Confidence limits for low induced frequencies of oncogenic transformation in the presence of a background

Microsatellite analysis for determination of the mutagenicity of extremely low-frequency electromagnetic fields and ionising radiation in vitro

Extremely low-frequency electromagnetic fields (ELF-EMF) have been reported to induce lesions in DNA and to enhance the mutagenicity of ionising radiation. However, the significance of these findings is uncertain because the determination of the carcinogenic potential of EMFs has largely been based on investigations of large chromosomal aberrations. Using a more sensitive method of detecting DNA damage involving microsatellite sequences, we observed that exposure of UVW human glioma cells to ELF-EMF alone at a field strength of 1 mT (50 Hz) for 12 h gave rise to 0.011 mutations/locus/cell. This was equivalent to a 3.75-fold increase in mutation induction compared with unexposed controls. Furthermore, ELF-EMF increased the mutagenic capacity of 0.3 and 3 Gy gamma-irradiation by factors of 2.6 and 2.75, respectively. These results suggest not only that ELF-EMF is mutagenic as a single agent but also that it can potentiate the mutagenicity of ionising radiation. Treatment with 0.3 Gy induced more than 10 times more mutations per unit dose than irradiation with 3 Gy, indicating hypermutability at low dose.

Risk coefficients for childhood cancer after intrauterine irradiation: a review

PURPOSE

To review the estimates of the risk of childhood cancer per unit dose of radiation received in utero derived from the largest case-control study of obstetric X-ray examinations and to compare them with the childhood cancer risk coefficients obtained from the cohorts of Japanese atomic bomb survivors irradiated either in utero or as young children.

MATERIALS AND METHODS

Data from the Oxford Survey of Childhood Cancers (OSCC) case-control study of foetal exposure to diagnostic X-rays and from the cohort studies of the Japanese survivors of the atomic bombings of Hiroshima and Nagasaki were used, together with associated dose estimates. Excess relative risk and excess absolute risk coefficients were compared, fully taking into consideration the various sources of uncertainty.

RESULTS

The excess relative risk coefficient for childhood (< 15 years of age) cancer obtained from the OSCC was around 50 Gy(-1), leading to an excess absolute risk coefficient for incident cases of about 8% Gy(-1). However, the statistical, dosimetry, modelling and other uncertainties associated with these risk estimates are appreciable, and there is reason to believe that these coefficients could be systematic overestimates. When these uncertainties and those associated with the equivalent risk coefficients derived from the Japanese cohort exposed in utero are taken into account, the risk estimates for childhood cancer obtained from these two sources are compatible. These coefficients are consistent with the high relative risk of childhood leukaemia among the Japanese survivors exposed as children. The absence of cases of childhood solid tumours among the Japanese children irradiated after birth in contrast to the significant excesses found in both intrauterine exposure studies might be explained by the cells from which these cancers originate being predominantly sensitive only to exposure in utero.

CONCLUSIONS

The consistency of the childhood cancer risk coefficients derived from the Oxford Survey and from the Japanese cohort irradiated in utero supports a causal explanation of the association between childhood cancer and an antenatal X-ray examination found in case-control studies. This implies that doses to the foetus in utero of the order of 10 mSv discernibly increase the risk of childhood cancer. However, uncertainties in risk estimates are such that it is difficult to conclude reliably from these epidemiological data what the level of risk at these low doses might be, beyond the inference that the risk is not zero or has been grossly underestimated.

Commentary 2 to Cox and Little: radiation-induced oncogenic transformation: the interplay between dose, dose protraction, and radiation quality

There is now a substantial body of evidence for end points such as oncogenic transformation in vitro, and carcinogenesis and life shortening in vivo, suggesting that dose protraction leads to an increase in effectiveness relative to a single, acute exposure--at least for radiations of medium linear energy transfer (LET) such as neutrons. Table I contains a summary of the pertinent data from studies in which the effect is seen. [table: see text] This phenomenon has come to be known as the "inverse dose rate effect," because it is in marked contrast to the situation at low LET, where protraction in delivery of a dose of radiation, either by fractionation or low dose rate, results in a decreased biological effect; additionally, at medium and high LET, for radiobiological end points such as clonogenic survival, the biological effectiveness is independent of protraction. The quantity and quality of the published reports on the "inverse dose rate effect" leaves little doubt that the effect is real, but the available evidence indicates that the magnitude of the effect is due to a complex interplay between dose, dose rate, and radiation quality. Here, we first summarize the available data on the inverse dose rate effect and suggest that it follows a consistent pattern in regard to dose, dose rate, and radiation quality; second, we describe a model that predicts these features; and, finally, we describe the significance of the effect for radiation protection.

