Cancer risks attributable to low doses of ionizing radiation: assessing what we really know

Surgeon's radiation exposure during percutaneous vertebroplasty

OBJECT

In this study the authors evaluated levels of radiation exposure to surgeons' protected and unprotected hands during fluoroscopically assisted vertebroplasty.

METHODS

The amount of radiation administered to 30 patients during 41 procedures in a controlled prospective trial over 6 months was assessed, comparing radiation exposure to the right and left hands in two neurosurgeons. Effective skin doses were evaluated using thermoluminescent finger dosimeters (ring dosimeters). The ratios of finger dosimeter exposure were compared between the glove-protected and unprotected left hands of two surgeons and both unprotected right hands. In addition, dose-area product (DAP) and fluoroscopy times were recorded in all patients. The mean treatment-effective dose to the surgeons' hands was 0.49 +/- 0.4 mSv in the glove-protected left hand and 1.81 +/- 1.31 mSv in the unprotected left hand (p < 0.05). The mean effective hand doses were 0.59 +/- 0.55 mSv in the unprotected right hand of the glove-protected surgeon and 0.62 +/- 0.55 mSv in the unprotected right hand of the control surgeon. The total corresponding fluoroscopy time was 38.55 minutes for the protected surgeon and 41.23 minutes for the unprotected one (p > 0.05). Lead glove shielding resulted in a radiation dose reduction of 75%. The total DAP for all procedures was 256,496 mGy/cm2 and 221,408 mGy/cm2 (p > 0.05) for the protected and unprotected surgeons, respectively.

CONCLUSIONS

This study emphasizes the importance of surgeons wearing lead glove protection on their leading hands during percutaneous vertebroplasty procedures and demonstrates a 75% reduction rate of exposure to radiation.

Infection, immune responses and the aetiology of childhood leukaemia

Childhood leukaemia is the principal subtype of paediatric cancer and, despite success in treatment, its causes remain enigmatic. A plethora of candidate environmental exposures have been proposed, but most lack a biological rationale or consistent epidemiological evidence. Although there might not be a single or exclusive cause, an abnormal immune response to common infection(s) has emerged as a plausible aetiological mechanism.

Estimating radiation-induced cancer risks at very low doses: rationale for using a linear no-threshold approach

The possible cancer risks caused by ionizing radiation doses of ~1 mSv or less are too small to be estimated directly from epidemiological data. The linear no-threshold (LNT) approach to estimating such risks involves using epidemiological data at higher (but still low) doses to establish an "anchor point", and then extrapolating the excess cancer risk linearly down from this point to the low dose of interest. The study in this issue by Professor Tubiana and colleagues, summarizing a French Academy of Sciences report, argues that such LNT extrapolations systematically give substantial overestimates of the excess cancer risk at very low doses. We suggest that, to the contrary, even if there are significant deviations from linearity in the relevant dose range, potentially caused by the effects of inter-cellular interactions or immune surveillance, we know almost nothing quantitatively about these effects. Consequently, we do not know the magnitude, nor even the direction of any such deviations from linearity-the risks could indeed be lower than those predicted by a linear extrapolation, but they could well be higher.

Recent reports on the effect of low doses of ionizing radiation and its dose-effect relationship

Recently, the risk associated with low doses of ionizing radiation has gained new interest. Here, we analyze and discuss the major differences between two reports recently published on this issue; the report of the French Academy of Sciences and of the French Academy of Medicine published in March 2005, and the BEIR VII-Phase 2 Report of the American National Academy of Sciences published as a preliminary version in July 2005. The conclusion of the French Report is that the linear no-threshold relationship (LNT) may greatly overestimate the carcinogenic effect of low doses (<100 mSv) and even more that of very low doses (<10 mSv), such as those delivered during X-ray examinations. Conversely, the conclusion of the BEIR VII report is that LNT should be used for assessing the detrimental effects of these low and very low doses. The causes of these diverging conclusions should be carefully examined. They seem to be mostly associated with the interpretation of recent biological data. The point of view of the French Report is that these recent data are incompatible with the postulate on which LNT is implicitly based, namely the constancy of the carcinogenic effect per unit dose, irrespective of dose and dose rate.

Extremely low dose ionizing radiation up-regulates CXC chemokines in normal human fibroblasts

Although the public today could be exposed to X-rays as high as 1 cGy due to diagnostic procedures, the biological effects of this low-dose range have not been well established. We searched through >23,000 transcripts in normal human fibroblasts, HFLIII, using a novel comprehensive expression analysis method. More than 200 genes were up-regulated transiently by 1 cGy of X-rays during the 1-hour period after irradiation. We determined the nucleotide sequence of 10 up-regulated transcripts with the greatest rate of increase in the irradiated HFLIII cells. Three of the 10 transcripts encoded CXC chemokines (CXCL1, CXCL2, and CXCL6). The rest included the transcripts of other secretory products (secretogranin II, thrombospondin type I domain containing 2, amphiregulin, and interleukin-6) and unknown genes. To test the involvement of CXC chemokines in cells irradiated with low doses, we irradiated HFLIII cells with 1 to 20 cGy X-rays and transferred the media from HFLIII culture to two melanoma cell lines characteristic of excessive numbers of the CXC chemokine-specific receptors. The growth of these melanoma lines were significantly stimulated by the medium from HFLIII irradiated at 1 to 5 cGy. Our results indicate that human cells respond to doses of radiation as low as 1 cGy, and mechanisms alternative to those involved in moderate/high-dose studies have to be considered in understanding the biological effects of diagnostic level radiation. In addition, our comprehensive approach using a novel expression profiling method is a powerful strategy to explore biological functions associated with very low levels of toxic agents.

