Leggett, R. W., Eckerman, K. F., Khokhryakov, V. F., Suslova, K. G., Krahenbuhl, M. P. and Miller, S. C. Mayak Worker Study: An Improved Biokinetic Model for Reconstructing Doses from Internally Deposited Plutonium. Radiat. Res. 164, 111–122 (2005).

The plutonium production facility known as the Mayak Production Association was put into operation in June 1948. A high incidence of cancer in the Mayak workers has been related to the level of exposure to plutonium, but uncertainties in tissue doses have hampered development of dose–risk relationships. As part of an effort to improve dose estimates for these workers, the systemic biokinetic model for plutonium currently recommended by the International Commission on Radiological Protection (ICRP) has been modified to reflect recently developed data and facilitate interpretation of case-specific information. This paper describes the proposed model and discusses its implications for dose reconstruction for the Mayak workers.

Newman, L. S., Mroz, M. M. and Ruttenber, A. J. Lung Fibrosis in Plutonium Workers. Radiat. Res. 164, 123–131 (2005).

There have been few systematic studies of the non-malignant health effects of α-particle radiation in humans. Animal studies and a report on plutonium-exposed workers from Russia suggest an association between high doses to the lung from plutonium exposure and the development of fibrotic lung disease. Prompted by a case of lung fibrosis in a retired plutonium worker, we tested the hypothesis that plutonium inhalation increases the risk for developing chest radiograph abnormalities consistent with pulmonary fibrosis. We conducted a retrospective study of nuclear weapons workers that included estimating absorbed doses to the lung with an internal dosimetry model. Our study population consisted of 326 plutonium-exposed workers with absorbed lung doses from 0 to 28 Sv and 194 unexposed workers. We compared the severity of chest radiograph interstitial abnormalities between the two groups using the International Labour Organization profusion scoring system. There was a significantly higher proportion of abnormal profusion scores among plutonium-exposed workers (17.5%) than among unexposed workers (7.2%), P < 0.01. Lung doses of 10 Sv or greater conferred a 5.3-fold risk (95% CI 1.2–23.4) of having an abnormal chest X ray consistent with pulmonary fibrosis when compared with unexposed individuals after controlling for the effects of age, smoking and asbestos exposure. This study shows that plutonium may cause lung fibrosis in humans at absorbed lung doses above 10 Sv.

Rzeszowska-Wolny, J., Polanska, J., Pietrowska, M., Palyvoda, O., Jaworska, J., Butkiewicz, D. and Hancock, R. Influence of Polymorphisms in DNA Repair Genes XPD, XRCC1 and MGMT on DNA Damage Induced by Gamma Radiation and its Repair in Lymphocytes In Vitro. Radiat. Res. 164, 132– 140 (2005).

DNA single-strand breaks (SSBs) were quantified by single-cell gel electrophoresis and micronucleated and apoptotic cells were quantified by microscopic assays in peripheral blood lymphocytes after irradiation on ice with 2 Gy of ⁶⁰Co γ radiation, and their association with polymorphisms of genes that encode proteins of different DNA repair pathways and influence cancer risk (XPD codon 312Asp → Asn and 751Lys → Gln, XRCC1 399Arg → Gln, and MGMT 84Leu → Phe) was studied. In unirradiated lymphocytes, SSBs were significantly more frequent in individuals older than the median age (52 years) (P = 0.015; n = 81), and the frequency of apoptotic or micronucleated cells was higher in individuals with alleles coding for Asn at XPD 312 or Gln at 751 (P = 0.030 or 0.023 ANOVA, respectively; n = 54). The only polymorphism associated with the background SSB level was MGMT 84Phe (P = 0.04, ANOVA; n = 66). After irradiation, SSB levels and repair parameters did not differ significantly with age or smoking habit. The SSB level varied more than twofold and the repair rate and level of unrepaired SSBs more than 10-fold between individuals. The presence of variant alleles coding for Asn at XPD 312 was associated with more radiation-induced SSBs (P = 0.014) and fewer unrepaired SSBs (P = 0.008), and the phenotype (>median induced SSBs/<median unrepaired SSBs) was seen in the majority of XPD 312Asn/Asn homozygotes; the odds ratio for variant homozygotes to show this phenotype was 5.2 (95% confidence interval 1.4–19.9). The hypothesis is discussed that XPD could participate in repair of ionizing radiation-induced DNA damage. While it cannot be excluded that the effects observed are due to cosegregating polymorphisms or that the responses of lymphocytes are not typical of other cell types, the results suggest that polymorphism of DNA repair genes, particularly XPD, is one factor implicated in the variability of responses to ionizing radiation between different individuals.

