Nontargeted effects of ionizing radiation: implications for low-dose exposures

The traditional thinking has been that the biological effects of ionizing radiation occur in irradiated cells as a consequence of the DNA damage they incur. This implies that: 1) biological effects occur only in irratiated cells, 2) radiation traversal through the nucleus of the cell is a prerequisite to produce a biological response, and 3) DNA is the target molecule in the cell. Evidence has been emerging, however, for non-DNA targeted effects of radiation; that is, effects including mutations, chromosomal aberrations, and changes in gene expression which occur in cells that in themselves receive no radiation exposure. Two of these phenomena will be described in this paper. The first is radiation-induced genomic instability whereby biological effects, including elevated frequencies of mutations and chromosomal aberrations, arise in the distant descendants of irradiated cells. The second phenomenon has been termed the "bystander effect", whereby in a mixed population of irradiated and nonirradiated cells, biological effects arise in those cells that receive no radiation exposure. The damage signals are transmitted from cell to cell through gap junction channels, and the genetic effects observed in bystander cells appear to result from an upregulation of oxidative stress. The possible influence of these non-targeted effects of radiation of the respounse to low-dose exposures is discussed.     

Cell-density dependent effects of low-dose ionizing radiation on E. coli cells

The changes in genome conformational state (GCS) induced by low-dose ionizing radiation in E. coli cells were measured by the method of anomalous viscosity time dependence (AVTD) in cellular lysates. Effects of X-rays at doses 0.1 cGy--1 Gy depended on post-irradiation time. Significant relaxation of DNA loops followed by a decrease in AVTD. The time of maximum relaxation was between 5-80 min depending on the dose of irradiation. U-shaped dose response was observed with increase of AVTD in the range of 0.1-4 Gy and decrease in AVTD at higher doses. No such increase in AVTD was seen upon irradiation of cells at the beginning of cell lysis while the AVTD decrease was the same. Significant differences in the effects of X-rays and gamma-rays at the same doses were observed suggesting a strong dependence of low-dose effects on LET. Effects of 0.01 cGy gamma-rays were studied at different cell densities during irradiation. We show that the radiation-induced changes in GCS lasted longer at higher cell density as compared to lower cell density. Only small amount of cells were hit at this dose and the data suggest cell-to-cell communication in response to low-dose ionizing radiation. This prolonged effect was also observed when cells were irradiated at high cell density and diluted to low cell density immediately after irradiation. These data suggest that cell-to-cell communication occur during irradiation or within 3 min post-irradiation. The cell-density dependent response to low-dose ionizing radiation was compared with previously reported data on exposure of E. coli cells to electromagnetic fields of extremely low frequency and extremely high frequency (millimeter waves). The body of our data show that cells can communicate in response to electromagnetic fields and ionizing radiation, presumably by reemission of secondary photons in infrared-submillimeter frequency range.     

Dosimetry for quantitative analysis of the effects of low-dose ionizing radiation in radiation therapy patients

We have developed and validated a practical approach to identifying the location on the skin surface that will receive a prespecified biopsy dose (ranging down to 1 cGy) in support of in vivo biological dosimetry in humans. This represents a significant technical challenge since the sites lie on the patient's surface outside the radiation fields. The PEREGRINE Monte Carlo simulation system was used to model radiation dose delivery, and TLDs were used for validation on phantoms and for confirmation during patient treatment. In the developmental studies, the Monte Carlo simulations consistently underestimated the dose at the biopsy site by approximately 15% (of the local dose) for a realistic treatment configuration, most likely due to lack of detail in the simulation of the linear accelerator outside the main beam line. Using a single, thickness-independent correction factor for the clinical calculations, the average of 36 measurements for the predicted 1-cGy point was 0.985 cGy (standard deviation: 0.110 cGy) despite patient breathing motion and other real-world challenges. Since the 10-cGy point is situated in the region of high-dose gradient at the edge of the field, patient motion had a greater effect, and the six measured points averaged 5.90 cGy (standard deviation: 1.01 cGy), a difference that is equivalent to approximately a 6-mm shift on the patient's surface.     

Effect of ionizing radiation on rat tissue: proteomic and biochemical analysis

Reactive oxygen species (ROS), generated by ionizing radiation, has been implicated in its effect on living tissues. We confirmed the changes in the oxidative stress markers upon irradiation. We characterized the changes in the proteome profile in rat liver after administering irradiation, and the affected proteins were identified by MALDI-TOF-MS and ESI-MS/MS. The identified proteins represent diverse sets of proteins participating in the cellular metabolism. Our results demonstrated that proteomics analysis is a useful method for characterization of a global proteome change caused by ionizing radiation to unravel the molecular mechanisms involved in the cellular responses to ionizing radiation.     