Health risks of low photon energy imaging

The major health risk associated with low photon energy imaging is thought to be the induction of cancer as a consequence of the radiation exposure and this is the focus of this paper. Low photon energy imaging typically involves exposure to a low dose (<50 mGy) of low linear energy transfer (LET) radiation delivered at high dose-rate. Since epidemiologic data cannot provide an accurate assessment of risk at the doses used in imaging, risk estimates are currently made by fitting a linear response to intermediate and high dose data for cancer induction in radiation-exposed human populations. This method assumes a linear no-threshold (LNT) response and implies that no dose of radiation is safe. This assumption is not borne out by many laboratory studies of cancer-related endpoints that would suggest that the risk at low doses is much less than would be estimated from linear extrapolation from intermediate to high doses. It is also well recognised that the dose-response from many epidemiologic studies could equally well be fit by threshold models. Through the study of radiation-induced neoplastic transformation in vitro J-shaped dose-response curves for a variety of low LET radiations, including those used in low photon energy imaging, have been demonstrated. The relative risks calculated from this data compare remarkably well with those for breast cancer and leukemia incidence in radiation-exposed populations. From this it is concluded that the LNT hypothesis is likely to overestimate the risk of cancer induction by low photon energy imaging, at least for certain tumors.

Diagnosis of abdominal aortic hypoplasia by state-of-the-art MR angiography

Abdominal aortic hypoplasia is a rare vascular variant with possible major clinical sequelae when the renal arteries are also involved. The condition is thought to result from embryonic overfusion of the two dorsal aortas. The diagnosis may be considered in patients presenting with hypertension in the neonatal period and severely reduced or absent arterial pulses in the groin. We present contrast-enhanced state-of-the-art magnetic resonance angiography imaging findings in an 8-year-old boy with abdominal aortic hypoplasia.

Polymorphisms in DNA repair genes, medical exposure to ionizing radiation, and breast cancer risk

An epidemiologic study was conducted to determine whether polymorphisms in DNA repair genes modify the association between breast cancer risk and exposure to ionizing radiation. Self-reported exposure to ionizing radiation from medical sources was evaluated as part of a population-based, case-control study of breast cancer in African-American (894 cases and 788 controls) and White (1,417 cases and 1,234 controls) women. Genotyping was conducted for polymorphisms in four genes involved in repair of radiation-induced DNA damage, the double-strand break repair pathway: X-ray cross-complementing group 3 (XRCC3) codon 241 Thr/Met, Nijmegen breakage syndrome 1 (NBS1) codon 185 Glu/Gln, X-ray cross-complementing group 2 (XRCC2) codon 188 Arg/His, and breast cancer susceptibility gene 2 (BRCH2) codon 372 Asn/His. Allele and genotype frequencies were not significantly different in cases compared with controls for all four genetic polymorphisms, and odds ratios for breast cancer were close to the null. Combining women with two, three, and four variant genotypes, a positive association was observed between breast cancer and number of lifetime mammograms (P(trend) < 0.0001). No association was observed among women with zero or one variant genotype (P = 0.86). Odds ratios for radiation treatments to the chest and number of lifetime chest X-rays were slightly elevated but not statistically significant among women with two to four variant genotypes. The study has several limitations, including inability to distinguish between diagnostic and screening mammograms or reliably classify prediagnostic mammograms and chest X-rays in cases. Prospective studies are needed to address whether common polymorphisms in DNA repair genes modify the effects of low-dose radiation exposure from medical sources.

Genomic instability in radiation-induced mouse lymphoma from p53 heterozygous mice

Although radiation can directly induce DNA damage and is a known human and animal carcinogen, the number of genetic changes in radiation-induced tumors, and the pathways responsible for generating them, are unknown. We have used high-density BAC arrays covering >95% of the mouse genome for analysis of genomic patterns of aberrations in spontaneous and radiation-induced mouse lymphomas. The majority of radiation-induced tumors exhibit one of three 'signatures' based on gene copy number changes. Some exhibit extensive scrambling of the genome, with very high numbers of recurrent gains and losses. Two other signatures are characterized by excess gains but relatively few losses, or vice versa. Changes in spontaneous tumors often involve whole chromosomes, whereas radiation-induced tumors exhibit a high frequency of localized deletion/amplification events. The number of copy number abnormalities does not correlate with the latency or pathology of the tumors. We propose that specific early events following radiation exposure induce changes in 'caretaker' genes that control specific downstream pathways involved in DNA damage repair. The nature of these early events may determine the overall genomic signature observed in the resulting tumor.

Mammographic screening before age 50 years in the UK: comparison of the radiation risks with the mortality benefits

Mammographic screening before age 50 years is less effective than at older ages and the associated radiation risks are higher. We estimated how many breast cancer deaths could be caused and how many could be prevented by a decade of annual two-view mammographic screening starting at ages 20, 30 and 40 years, respectively, in the UK; for all women, and for women with first-degree relatives affected with breast cancer. We extrapolated from a radiation risk model to estimate the number of radiation-induced breast cancer deaths, and used results from randomised trials, which suggest a reduction in breast cancer mortality of 10–20% in women invited to screening before age 50 years, to estimate the number of deaths that could be prevented. The net change in breast cancer deaths was defined as the number of radiation-induced deaths minus the number of prevented deaths. For all women, assuming a reduction in mortality from screening of 20%, a decade of annual screening was estimated to induce more deaths than it prevents if started at age 20 years and at age 30 years (net increase=0.86 and 0.37 breast cancer deaths, respectively, per 1000 women screened). The corresponding estimate for screening starting at age 40 years was a net decrease of 0.46 deaths/1000 women screened and a zero net change assuming a 10% mortality reduction. Results for women with first-degree relatives with breast cancer were generally in the same direction but, because their background incidence rates are higher, the net increases or decreases were greater. In conclusion, our estimates suggest that a decade of annual two-view mammographic screening before age 40 years would result in a net increase in breast cancer deaths, and that starting at age 40 years could result in a material net decrease only if breast cancer mortality is reduced by about 20% or more in women screened. Although these calculations were based on a number of uncertain parameters, in general, the conclusions were not altered when these parameters were varied within a feasible range.