Adaptive response and the bystander effect induced by radiation in C3H 10T(1/2) cells in culture

This paper discusses two phenomena of importance at low doses that have an impact on the shape of the dose-response relationship. First, there is the bystander effect, the term used to describe the biological effects observed in cells that are not themselves traversed by a charged particle, but are neighbors of cells that are; this exaggerates the effect of small doses of radiation. Second, there is the adaptive response, whereby exposure to a low level of DNA stress renders cells resistant to a subsequent exposure; this reduces the effect of low doses of radiation. The present work was undertaken to assess the relative importance of the adaptive response and the bystander effect induced by radiation in C3H 10T(1/2) cells in culture. When the single-cell microbeam delivered from 1 to 12 alpha particles through the nuclei of 10% of C3H 10T(1/2) cells, more cells were inactivated than were actually traversed by alpha particles. The magnitude of this bystander effect increased with the number of particles per cell. An adaptive dose of 2 cGy of gamma rays, delivered 6 h beforehand, canceled out about half of the bystander effect produced by the alpha particles.

The bystander effect in radiation oncogenesis: I. Transformation in C3H 10T1/2 cells in vitro can be initiated in the unirradiated neighbors of irradiated cells

It has long been accepted that radiation-induced genetic effects require that DNA be hit and damaged directly by the radiation. Recently, evidence has accumulated that in cell populations exposed to low doses of alpha particles, biological effects occur in a larger proportion of cells than are estimated to have been traversed by alpha particles. The end points observed include chromosome aberrations, mutations and gene expression. The development of a fast single-cell microbeam now makes it possible to expose a precisely known proportion of cells in a population to exactly defined numbers of alpha particles, and to assay for oncogenic transformation. The single-cell microbeam delivered no, one, two, four or eight alpha particles through the nuclei of all or just 10% of C3H 10T1/2 cells. We show that (a) more cells can be inactivated than were actually traversed by alpha particles and (b) when 10% of the cells on a dish are exposed to alpha particles, the resulting frequency of induced transformation is not less than that observed when every cell on the dish is exposed to the same number of alpha particles. These observations constitute evidence suggesting a bystander effect, i.e., that unirradiated cells are responding to damage induced in irradiated cells. This bystander effect in a biological system of relevance to carcinogenesis could have significant implications for risk estimation for low-dose radiation.

The oncogenic transforming potential of the passage of single alpha particles through mammalian cell nuclei

Domestic, low-level exposure to radon gas is considered a major environmental lung-cancer hazard involving DNA damage to bronchial cells by alpha particles from radon progeny. At domestic exposure levels, the relevant bronchial cells are very rarely traversed by more than one alpha particle, whereas at higher radon levels-at which epidemiological studies in uranium miners allow lung-cancer risks to be quantified with reasonable precision-these bronchial cells are frequently exposed to multiple alpha-particle traversals. Measuring the oncogenic transforming effects of exactly one alpha particle without the confounding effects of multiple traversals has hitherto been unfeasible, resulting in uncertainty in extrapolations of risk from high to domestic radon levels. A technique to assess the effects of single alpha particles uses a charged-particle microbeam, which irradiates individual cells or cell nuclei with predefined exact numbers of particles. Although previously too slow to assess the relevant small oncogenic risks, recent improvements in throughput now permit microbeam irradiation of large cell numbers, allowing the first oncogenic risk measurements for the traversal of exactly one alpha particle through a cell nucleus. Given positive controls to ensure that the dosimetry and biological controls were comparable, the measured oncogenicity from exactly one alpha particle was significantly lower than for a Poisson-distributed mean of one alpha particle, implying that cells traversed by multiple alpha particles contribute most of the risk. If this result applies generally, extrapolation from high-level radon risks (involving cellular traversal by multiple alpha particles) may overestimate low-level (involving only single alpha particles) radon risks.

Leukaemia and non-Hodgkin's lymphoma in young persons resident in small areas of West Cumbria in relation to paternal preconceptional irradiation

