancer risk from low- and high-dose radiation, their effects on DNA repair response pathways, and the best course of action for patients and providers to minimize risk.

Assessing the risk of second malignancies after modern radiotherapy

Recent advances in radiotherapy have enabled the use of different types of particles, such as protons and heavy ions, as well as refinements to the treatment of tumours with standard sources (photons). However, the risk of second cancers arising in long-term survivors continues to be a problem. The long-term risks from treatments such as particle therapy have not yet been determined and are unlikely to become apparent for many years. Therefore, there is a need to develop risk assessments based on our current knowledge of radiation-induced carcinogenesis.

Gain of chromosome band 7q11 in papillary thyroid carcinomas of young patients is associated with exposure to low-dose irradiation

The main consequence of the Chernobyl accident has been an increase in papillary thyroid carcinomas (PTCs) in those exposed to radioactive fallout as young children. Our aim was to identify genomic alterations that are associated with exposure to radiation. We used array comparative genomic hybridization to analyze a main (n = 52) and a validation cohort (n = 28) of PTC from patients aged <25 y at operation and matched for age at diagnosis and residency. Both cohorts consisted of patients exposed and not exposed to radioiodine fallout. The study showed association of a gain on chromosome 7 (7q11.22-11.23) with exposure (false discovery rate = 0.035). Thirty-nine percent of the exposed group showed the alteration; however, it was not found in a single case from the unexposed group. This was confirmed in the validation set. Because only a subgroup of cases in the exposed groups showed gain of 7q11.22-11.23, it is likely that different molecular subgroups and routes of radiation-induced carcinogenesis exist. The candidate gene CLIP2 was specifically overexpressed in the exposed cases. In addition, the expression of the genes PMS2L11, PMS2L3, and STAG3L3 correlated with gain of 7q11.22-11.23. An enrichment of Gene Ontology terms "DNA repair" (PMS2L3, PMS2L5), "response to DNA damage stimulus" (BAZ1B, PMS2L3, PMS2L5, RFC2), and "cell-cell adhesion" (CLDN3, CLDN4) was found. This study, using matched exposed and unexposed cohorts, provides insights into the radiation-related carcinogenesis of young-onset PTC and, with the exposure-specific gain of 7q11 and overexpression of the CLIP2 gene, radiation-specific molecular markers.

Therapy-related myeloid neoplasms: pathobiology and clinical characteristics

Therapy-related myeloid neoplasms (t-MNs) are serious long-term consequences of cytotoxic treatments for an antecedent disorder. t-MNs are observed after ionizing radiation as well as conventional chemotherapy including alkylating agents, topoisomerase-II-inhibitors and antimetabolites. In addition, adjuvant use of recombinant human granulocyte-colony stimulating factor may also increase the risk of t-MNs. There is clinical and biological overlap between t-MNs and high-risk de novo myelodysplastic syndromes and acute myeloid leukaemia suggesting similar mechanisms of leukaemogenesis. Human studies and animal models point to a prominent role of genetic susceptibilty in the pathogenesis of t-MNs. Common genetic variants have been identified that modulate t-MN risk, and t-MNs have been observed in some cancer predisposition syndromes. In either case, establishing a leukaemic phenotype requires acquisition of somatic mutations - most likely induced by the cytotoxic treatment. Knowledge of the specific nature of the initiating exposure has allowed the identification of crucial pathogenetic mechanisms and for these to be modelled in vitro and in vivo. Prognosis of patients with t-MNs is dismal and at present, the only curative approach for the majority of these individuals is haematopoietic stem cell transplantation, which is characterized by high transplant-related mortality rates. Novel transplantation strategies using reduced intensity conditioning regimens as well as novel drugs - demethylating agents and targeted therapies - await clinical testing and may improve outcome. Ultimately, individual assessment of genetic risk factors may translate into tailored therapies and establish a strategy for reducing t-MN incidences without jeopardizing therapeutic success rates for the primary disorders.

Track structures, DNA targets and radiation effects in the biophysical Monte Carlo simulation code PARTRAC

This review describes the PARTRAC suite of comprehensive Monte Carlo simulation tools for calculations of track structures of a variety of ionizing radiation qualities and their biological effects. A multi-scale target model characterizes essential structures of the whole genomic DNA within human fibroblasts and lymphocytes in atomic resolution. Calculation methods and essential results are recapitulated regarding the physical, physico-chemical and chemical stage of track structure development of radiation damage induction. Recent model extension towards DNA repair processes extends the time dimension by about 12 orders of magnitude and paves the way for superior predictions of radiation risks.