Complex chromosome aberrations persist in individuals many years after occupational exposure to densely ionizing radiation: an mFISH study

Long-lived, sensitive, and specific biomarkers of particular mutagenic agents are much sought after and potentially have broad applications in the fields of cancer biology, epidemiology, and prevention. Many clastogens induce a spectrum of chromosome aberrations, and some of them can be exploited as biomarkers of exposure. Densely ionizing radiation, for example, alpha particle radiation (from radon or plutonium) and neutron radiation, preferentially induces complex chromosome aberrations, which can be detected by the 24-color multifluor fluorescence in situ hybridization (mFISH) technique. We report the detection and quantification of stable complex chromosome aberrations in lymphocytes of healthy former nuclear-weapons workers, who were exposed many years ago to plutonium, gamma rays, or both, at the Mayak weapons complex in Russia. We analyzed peripheral-blood lymphocytes from these individuals for the presence of persistent complex chromosome aberrations. A significantly elevated frequency of complex chromosome translocations was detected in the highly exposed plutonium workers but not in the group exposed only to high doses of gamma radiation. No such differences were found for simple chromosomal aberrations. The results suggest that stable complex chromosomal translocations represent a long-lived, quantitative, low-background biomarker of densely ionizing radiation for human populations exposed many years ago.

Modeling Intercellular Interactions during Carcinogenesis

By modulating the microenvironment of malignant or premalignant cells, inhibitory or stimulatory signals from nearby cells can play a key role in carcinogenesis. However, current commonly used quantitative models for induction of cancers by ionizing radiation focus on single cells and their progeny. Intercellular interactions are neglected or assumed to be confined to unidirectional radiation bystander effect signals from cells of the same tissue type. We here formulate a parsimoniously parameterized two-stage logistic (TSL) carcinogenesis model that incorporates some effects of intercellular interactions during the growth of premalignant cells. We show that for baseline tumor rates, involving no radiation apart from background radiation, this TSL model gives acceptable fits to a number of data sets. Specifically, it gives the same baseline hazard function, using the same number of adjustable parameters, as does the commonly used two-stage clonal expansion (TSCE) model, so it is automatically applicable to the many data sets on baseline cancer that have been analyzed using the TSCE model. For perturbations of baseline rates due to radiation, the models differ. We argue from epidemiological and laboratory evidence, especially results for the atomic bomb survivors, that for radiation carcinogenesis the TSL model gives results at least as realistic as the TSCE or similar models, despite involving fewer adjustable parameters in many cases.

CT dose reduction in children

World wide, the number of CT studies in children and the radiation exposure by CT increases. The same energy dose has a greater biological impact in children than in adults, and scan parameters have to be adapted to the smaller diameter of the juvenile body. Based on seven rules, a practical approach to paediatric CT is shown: Justification and patient preparation are important steps before scanning, and they differ from the preparation of adult patients. The subsequent choice of scan parameters aims at obtaining the minimal signal-to-noise ratio and volume coverage needed in a specific medical situation; exposure can be divided in two aspects: the CT dose index determining energy deposition per rotation and the dose-length product (DLP) determining the volume dose. DLP closely parallels the effective dose, the best parameter of the biological impact. Modern scanners offer dose modulation to locally minimise exposure while maintaining image quality. Beyond the selection of the physical parameters, the dose can be kept low by scanning the minimal length of the body and by avoiding any non-qualified repeated scanning of parts of the body. Following these rules, paediatric CT examinations of good quality can be obtained at a reasonable cost of radiation exposure.

Neoplastic transformation in vitro induced by low doses of 232 MeV protons

The aim was to define the dose--response curve for high-energy proton-induced neoplastic transformation in vitro. The HeLa x skin fibroblast human hybrid cell assay was used to determine the frequency of neoplastic transformation following doses of 232 MeV protons (mean linear energy transfer, LET=0.44 keV microm(-1)) in the range 5-600 mGy. Proton irradiations were carried out at the Loma Linda University Proton Treatment Facility, CA, USA. The data indicate no evidence for induction of transformation below a dose of 100 mGy. At doses of 5 and 50 mGy, there is evidence for a possible suppression of transformation frequencies below that for spontaneous transformation. The shape of the dose--response curve for high-energy proton-induced transformation of the human hybrid cell line CGL1 does not follow a linear no-threshold model and shows evidence for a threshold as well as for possible suppression of transformation at doses <100 mGy, similar to that seen for other low-LET radiations.