Some long-term cell kinetic effects of ionizing radiation on mouse bladder urothelium

The cell kinetics of the mouse bladder urothelium were followed with tritiated thymidine pulse labelling and flow cytometry for one year after irradiation with electrons. No perturbations were registered after 10 Gy. Three to four weeks after 20 Gy an elevation of the labelling index with a subsequent return to normal was seen. Flow cytometry revealed some increase in the proportion of octaploid nuclei at the same time. From about six months after irradiation the normal polyploidization decreased. The urothelium turned into a mainly diploid cell population. The proportion of diploid S phase cells also increased. The data give some support to the model hypothesis of reactive proliferation in a 'flexible' tissue, according to Wheldon et al. (1982).     

Some short-term cell kinetic effects of ionizing radiation on mouse bladder urothelium

Early morphological changes and the pattern of reactive proliferation of the hairless mouse urinary bladder urothelium after irradiation are reported. Groups of female hairless mice were anaesthetized with sodium pentobarbital and irradiated over the bladder region with 0, 10, 20 and 30 Gy. Control groups were sham-treated. Short-term cell kinetic changes were monitored using incorporation of tritiated thymidine and flow cytometry. Only minor radiation-induced alterations in the cell kinetic pattern were recorded, and no significant histomorphological changes were seen. However, a marked increase in the thymidine incorporation was seen in the control animals on the first day after anaesthesia. Radiation proctitis induced early deaths in the 30 Gy irradiated animals. The present results are in accordance with commonly accepted radiobiological theories, but not in agreement with results previously published by others.     

Untargeted effects of ionizing radiation: implications for radiation pathology

The dogma that genetic alterations are restricted to directly irradiated cells has been challenged by observations in which effects of ionizing radiation, characteristically associated with the consequences of energy deposition in the cell nucleus, arise in non-irradiated cells. These, so called, untargeted effects are demonstrated in cells that have received damaging signals produced by irradiated cells (radiation-induced bystander effects) or that are the descendants of irradiated cells (radiation-induced genomic instability). Radiation-induced genomic instability is characterized by a number of delayed adverse responses including chromosomal abnormalities, gene mutations and cell death. Similar effects, as well as responses that may be regarded as protective, have been attributed to bystander mechanisms. Whilst the majority of studies to date have used in vitro systems, some adverse non-targeted effects have been demonstrated in vivo. However, at least for haemopoietic tissues, radiation-induced genomic instability in vivo may not necessarily be a reflection of genomically unstable cells. Rather the damage may reflect responses to ongoing production of damaging signals; i.e. bystander responses, but not in the sense used to describe the rapidly induced effects resulting from direct interaction of irradiated and non-irradiated cells. The findings are consistent with a delayed and long-lived tissue reaction to radiation injury characteristic of an inflammatory response with the potential for persisting bystander-mediated damage. An important implication of the findings is that contrary to conventional radiobiological dogma and interpretation of epidemiologically-based risk estimates, ionizing radiation may contribute to malignancy and particularly childhood leukaemia by promoting initiated cells rather than being the initiating agent. Untargeted mechanisms may also contribute to other pathological consequences.     

The acute effects of ionizing radiation on DNA synthesis and the development of antibody-producing cells

Ionizing radiation inhibited the development of specific haemolysin-producing cells (PFC) and depressed the incorporation of (3H) thymidine by rabbit spleen explants responding to SRC in the culture medium. In contrast to these effects, the rates of incorporation of precursors for protein and RNA synthesis were much less affected. The depression of (3H) thymidine incorporation was found to result from a quantitative reduction of new DNA synthesis, without any change in the proportion of labelled cells, at any time after irradiation. The DNA synthesis occurring in these cells preparing to develop antibody-producing capacity was thus radio-sensitive, but the exact nature of the defect resulting from exposure to radiation requires further study.     

Hypothesis regarding a membrane-associated mechanism of biological action due to low-dose ionizing radiation

A novel theory is proposed regarding the action of ionizing radiation in the range of very low doses. The basic premise of the theory presented is that the low-dose effect cannot be explained by direct damage to the DNA (as has generally been assumed) and that effects on cellular membranes should be considered instead. Low-dose radiation damaging the plasma membrane decreases the concentration of low-molecular weight compounds (LMWC) inside the cell, which through an unspecific mechanism induces an activation of all enzymes. The mechanism described here has been well substantiated. The changes in the intracellular contents of LMWC and the increase of pH_in cause chromatin rearrangements, alterations in DNA folding and finally, if the latter are strong enough, expression of various ”silent” genes including repair enzyme genes. Received: 2 February 1999 / Accepted: 15 May 2000     

Cancer risk related to low-dose ionizing radiation from cardiac imaging in patients after acute myocardial infarction

Background

Patients exposed to low-dose ionizing radiation from cardiac imaging and therapeutic procedures after acute myocardial infarction may be at increased risk of cancer.

Methods

Using an administrative database, we selected a cohort of patients who had an acute myocardial infarction between April 1996 and March 2006 and no history of cancer. We documented all cardiac imaging and therapeutic procedures involving low-dose ionizing radiation. The primary outcome was risk of cancer. Statistical analyses were performed using a time-dependent Cox model adjusted for age, sex and exposure to low-dose ionizing radiation from noncardiac imaging to account for work-up of cancer.