Meta-analysis of non-tumour doses for radiation-induced cancer on the basis of dose-rate

Purpose: Quantitative analysis of cancer risk of ionising radiation as a function of dose-rate.

Materials and methods: Non-tumour dose, D_nt, defined as the highest dose of radiation at which no statistically significant tumour increase was observed above the control level, was analysed as a function of dose-rate of radiation.

Results: An inverse correlation was found between D_nt and dose-rate of the radiation. D_nt increased 20-fold with decreasing dose-rate from 1-10⁻⁸ Gy/min for whole body irradiation with low linear energy transfer (LET) radiation. Partial body radiation also showed a dose-rate dependence with a 5- to 10-fold larger D_nt as dose rate decreased. The dose-rate effect was also found for high LET radiation but at 10-fold lower D_nt levels.

Conclusions: The cancer risk of ionising radiation varies 1000-fold depending on the dose-rate of radiation and exposure conditions. This analysis explains the discrepancy of cancer risk between A-bomb survivors and radium dial painters.

Mutagenic adaptive response to high-LET radiation in human lymphoblastoid cells exposed to X-rays

The ability of cells to adapt low-dose or low-dose rate radiation is well known. High-LET radiation has unique characteristics, and the data concerning low doses effects and high-LET radiation remain fragmented. In this study, we assessed in vitro the ability of low doses of X-rays to induce an adaptive response (AR) to a subsequent challenging dose of heavy-ion radiation. Lymphoblastoid cells (TK6, AHH-1, NH32) were exposed to priming 0.02-0.1Gy X-rays, followed 6h later by challenging 1Gy heavy-ion radiation (carbon-ion: 20 and 40keV/μm, neon-ion: 150keV/μm). Pre-exposure of p53-competent cells resulted in decreased mutation frequencies at hypoxanthine-guanine phosphoribosyl transferase locus and different H2AX phosphorylation kinetics, as compared to cells exposed to challenging radiation alone. This phenomenon did not seem to be linked with cell cycle effects or radiation-induced apoptosis. Taken together, our results suggested the existence of an AR to mutagenic effects of heavy-ion radiation in lymphoblastoid cells and the involvement of double-strand break repair mechanisms.

The Ku-dependent non-homologous end-joining pathway contributes to low-dose radiation-stimulated cell survival

Low-dose (≤0.1 Gy) radiation-induced adaptive responses could protect cells from high-challenge dose radiation-induced killing. The protective role is believed to promote the repair of DNA double-strand breaks (DSBs) that are a severe threat to cell survival. However, it remains unclear which repair pathway, homologous recombination repair (HRR) or non-homologous end-joining (NHEJ), is promoted by low-dose radiation. To address this question, we examined the effects of low-dose (0.1 Gy) on high-challenge dose (2-4 Gy) induced killing in NHEJ- or HRR-deficient cell lines. We showed that 0.1 Gy reduced the high-dose radiation-induced killing for wild-type or HRR-deficient cells, but enhanced the killing for NHEJ-deficient cells. Interestingly, low-dose radiation also enhanced the killing for wild-type cells exposed to high-challenge dose radiation with high-linear energy transfer (LET). Because it is known that high-LET radiation induces an inefficient NHEJ, these results support that the low-dose radiation-stimulated protective role in reducing high-challenge dose (low-LET)-induced cell killing might depend on NHEJ. In addition, we showed that low-dose radiation activated the DNA-PK catalytic subunit (DNA-PKcs) and the inhibitor of DNA-PKcs destroyed the low-dose radiation-induced protective role. These results suggest that low-dose radiation might promote NHEJ through the stimulation of DNA-PKcs activity and; therefore, increase the resistance of cells to high-challenge dose radiation-induced killing.

Low-level ionizing radiation from noninvasive cardiac imaging: can we extrapolate estimated risks from epidemiologic data to the clinical setting?

Clinical decision-making regarding the use of low-level ionizing radiation for diagnostic and/or therapeutic purposes in patients with cardiovascular disease must, as in all other clinical scenarios, encompass the broad range of the risk-benefit ratio. Concerns regarding the late carcinogenic effects of exposure to low levels, i.e., <100 mSv, of ionizing radiation stem from extrapolation of exposure-outcome data in survivors of World War II atomic bomb explosions. However, ongoing debate regarding the true incremental risk to subjects exposed to doses currently administered in cardiovascular procedures fails to take into account the uncertainty of the dose-response relationship in this lower range, as well as tissue-specific reparative responses, also manifest at lower levels of exposure. The present discussion draws attention to both of these aspects as they relate to clinical decision-making.