Phenomena leading to cell survival values which deviate from linear-quadratic models

For several decades, the prevailing paradigm for modeling the effects of ionizing radiation (IR) on living systems was the target model with its inherent assumptions--that only those cells in the radiation path whose molecules sustained collisions with high energy particles and rays were damaged, that the damage was proportional to the energy absorbed by each cell and to the number of cells absorbing energy, and that all cells had identical sensitivities to radiation. However, evidence has accumulated that cells exhibit phenomena at low radiation exposures that appear to contradict at least one of these assumptions. Some of these phenomena currently under active study include low-dose hypersensitivity (HRS), increased radiation radioresistance (IRR), the adaptive response (AR), the bystander effect (BE), and death-inducing factor (DIE). These effects may interact to give rise to other phenomena such as hormesis, in which small amounts of otherwise toxic agent appear to be beneficial. Elucidating the cellular and molecular bases for these phenomena will lead to greater understanding of the relationships of these processes, including hormesis, to human health.

Lymphoid gene expression as a predictor of risk of secondary brain tumors

Gene expression profiles are tissue-specific but may also reflect germ-line-driven expression patterns across tissue types. Previously, using a targeted pharmacologic approach, we identified germ-line polymorphisms in a single gene (thiopurine methyltransferase) associated with the risk of irradiation- and chemotherapy-induced secondary brain tumors in children with acute lymphoblastic leukemia (ALL). To identify additional candidate genetic risk factors, in identically treated patients, we compared the gene expression profiles of diagnostic ALL blasts of those who did develop irradiation-associated brain tumors (n = 9) with the profiles from those who did not (n = 33). Weighted rank regression was used to identify 33 probe sets associated with the time-dependent development of brain tumors; k-means clustering (k = 2) identified 2 groups that differed significantly in cumulative incidence of brain tumors (P = 0.012). Permutation analysis was used to estimate the probability (P = 0.18) of obtaining 2 such clusters by chance. Linear discriminant analysis (time-independent categorization of outcome) was used to identify 70 probe sets whose expression differentiated between the 2 groups of patients. Permutation analyses (n = 1,000) was used to estimate the probability of selecting these probe sets by chance (P = 0.055). Five probe sets were in common between the time-independent and time-dependent methods. The distinguishing genes are involved in neural growth (FGFR1) and in nuclear trafficking (HNRPL, KPNB1). These data suggest that gene expression profiling from accessible tissues may identify targets involved in therapy-related malignancies in unrelated tissues.

Mass screening with CT colonography: should the radiation exposure be of concern?

BACKGROUND & AIMS

Computed tomography colonography (CTC), particularly using noncathartic techniques, has the clear potential to increase compliance for colorectal cancer screening. Because the geometry for CTC is highly advantageous, it can be performed with lower radiation doses than almost any other CT examination. If CTC were to become a standard screening tool for the population age 50 years and older, the potential market in the United States would soon be over 100 million people. Therefore, it is pertinent to consider the radiation exposure and any potential radiation risk to the population from such a mass CTC screening program.

METHODS

Organ doses from CTC examinations can be estimated with standard techniques. These doses can be applied to organ- and dose-specific radiation cancer risk estimates to estimate the excess cancer risk resulting from the radiation from a paired (supine and prone) CTC examination.

RESULTS

The cancer risks associated with the radiation exposure from CTC are unlikely to be zero, but they are small. A best estimate for the absolute lifetime cancer risk associated with the radiation exposure using typical current scanner techniques is about 0.14% for paired CTC scans for a 50-year-old, and about half that for a 70-year-old. These values probably could be reduced by factors of 5 or 10 with optimized CTC protocols.

CONCLUSIONS

In terms of the radiation exposure, the benefit-risk ratio potentially is large for CTC.

In vivo formation and repair of DNA double-strand breaks after computed tomography examinations

Ionizing radiation can lead to a variety of deleterious effects in humans, most importantly to the induction of cancer. DNA double-strand breaks (DSBs) are among the most significant genetic lesions introduced by ionizing radiation that can initiate carcinogenesis. We have enumerated gamma-H2AX foci as a measure for DSBs in lymphocytes from individuals undergoing computed tomography examination of the thorax and/or the abdomen. The number of DSBs induced by computed tomography examination was found to depend linearly on the dose-length product, a radiodiagnostic unit that is proportional to both the local dose delivered and the length of the body exposed. Analysis of lymphocytes sampled up to 1 day postirradiation provided kinetics for the in vivo loss of gamma-H2AX foci that correlated with DSB repair. Interestingly, in contrast to results obtained in vitro, normal individuals repair DSBs to background levels. A patient who had previously shown severe side effects after radiotherapy displayed levels of gamma-H2AX foci at various sampling times postirradiation that were several times higher than those of normal individuals. Gamma-H2AX and pulsed-field gel electrophoresis analysis of fibroblasts obtained from this patient confirmed a substantial DSB repair defect. Additionally, these fibroblasts showed significant in vitro radiosensitivity. These data show that the in vivo induction and repair of DSBs can be assessed in individuals exposed to low radiation doses, adding a further dimension to DSB repair studies and providing the opportunity to identify repair-compromised individuals after diagnostic irradiation procedures.

Reducing radiation dose in emergency computed tomography with automatic exposure control techniques

Computed tomography (CT) scanning is being increasingly used for evaluation of trauma, which most commonly involves younger individuals. As younger patients are at higher risk for radiation-induced cancer compared to older patients, radiation dose reduction is an important issue in emergency CT scanning. With automatic exposure control techniques, users select a desired image quality and the system adapts tube current to obtain the desired image quality with greater radiation dose efficiency. These techniques can help in reducing radiation dose by 10-60% in most instances. This review article presents a comprehensive description of fundamentals, clinical applications and radiation dose benefits of automatic exposure control in emergency CT scanning.