Nagasawa, H., Peng, Y., Wilson, P. F., Lio, Y-C., Chen, D. J., Bedford, J. S. and Little, J. B. Role of Homologous Recombination in the Alpha-Particle-Induced Bystander Effect for Sister Chromatid Exchanges and Chromosomal Aberrations. Radiat. Res. 164, 141–147 (2005).

The bystander effect for sister chromatid exchanges (SCEs) and chromosomal aberrations was examined in hamster cell lines deficient in either DNA-PKcs (V3 cells, deficient in nonhomologous end joining, NHEJ) or RAD51C (irs3 cells, deficient in homologous recombination, HR). Cells synchronized in G₀/G₁ phase were irradiated with very low fluences of α particles such that <1% of the nuclei were traversed by an α particle. Wild-type cells showed a prominent bystander response for SCE induction; an even greater effect was observed in V3 cells. On the other hand, no significant induction of SCE was observed in the irs3 RAD51C-deficient bystander cells irradiated at various stages in the cell cycle. Whereas a marked bystander effect for chromosomal aberrations occurred in V3 cells, the induction of chromosomal aberrations in irs3 bystander cells was minimal and similar to that of wild-type cells. Based on these findings, we hypothesize that HR is essential for the induction of SCE in bystander cells; however, HR is unable to repair the DNA damage induced in NHEJ-deficient bystander cells that leads to either SCE or chromosomal aberrations.

Virsik-Köpp, P., Hofman-Hüther, H., Rave-Fränk, M. and Schmidberger, H. The Effect of Wortmannin on Radiation-Induced Chromosome Aberration Formation in the Radioresistant Tumor Cell Line WiDr. Radiat. Res. 164, 148–156 (2005).

We analyzed the formation of radiation-induced chromosome aberrations in the cells of the radioresistant colon carcinoma cell line WiDr after treatment with wortmannin, an inhibitor of PI-3 kinases, including DNA-PK. Cells irradiated in G₀/G₁ phase with 200 kV X rays were treated with wortmannin before or after irradiation. Chromosome-type and chromatid-type aberrations were scored in metaphase cells by either Giemsa staining or FISH. Moreover, DNA-PK activity was measured in the absence and presence of wortmannin. In irradiated G₀/G₁-phase WiDr cells, only chromosome-type aberrations, including simple and complex exchanges and excess acentrics, were observed. After addition of 1 to 20 μM wortmannin, the formation of chromosome-type exchange aberrations was completely suppressed. The irradiated cells displayed exclusively chromatid-type aberrations including simple and complex chromatid exchanges and chromatid/isochromatid breaks. Whether the chromatid-type aberrations arise during G₀/G₁ as a result of homologous recombination processes coping with damaged DNA or whether DNA damage induced during G₀/G₁ phase persists until S and G₂ phase and is then processed by homologous recombination pathways must be investigated further.

Guzmán-Rincón, J., Delfín-Loya, A., Ureña-Núñez, F., Paredes, L. C., Zambrano-Achirica, F. and Graf, U. Genotoxicity of Neutrons in Drosophila melanogaster. Somatic Mutation and Recombination Induced by Reactor Neutrons. Radiat. Res. 164, 157–162 (2005).

This paper describes the observation of a direct relationship between the absorbed doses of neutrons and the frequencies of somatic mutation and recombination using the wing somatic mutation and recombination test (SMART) of Drosophila melanogaster. This test was used for evaluating the biological effects induced by neutrons from the Triga Mark III reactor of Mexico. Two different reactor power levels were used, 300 and 1000 kW, and two absorbed doses were tested for each power level: 1.6 and 3.2 Gy for 300 kW and 0.84 and 1.7 Gy for 1000 kW. A linear relationship was observed between the absorbed dose and the somatic mutation and recombination frequencies. Furthermore, these frequencies were dependent on larval age: In 96-h-old larvae, the frequencies were increased considerably but the sizes of the spots were smaller than in 72-h-old larvae. The analysis of the balancer-heterozygous progeny showed a linear absorbed dose– response relationship, although the responses were clearly lower than found in the marker-trans-heterozygous flies. Approximately 65% of the genotoxicity observed is due to recombinational events. The results of the study indicate that thermal and fast neutrons are both mutagenic and recombinagenic in the D. melanogaster wing SMART, and that the frequencies are dependent on neutron dose, reactor power, and the age of the treated larvae.