The results of a previous study suggested that an association between childhood leukaemia and the radiation dose received occupationally by a father before the conception of his child might provide the explanation for the marked excess of childhood leukaemia and non-Hodgkin's lymphoma in the village of Seascale, West Cumbria. The present study identifies other small areas (electoral wards) in West Cumbria where excess cases of leukaemia and non-Hodgkin's lymphoma in young people have occurred and determines whether a recorded dose of radiation was received occupationally by the father before the conception of each of the affected individuals. Forty-one cases of leukaemia and non-Hodgkin's lymphoma were diagnosed during 1968-85 in young people under 25 years of age resident in the 49 electoral wards lying within the boundary of West Cumbria and the adjacent ward of Broughton. Raised incidence rate ratios (two-sided P<0.01) were found for acute lymphoblastic leukaemia among those aged 0-14 years (concentrated among those aged 0-4 years) in Seascale ward and among those aged 0-24 years (also concentrated among those aged 0-4 years) in Egremont North ward, for acute myeloid leukaemia among those aged 0-14 years in Sandwith ward, for all leukaemias among those aged 0-14 years in Broughton ward (South Lakeland) and for non-Hodgkin's lymphoma among those aged 0-14 years in Seascale ward. For West Cumbria as a whole, incidence rates were not usual. Apart from Seascale, for none of these electoral wards has a father of an affected child been linked definitely to an occupational dose of radiation recorded before the conception of the child. Particularly striking are the excesses of acute lymphoblastic leukaemia cases among young children living in the wards of Seascale and Egremont North, situated 11 km apart. The cases in Egremont North are not associated with recorded doses of radiation received occupationally by fathers before the conception of the affected children, even though the total numbers of children associated with such doses born in Seascale and Egremont North wards are similar. This finding is further evidence against a causal role for paternal preconceptional radiation exposure in the cases of childhood leukaemia in Seascale.

Geographical distribution of preconceptional radiation doses to fathers employed at the Sellafield nuclear installation, West Cumbria

OBJECTIVE

To examine whether the geographical distribution of births associated with preconceptional exposure of fathers to radiation at the Sellafield nuclear installation is consistent with the suggestion that this exposure explains the excess of childhood lymphoid malignancy in the adjacent village of Seascale.

DESIGN

Retrospective birth cohort study.

SETTING

Cumbria, West Cumbria health district, and Seascale civil parish.

SUBJECTS

The 10,363 children born in Cumbria during 1950-89 to fathers employed at Sellafield.

MAIN OUTCOME MEASURES

The doses of external whole body ionising radiation received by fathers at Sellafield in the total time and in the six months before conception of their children; the proportions of the collective doses associated with Seascale and the rest of West Cumbria.

RESULTS

9256 children were born to fathers who had been exposed to radiation before the child's conception. Of these, 7318 had fathers who were exposed in the six months before conception. Overall 7% (38 person-Sv) of the collective total preconceptional dose and 7% (3 person-Sv) of the collective dose for the six months before conception were associated with children born in Seascale. Of all the children whose fathers worked at Sellafield, 842 (8%) were born in Seascale. The mean individual doses before conception were consistently lower in Seascale than in the rest of West Cumbria.

CONCLUSIONS

The distribution of the paternal preconceptional radiation dose is statistically incompatible with this exposure providing a causal explanation for the cluster of childhood leukaemias in Seascale.

Cytogenetic and survival adaptive responses in G1 phase human lymphocytes

Human lymphocytes from six donors were treated with 5 cGy of X-rays followed by 200 or 400 cGy in the first G1 phase after PHA stimulation, and assayed for cytogenetic aberrations and cell survival. Four donors showed cytogenetic adaptive responses with 400 cGy, and one with both 200 cGy and 400 cGy. Both exchanges and deletions were reduced, indicating that the cytogenetic adaptive response acts by restitution of chromosome breaks. Good correlations were found between nonaberrant cells and survival, although the former were often higher than the later, especially with the 400 cGy dose. In four of six donors, 5 cGy alone had significant effects on cell survival; however, this was independent of the 5 cGy effect on high-dose-induced responses. Two donors had survival adaptive responses with 5 + 200 cGy, while none were found with the 5 + 400 cGy treatment. The comparisons between the cytogenetic and survival responses suggests that a survival adaptive response will only be seen with a sufficient increase in nonaberrant cells. To date, adaptive responses to ionizing radiation have been reported to occur in G0, G1 and late S/early G2 human lymphocytes.

The inverse dose-rate effect for oncogenic transformation by neutrons and charged particles: a plausible interpretation consistent with published data

The enhancement of oncogenic transformation in the C3H10T1/2 system by protraction of a high-LET irradiation has been widely reported. Prima facie, the results are inconsistent in that some but not all experiments have shown an enhancement. That the reported data follow a clear pattern is shown, and a model whose predictions are quantitatively consistent with these trends is discussed. The model, developed from that originally suggested by Rossi and Kellerer (1986), postulates that cells are especially sensitive to radiation during some period of their cycle. A sensitive period of about 1 h is shown to yield predictions consistent with all available data. If the suggested model is realistic and applicable to human cells in vivo, little enhancement would be expected for high-LET radiations such as from radon daughters or HZE cosmic rays, though an effect might be expected from trapped protons on astronauts in earth orbit. For fission neutrons a time-dependent factor of N = 2 in the formula for dose equivalent (H = DQN) might be appropriate for very low dose rates, if a quality factor Q = 10 were applied. If Q was taken as 20, then a value of N = 1 would probably be adequate.