Low-dose ionizing radiation: induction of differential intracellular signalling possibly affecting intercellular communication

Given the complexity of the carcinogenic process and the lack of any mechanistic understanding of how ionizing radiation at low-level exposures affects the multistage, multimechanism processes of carcinogenesis, it is imperative that concepts and paradigms be reexamined when extrapolating from high dose to low dose. Any health effect directly linked to low-dose radiation exposure must have molecular/biochemical and biological bases. On the other hand, demonstrating some molecular/biochemical or cellular effect, using surrogate systems for the human being, does not necessarily imply a corresponding health effect. Given the general acceptance of an extrapolated LNT model, our current understanding of carcinogenesis cries out for a resolution of a real problem. How can a low-level acute, or even a chronic, exposure of ionizing radiation bring about all the different mechanisms (mutagenic, cytotoxic, and epigenetic) and genotypic/phenotypic changes needed to convert normal cells to an invasive, malignant cell, given all the protective, repair, and suppressive systems known to exist in the human body? Until recently, the prevailing paradigm that ionizing radiation brings about cancer primarily by DNA damage and its conversion to gene and chromosomal mutations, drove our interpretation of radiation carcinogenesis. Today, our knowledge includes the facts both that epigenetic events play a major role in carcinogenesis and that low-dose radiation can also induce epigenetic events in and between cells in tissues. This challenges any simple extrapolation of the LNT model. Although a recent delineation of "hallmarks" of the cancer process has helped to focus on how ionizing radiation might contribute to the induction of cancers, several other hallmarks, previously ignored--namely, the stem cells in tissues as targets for carcinogenesis and the role of cell-cell communication processes in modulating the radiation effects on the target cell--must be considered, particularly for the adaptive response, bystander effects, and genomic instability phenomena.

Uncertainty and sensitivity analysis in assessment of the thyroid cancer risk related to Chernobyl fallout in Eastern France

The increase in the thyroid cancer incidence in France observed over the last 20 years has raised public concern about its association with the 1986 nuclear power plant accident at Chernobyl. At the request of French authorities, a first study sought to quantify the possible risk of thyroid cancer associated with the Chernobyl fallout in France. This study suffered from two limitations. The first involved the lack of knowledge of spontaneous thyroid cancer incidence rates (in the absence of exposure), which was especially necessary to take their trends into account for projections over time; the second was the failure to consider the uncertainties. The aim of this article is to enhance the initial thyroid cancer risk assessment for the period 1991-2007 in the area of France most exposed to the fallout (i.e., eastern France) and thereby mitigate these limitations. We consider the changes over time in the incidence of spontaneous thyroid cancer and conduct both uncertainty and sensitivity analyses. The number of spontaneous thyroid cancers was estimated from French cancer registries on the basis of two scenarios: one with a constant incidence, the other using the trend observed. Thyroid doses were estimated from all available data about contamination in France from Chernobyl fallout. Results from a 1995 pooled analysis published by Ron et al. were used to determine the dose-response relation. Depending on the scenario, the number of spontaneous thyroid cancer cases ranges from 894 (90% CI: 869-920) to 1,716 (90% CI: 1,691-1,741). The number of excess thyroid cancer cases predicted ranges from 5 (90% UI: 1-15) to 63 (90% UI: 12-180). All of the assumptions underlying the thyroid cancer risk assessment are discussed.

Uncertainties in doses and risks from internal radiation

Many uncertainties exist in current estimates of radiation doses and risks: larger uncertainties exist with internal radiation. These arise mainly from the many steps used to derive doses, and partly from lack of statistical precision in deriving risks from epidemiology studies. The size of these uncertainties has been estimated by a number of expert dosimetrists: for some nuclides these are very large. The recent report by the CERRIE committee recommended that uncertainties should be acknowledged and dealt with by the government. Its parent committee, COMARE, backed these findings. A number of practical recommendations are suggested for government action in the light of the CERRIE report.

Somatic DNA damage in interventional cardiologists: a case-control study

Interventional cardiologists who work in cardiac catheterization laboratories are exposed to low doses of ionizing radiation that could pose a health hazard. DNA damage is considered to be the main initiating event by which radiation damage to cells results in development of cancer and hereditary disease. The aim of the present study was to assess the effects of chronic low-dose X-ray radiation exposure on somatic DNA damage of interventional cardiologists working in high-volume cardiac catheterization laboratories. For this analysis, we used peripheral lymphocytes and the assay for micronuclei (MNs), which is considered to be a reliable biological dosimeter for radiation exposure. We obtained peripheral blood from 62 physicians (mean age+/-se = 40.6+/-1.5 years): 31 interventional cardiologists (group I, exposed) and 31 age- and sex-matched clinical cardiologists (group II, nonexposed). Interventional cardiologists showed higher MN values (group I=20.5+/-1.6 vs. group II=12.8+/-1.3, P=0.001), although some overlap was apparent in the individual subject analysis. A correlation between years of professional activity and MN frequency value was detectable for interventional cardiologists (r=0.428, P=0.02) but not for clinical cardiologists (r=0.253, P=0.17). The results indicated that, overall, interventional cardiologists working in a high-volume catheterization laboratory have higher levels of somatic DNA damage when compared with clinical cardiologists working outside the catheterization laboratory. The amount of this damage varies and is only weakly related to the duration of professional exposure, which suggests that a dominant modulation of the underlying genetic substrate by environmental factors has a role in determining the harm in individual physicians.