Laszlo, A., Moros, E. G., Davidson, T., Bradbury, M., Straube, W. and Roti Roti, J. The Heat-Shock Factor is not Activated in Mammalian Cells Exposed to Cellular Phone Frequency Microwaves. Radiat. Res. 164, 163–172 (2005).

There has been considerable interest in the biological effects of exposure to radiofrequency electromagnetic radiation, given the explosive growth of cellular telephone use, with the possible induction of malignancy being a significant concern. Thus the determination of whether nonthermal effects of radiofrequency electromagnetic radiation contribute to the process leading to malignancy is an important task. One proposed pathway to malignancy involves the induction of the stress response by exposures to cell phone frequency microwaves. The first step in the induction of the stress response is the activation of the DNA-binding activity of the specific transcription factor involved in this response, the heat-shock factor (HSF). The DNA-binding activity of HSF in hamster, mouse and human cells was determined after acute and continuous exposures to frequency domain multiple access (FDMA)- or code domain multiple access (CDMA)-modulated microwaves at low (0.6 W/kg) or high (∼5 W/kg) SARs at frequencies used for mobile communication. The DNA-binding activity of HSF was monitored using a gel shift assay; the calibration of this assay indicated that an increase of ∼10% in the activation of the DNA-binding activity of HSF after a 1°C increase in temperature could be detected. We failed to detect any increase in the DNA-binding ability of HSF in cultured mammalian cells as a consequence of any exposure tested, within the sensitivity of our assay. Our results do not support the notion that the stress response is activated as a consequence of exposure to microwaves of frequencies associated with mobile communication devices.

Cai, Z., Cloutier, P., Sanche, L. and Hunting, D. DNA Interduplex Crosslinks Induced by Al_Kα X Rays under Vacuum. Radiat. Res. 164, 173–179 (2005).

Dry pGEM®-3Zf(-) plasmid DNA was exposed to Al_kα X rays (1.5 keV) for various times in an ultra-high vacuum chamber with mean absorbed dose rates ranging from 1.8 to 41.7 Gy s⁻¹. The different forms of plasmid DNA were separated by neutral agarose gel electrophoresis and quantified by staining and laser scanning. In addition to the bands for supercoiled, nicked circular, linear and concatameric forms of plasmid DNA, two additional bands were observed in X-irradiated samples; these migrated at rates similar to those for 8-kb and >10-kb linear double-stranded DNA. Digestion of irradiated DNA with the restriction enzymes EcoR1 and PvuI suggested that the two slowly migrating bands were interduplex crosslinked DNA. Alkaline agarose gel electrophoresis of irradiated DNA digested with EcoR1 confirmed that the interduplex crosslink was covalent. Exposure–response curves were determined for the formation of nicked circular, linear and interduplex crosslinked DNA as well as for the loss of supercoiled and concatameric DNA. Formation and loss of these species were independent of absorbed dose rate over a 20-fold range. The G values for DNA single-strand breaks, double-strand breaks and crosslinks were determined to be 62 ± 6, 5.6 ± 0.6 and 16 ± 4 nmol J⁻¹, respectively. The formation of DNA interduplex crosslinks appears to be due to single event. The mechanism responsible for the formation of DNA interduplex crosslinks is discussed with emphasis on its implications in vivo.

Semenenko, V. A., Stewart, R. D. and Ackerman, E. J. Monte Carlo Simulation of Base and Nucleotide Excision Repair of Clustered DNA Damage Sites. I. Model Properties and Predicted Trends. Radiat. Res. 164, 180–193 (2005).

DNA is constantly damaged through endogenous processes and by exogenous agents, such as ionizing radiation. Base excision repair (BER) and nucleotide excision repair (NER) help maintain the stability of the genome by removing many different types of DNA damage. We present a Monte Carlo excision repair (MCER) model that simulates key steps in the short-patch and long-patch BER pathways and the NER pathway. The repair of both single and clustered damages, except double-strand breaks (DSBs), is simulated in the MCER model. Output from the model includes estimates of the probability that a cluster is repaired correctly, the fraction of the clusters converted into DSBs through the action of excision repair enzymes, the fraction of the clusters repaired with mutations, and the expected number of repair cycles needed to completely remove a clustered damage site. The quantitative implications of alternative hypotheses regarding the postulated repair mechanisms are investigated through a series of parameter sensitivity studies. These sensitivity studies are also used to help define the putative repair characteristics of clustered damage sites other than DSBs.