Results

Of the 82 861 patients included in the cohort, 77% underwent at least one cardiac imaging or therapeutic procedure involving low-dose ionizing radiation in the first year after acute myocardial infarction. The cumulative exposure to radiation from cardiac procedures was 5.3 milliSieverts (mSv) per patient-year, of which 84% occurred during the first year after acute myocardial infarction. A total of 12 020 incident cancers were diagnosed during the follow-up period. There was a dose-dependent relation between exposure to radiation from cardiac procedures and subsequent risk of cancer. For every 10 mSv of low-dose ionizing radiation, there was a 3% increase in the risk of age- and sex-adjusted cancer over a mean follow-up period of five years (hazard ratio 1.003 per milliSievert, 95% confidence interval 1.002–1.004).

Interpretation

Exposure to low-dose ionizing radiation from cardiac imaging and therapeutic procedures after acute myocardial infarction is associated with an increased risk of cancer.Studies involving atomic bomb survivors have documented an increased incidence of malignant neoplasm related to the radiation exposure.^1–4 Survivors who were farther from the epicentre of the blast had a lower incidence of cancer, whereas those who were closer had a higher incidence.⁵ Similar risk estimates have been reported among workers in nuclear plants.⁶ However, little is known about the relation between exposure to low-dose ionizing radiation from medical procedures and the risk of cancer.In the past six decades since the atomic bomb explosions, most individuals worldwide have had minimal exposure to ionizing radiation. However, the recent increase in the use of medical imaging and therapeutic procedures involving low-dose ionizing radiation has led to a growing concern that individual patients may be at increased risk of cancer.^7–12 Whereas strict regulatory control is placed on occupational exposure at work sites, no such control exists among patients who are exposed to such radiation.^13–16It is not only the frequency of these procedures that is increasing. Newer types of imaging procedures are using higher doses of low-dose ionizing radiation than those used with more traditional procedures.^8,11 Among patients being evaluated for coronary artery disease, for example, coronary computed tomography is increasingly being used. This test may be used in addition to other tests such as nuclear scans, coronary angiography and percutaneous coronary intervention, each of which exposes the patient to low-dose ionizing radiation.^12,17–21 Imaging procedures provide information that can be used to predict the prognosis of patients with coronary artery disease. Since such predictions do not necessarily translate into better clinical outcomes,^8,12 the prognostic value obtained from imaging procedures using low-dose ionizing radiation needs to be balanced against the potential for risk.Authors of several studies have estimated that the risk of cancer is not negligible among patients exposed to low-dose ionizing radiation.^22–27 To our knowledge, none of these studies directly linked cumulative exposure and cancer risk. We examined a cohort of patients who had acute myocardial infarction and measured the association between low-dose ionizing radiation from cardiac imaging and therapeutic procedures and the risk of cancer.     

The psychological effects of ionizing radiation

This paper presents a case study of eleven men who were exposed to non-background ionizing radiation as active participants in the United States' atmospheric nuclear tests. Each of the subjects has developed a virtually identical complex of debilitating psychiatric symptoms. The content of these symptoms is almost entirely focused upon the health effects of the radiation to which each of the subjects was exposed. This symptom complex appears to comprise a syndrome. The symptom structure and course of this syndrome suggests three hypotheses: The syndrome appears to be a pathological development of the self diagnostic belief (that one has been physically harmed by radiation) into a set of symptoms that elaborate upon and express this belief. The self diagnostic belief develops as a means of resolving any one of the various medical mysteries that an individual can experience subsequent to exposure to radiation. Development of the syndrome is a consequence of exposure to non-background ionizing radiation. The paper discusses the evidence for these hypotheses and suggests future research directions.     

Long-term effects of postovulatory aging of mouse oocytes on offspring: a two-generational study.

Aims of this study were to analyze the long-term effects of postovulatory aging of mouse oocytes on 1) reproductive traits of parental (F(0)) and first (F(1))-generation females (pregnancy rate, gestation length, litter size, perinatal death, and sex ratio of offspring) and 2) developmental and behavioral variables of F(1) and second-generation (F(2)) offspring (birth weight and weight gain during preweaning development, postnatal day of attainment of immediate righting, spontaneous motor activity, and passive and active conditioned learning ability). Hybrid (C57BL/6JIco x CBA/JIco) females were artificially inseminated at 13 h (control group) or 22 h (oocyte-aged group) after GnRH injection. Experimental (oocyte-aged group) F(0) females exhibited lower pregnancy rate, shortened gestation length, decreased litter size, higher perinatal death of their pups, and increased percentage of male offspring compared to control F(0) females. Postovulatory aging of oocytes was also associated with increased number of growth-retarded pups, delayed development of the righting reflex, and higher spontaneous motor activity and emotionality of F(1) offspring. Postovulatory aging of F(0) oocytes did not affect birth weight, weight gain during preweaning development, passive and active conditioned learning ability of F(1) offspring, or reproductive traits of F(1) females or developmental and behavior variables of F(2) offspring.