Micronuclei in humans induced by exposure to low level of ionizing radiation: influence of polymorphisms in DNA repair genes

Understanding the risks deriving from protracted exposure to low doses of ionizing radiation has remarkable societal importance in view of the large number of work settings in which sources of IR are encountered. To address this question, we studied the frequency of micronuclei (MN), which is an indicator of DNA damage, in a population exposed to low levels of ionizing radiation and in matched controls. In both exposed population and controls, the possible influence of single nucleotide polymorphisms in XRCC1, XRCC3 and XPD genes on the frequency of micronuclei was also evaluated. We also considered the effects of confounding factors, like smoking status, age and gender. The results indicated that MN frequency was significantly higher in the exposed workers than in the controls [8.62+/-2.80 versus 6.86+/-2.65; P=0.019]. Radiological workers with variant alleles for XRCC1 or XRCC3 polymorphisms or wild-type alleles for XPD exon 23 or 10 polymorphisms showed a significantly higher MN frequency than controls with the same genotypes. Smoking status did not affect micronuclei frequency either in exposed workers or controls, while age was associated with increased MN frequency in the exposed only. In the combined population, gender but not age exerted an influence on the yield of MN, being higher in females than in males. Even though there is a limitation in this study due to the small number of subjects, these results suggest that even exposures to low level of ionizing radiation could have genotoxic effects and that XRCC3, XRCC1 and XPD polymorphisms might contribute to the increased genetic damage in susceptible individuals occupationally exposed to chronic low levels of ionizing radiation. For a clear conclusion on the induction of DNA damage caused by protracted exposure to low doses of ionizing radiation and the possible influence of genetic polymorphism in DNA repair genes larger studies are needed.

Genomic Instability in Human Lymphocytes Irradiated with Individual Charged Particles: Involvement of Tumor Necrosis Factor α in Irradiated Cells but not Bystander Cells

Exposure to ionizing radiation can increase the risk of cancer, which is often characterized by genomic instability. In environmental exposures to high-LET radiation (e.g. 222Ra), it is unlikely that many cells will be traversed or that any cell will be traversed by more than one alpha particle, resulting in an in vivo bystander situation, potentially involving inflammation. Here primary human lymphocytes were irradiated with precise numbers of 3He2+ ions delivered to defined cell population fractions, to as low as a single cell being traversed, resembling in vivo conditions. Also, we assessed the contribution to genomic instability of the pro-inflammatory cytokine tumor necrosis factor alpha (TNFA). Genomic instability was significantly elevated in irradiated groups (> or = two-fold over controls) and was comparable whether cells were traversed by one or two 3He2+ ions. Interestingly, substantial heterogeneity in genomic instability between experiments was observed when only one cell was traversed. Genomic instability was significantly reduced (60%) in cultures in which all cells were irradiated in the presence of TNFA antibody, but not when fractions were irradiated under the same conditions, suggesting that TNFA may have a role in the initiation of genomic instability in irradiated cells but not bystander cells. These results have implications for low-dose exposure risks and cancer.

Nonlinear response for neoplastic transformation following low doses of low let radiation

There are now several independent studies that indicate that the dose-response for the endpoint of radiation-induced neoplastic transformation in vitro is non-linear for low linear energy transfer (LET) radiation. At low doses (<10 cGy) the transformation frequency drops below that seen spontaneously. Importantly, this observation has been made using fluoroscopic energy x-rays, a commonly used modality in diagnostic radiology, the practice of which is responsible for the majority of radiation exposure to the general public. Since the transformation frequency is reduced over a large dose range (0.1 to 10cGy) it is likely that multiple mechanisms are involved and that the relative contribution of these may vary with dose. These include the killing of a subpopulation of cells prone to spontaneous transformation at the lowest doses, and the induction of DNA repair at somewhat higher doses. Protective effects of low doses of low LET radiation on other cancer-relevant endpoints in vitro and in vivo have also been observed by several independent laboratories. These observations strongly suggest that the linear-nonthreshold dose-response model is unlikely to apply to the induction of cancer by low doses of low LET radiation in humans.

Radiation risk is linear with dose at low doses

This paper presents a brief argument, based on a mechanistic approach, to show that radiation risk is linear with radiation dose from zero dose up. Similarities in cellular effects lead to the assumption of a common mechanism and the DNA double strand break is identified as the crucial radiation-induced lesion. A cancer model extends the cellular effects to the main radiation risk providing confirmation of the dose effect for cancer at low doses.

Prenatal protracted irradiation at very low dose rate induces severe neuronal loss in rat hippocampus and cerebellum

Prenatal irradiation is known to damage the developing brain. However, little is known about the consequences of very low dose rate prenatal protracted irradiation over several days on neuron numbers in the offspring brain, and on volumes of the corresponding brain regions. Pregnant Wistar rats were exposed either to a protracted gamma irradiation from embryonic day (E) 13 to E16 (0.7 mGy/min; total cumulative dose approximately 3 Gy) or were sham-irradiated. Thirty months old male and female offspring were then analyzed for alterations in hippocampal and cerebellar morphology. Using design-based stereology and the analysis of sets of sections systematically and randomly sampled to span the entire brain region of interest, a statistically significant decrease in numbers of hippocampal pyramidal and granule cells as well as of cerebellar Purkinje and granule cells (approximately 50%) was found in male and female irradiated offspring. The volumes of these brain regions were comparably altered. The analysis of only a "representative" section per animal yielded mostly non-significant trends. Evaluation of neuron densities showed no differences between prenatally irradiated and sham-irradiated offspring. Most importantly, very low dose rate prenatal protracted gamma irradiation did not result in the same morphologic alterations in the offspring brain as previously observed after prenatal single irradiation such as derangement of the laminar structure of pyramidal cells within the hippocampus or malformation of cerebellar lobules.

MDCT angiography of pediatric vascular diseases of the abdomen, pelvis, and extremities

Multi-detector-row computed tomography (MDCT) enables rapid, noninvasive, high-resolution, and three-dimensional imaging of pediatric vascular diseases. In this paper, we explore the adaptation of the MDCT angiographic principles to pediatric patients for vascular diseases of the abdomen, pelvis, and extremities. Special emphasis is placed on the practical aspects of how to perform these studies. Optimizations of scan parameters, contrast medium usage, radiation dose, and three-dimensional image processing are discussed in detail. We provide practical guidance on how to choose between MR angiography and CT angiography. Finally, we review important pediatric vascular diseases, categorized into traumatic injuries, inherited vascular diseases, congenital vascular diseases, vasculitides, and surgical planning and assessment. In each category, we discuss how CT angiography can be tailored to maximize its clinical benefits.

Radiation exposure and cause specific mortality among nuclear workers in Belgium (1969-1994)

Cause specific mortality was studied in nuclear workers from five nuclear facilities in Belgium and compared to the general population. For the 1969-1994 period, mortality in male nuclear workers is significantly lower for all causes of death and for all cancer deaths. The same conclusions are reached if one assumes a latency period of 20 y between the first irradiation and cancer induction. In female workers, mortality due to all causes and all cancer deaths is not different from that of the general population. Analysis of cause specific mortality was performed for male and female workers for three endpoints: specific cancer sites, cardiovascular and respiratory diseases. No significant increase in mortality was observed. In male workers, the influence of cumulative dose was also investigated using four dose levels: no significant correlation was found. Smoking habits may be a confounding factor in smoking related health conditions.

Interrelationships amongst radiation-induced genomic instability, bystander effects, and the adaptive response

Over the past two decades, our understanding of radiation biology has undergone a fundamental shift in paradigms away from deterministic "hit-effect" relationships and towards complex ongoing "cellular responses". These responses include now familiar, but still poorly understood, phenomena associated with radiation exposure such as bystander effects, genomic instability, and adaptive responses. All three have been observed at very low doses, and at time points far removed from the initial radiation exposure, and are extremely relevant for linear extrapolation to low doses; the adaptive response is particularly relevant when exposure is spread over a period of time. These are precisely the circumstances that are most relevant to understanding cancer risk associated with environmental and occupational radiation exposures. This review will provide a synthesis of the known, and proposed, interrelationships amongst low-dose cellular responses to radiation. It also will examine the potential importance of non-targeted cellular responses to ionizing radiation in setting acceptable exposure limits especially to low-LET radiations.

Projecting the time trend of thyroid cancers: its impact on assessment of radiation-induced cancer risks

The incidence of thyroid cancer, which may be induced by ionizing radiation, has been rising in most Western countries for more than 20 years. In France, public worry about this increase and its possible connection with the fallout from Chernobyl led the government to ask for an evaluation of the health impact of this accident and an assessment of the feasibility of an epidemiological study. These requests raise two methodological questions: Which risk model should be used to relate exposure to risk? What is known about the spontaneous incidence rate of thyroid cancers? This article analyzes the impact of the time trend in the spontaneous incidence of thyroid cancers over the past 20 years in France when evaluating the risk of radiation-induced cancer. Age-period-cohort models were used to model the trend of spontaneous incidence from 1978 through 1997 and then to apply two scenarios for projections up to 2007: one with a constant incidence, the other using the trend observed over the past 20 years. Then the risk was assessed for a hypothetical population of 30,000 children aged 0 to 15 y, exposed to a hypothetical 0.1 Gy thyroid dose. The analysis shows that consideration of the trend instead of a constant spontaneous incidence can yield substantial differences in the risk estimates for thyroid cancer.

Imaging perinatal brain injury in premature infants

The primary methods currently in use for imaging the infant brain are cranial ultrasound (CUS), computed tomography (CT) and magnetic resonance imaging (MRI). This review outlines the relative strengths and weaknesses of these modalities in relation to the premature infant, with specific focus on the correlations between imaging findings and neurodevelopmental outcome. Since MRI is undergoing rapid development at this time, the newer MRI methods of brain volume measurement and diffusion tensor imaging are reviewed in more detail. Current guidelines regarding the application of these neuroimaging methods to the premature infant are discussed.

Radiation dose determines the degree of myeloid engraftment after nonmyeloablative stem cell transplantation

A multivariate analysis of 121 dogs conditioned with 200, 100, or 50 cGy of total body irradiation (TBI) followed by hematopoietic stem cell transplantation from matched littermates showed that TBI dose was the only factor examined that was statistically significantly associated with the percentage of donor myeloid engraftment in stable long-term chimeras ( P = .008). To understand the direct effects of low-dose irradiation on hematopoietic stem/progenitor cells, nonirradiated and irradiated human CD34 + cells were evaluated for competitive repopulating ability in nonobese diabetic/severe combined immunodeficiency beta2m -/- mice. As expected, the results showed a radiation dose-dependent loss of competitive repopulating ability. Flow cytometric analysis indicated that, within a viable cell gate, there was reduced expression of P-selectin glycoprotein ligand-1 and L selectin on irradiated compared with nonirradiated CD34 + cells; this suggests that irradiated stem/progenitor cells may be compromised in their ability to home to or interact with the marrow microenvironment. However, the CD34 + /P-selectin glycoprotein ligand-1 dim cells also showed activation of caspase-3, indicating that they were destined to die. These results suggest that the TBI dose determines the degree of myeloid engraftment by compromising the resident stem/progenitor cell compartment.

Neoplastic TransformationIn Vitroafter Exposure to Low Doses of Mammographic-Energy X Rays: Quantitative and Mechanistic Aspects

The induction of neoplastic transformation in vitro after exposure of HeLa x skin fibroblast hybrid cells to low doses of mammography-energy (28 kVp) X rays has been studied. The data indicate no evidence of an increase in transformation frequency over the range 0.05 to 22 cGy, and doses in the range 0.05 to 1.1 cGy may result in suppression of transformation frequencies to levels below that seen spontaneously. This finding is not consistent with a linear, no-threshold dose- response curve. The dose range at which possible suppression is evident includes doses typically experienced in mammographic examination of the human breast. Experiments are described that attempt to elucidate any possible role of bystander effects in modulating this low-dose radiation response. Not unexpectedly, inhibition of gap junction intercellular communication (GJIC) with the inhibitor lindane did not result in any significant alteration of transformation frequencies seen at doses of 0.27 or 5.4 cGy in these subconfluent cultures. Furthermore, no evidence of a bystander effect associated with factors secreted into the extracellular medium was seen in medium transfer experiments. Thus, in this system and under the experimental conditions used, bystander effects would not appear to be playing a major role in modulating the shape of the dose-response curve.

The biological effects of diagnostic cardiac imaging on chronically exposed physicians: the importance of being non-ionizing

Ultrasounds and ionizing radiation are extensively used for diagnostic applications in the cardiology clinical practice. This paper reviewed the available information on occupational risk of the cardiologists who perform, every day, cardiac imaging procedures. At the moment, there are no consistent evidence that exposure to medical ultrasound is capable of inducing genetic effects, and representing a serious health hazard for clinical staff. In contrast, exposure to ionizing radiation may result in adverse health effect on clinical cardiologists. Although the current risk estimates are clouded by approximations and extrapolations, most data from cytogenetic studies have reported a detrimental effect on somatic DNA of professionally exposed personnel to chronic low doses of ionizing radiation. Since interventional cardiologists and electro-physiologists have the highest radiation exposure among health professionals, a major awareness is crucial for improving occupational protection. Furthermore, the use of a biological dosimeter could be a reliable tool for the risk quantification on an individual basis.

The cancer epidemiology of radiation

Ionizing radiation has been the subject of intense epidemiological investigation. Studies have demonstrated that exposure to moderate-to-high levels can cause most forms of cancer, leukaemia and cancers of the breast, lung and thyroid being particularly sensitive to induction by radiation, especially at young ages at exposure. Predominant among these studies is the Life Span Study of the cohort of survivors of the atomic bombings of Japan in 1945, but substantial evidence is derived from groups exposed for medical reasons, occupationally or environmentally. Notable among these other groups are underground hard rock miners who inhaled radioactive radon gas and its decay products, large numbers of patients irradiated therapeutically and workers who received high doses in the nuclear weapons programme of the former USSR. The degree of carcinogenic risk arising from low levels of exposure is more contentious, but the available evidence points to an increased risk that is approximately proportional to the dose received. Epidemiological investigations of nonionizing radiation have established ultraviolet radiation as a cause of skin cancer. However, the evidence for a carcinogenic effect of other forms of nonionizing radiation, such as those associated with mobile telephones or electricity transmission lines, is not convincing, although the possibility of a link between childhood leukaemia and extremely low-frequency electromagnetic fields cannot be dismissed entirely.

Targeted cytoplasmic irradiation induces bystander responses

The observation of radiation-induced bystander responses, in which cells respond to their neighbors being irradiated, has important implications for understanding mechanisms of radiation action particularly after low-dose exposure. Much of this questions the current dogma of direct DNA damage driving response in irradiated systems. In this study, we have used a charged-particle microbeam to target individual helium ions ((3)He(2+)) to individual cells within a population of radioresistant glioma cells cultured alone or in coculture with primary human fibroblasts. We found that even when a single cell within the glioma population was precisely traversed through its cytoplasm with one (3)He(2+) ion, bystander responses were induced in the neighboring nonirradiated glioma or fibroblasts so that the yield of micronuclei was increased by 36% for the glioma population and 78% for the bystander fibroblast population. Importantly, the yield of bystander-induced micronuclei was independent of whether the cytoplasm or nucleus of a cell was targeted. The bystander responses were fully eliminated when the populations were treated with 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide or filipin, which scavenge nitric oxide (NO) and disrupt membrane rafts, respectively. By using the probe 4-amino-5-methylamino-2',7'-difluorofluorescein, it was found that the NO level in the glioma population was increased by 15% after 1 or 10 cytoplasmic traversals, and this NO production was inhibited by filipin. This finding shows that direct DNA damage is not required for switching on of important cell-signaling mechanisms after low-dose irradiation and that, under these conditions, the whole cell should be considered a sensor of radiation exposure.

Oxidative stress, radiation-adaptive responses, and aging

Organisms living in an aerobic environment were forced to evolve effective cellular strategies to detoxify reactive oxygen species. Besides diverse antioxidant enzymes and compounds, DNA repair enzymes, and disassembly systems, which remove damaged proteins, regulation systems that control transcription, translation, and activation have also been developed. The adaptive responses, especially those to radiation, are defensive regulation mechanisms by which oxidative stress (conditioning irradiation) elicits a response against damage because of subsequent stress (challenging irradiation). Although many researchers have investigated these molecular mechanisms, they remain obscure because of their complex signaling pathways and the involvement of various proteins. This article reviews the factors concerned with radiation-adaptive response, the signaling pathways activated by conditioning irradiation, and the effects of aging on radiation-adaptive response. The proteomics approach is also introduced, which is a useful method for studying stress response in cells.