Semenenko, V. A. and Stewart, R. D. Monte Carlo Simulation of Base and Nucleotide Excision Repair of Clustered DNA Damage Sites. II. Comparisons of Model Predictions to Measured Data. Radiat. Res. 164, 194–201 (2005).

Clustered damage sites other than double-strand breaks (DSBs) have the potential to contribute to deleterious effects of ionizing radiation, such as cell killing and mutagenesis. In the companion article (Semenenko et al., Radiat. Res. 164, 180–193, 2005), a general Monte Carlo framework to simulate key steps in the base and nucleotide excision repair of DNA damage other than DSBs is proposed. In this article, model predictions are compared to measured data for selected low-and high-LET radiations. The Monte Carlo model reproduces experimental observations for the formation of enzymatic DSBs in Escherichia coli and cells of two Chinese hamster cell lines (V79 and xrs5). Comparisons of model predictions with experimental values for low-LET radiation suggest that an inhibition of DNA backbone incision at the sites of base damage by opposing strand breaks is active over longer distances between the damaged base and the strand break in hamster cells (8 bp) compared to E. coli (3 bp). Model estimates for the induction of point mutations in the human hypoxanthine guanine phosphoribosyl transferase (HPRT) gene by ionizing radiation are of the same order of magnitude as the measured mutation frequencies. Trends in the mutation frequency for low- and high-LET radiation are predicted correctly by the model. The agreement between selected experimental data sets and simulation results provides some confidence in postulated mechanisms for excision repair of DNA damage other than DSBs and suggests that the proposed Monte Carlo scheme is useful for predicting repair outcomes.

Emfietzoglou, D., Cucinotta, F. A. and Nikjoo, H. A Complete Dielectric Response Model for Liquid Water: A Solution of the Bethe Ridge Problem. Radiat. Res. 164, 202–211 (2005).

We present a complete yet computationally simple model for the dielectric response function of liquid water over the energy-momentum plane, which, in contrast to earlier models, is consistent with the recent inelastic X-ray scattering spectroscopy data at both zero and finite momentum transfer values. The model follows Ritchie's extended-Drude algorithm and is particularly effective at the region of the Bethe ridge, substantially improving previous models. The present development allows for a more accurate simulation of the inelastic scattering and energy deposition process of low-energy electrons in liquid water and other biomaterials. As an example, we calculate the stopping power of liquid water for electrons over the 0.1–10 keV range where direct experimental measurements are still impractical and the Bethe stopping formula is inaccurate. The new stopping power values are up to 30–40% lower than previous calculations. Within the range of validity of the first Born approximation, the new values are accurate to within the experimental uncertainties (a few percent). At the low end, the introduction of Born corrections raises the uncertainty to perhaps ∼10%. Thus the present model helps extend the ICRU electron stopping power database for liquid water down to about two orders of magnitude with a comparable level of uncertainty.

Binns, P. J., Riley, K. J. and Harling, O. K. Epithermal Neutron Beams for Clinical Studies of Boron Neutron Capture Therapy: A Dosimetric Comparison of Seven Beams. Radiat. Res. 164, 212–220 (2005).

A comparison of seven epithermal neutron beams used in clinical studies of boron neutron capture therapy (BNCT) in Sweden (Studsvik), Finland (Espoo), Czech Republic (ReZ), The Netherlands (Petten) and the U.S. (Brookhaven and Cambridge) was performed to facilitate sharing of preclinical and clinical results. The physical performance of each beam was measured using a common dosimetry method under conditions pertinent to brain irradiations. Neutron fluence and absorbed dose measurements were performed with activation foils and paired ionization chambers on the central axis both in air and in an ellipsoidal water phantom. The overall quality of each beam was assessed by figures of merit determined from the total weighted dose profiles that assumed the presence of boron in tissue. The in-air specific beam contamination from both fast neutrons and γ rays ranged between 8 and 65 × 10⁻¹¹ cGy(w) cm² for the different beams and the epithermal neutron flux intensities available at the patient position differed by more than a factor of 20 from 0.2–4.3 × 10⁹ n cm⁻² s⁻¹. Percentage depth dose profiles measured in-phantom for the individual photon, thermal and fast-neutron dose components differed only subtly in shape between facilities. Assuming uptake characteristics consistent with the currently used boronated phenylalanine, all the epithermal beams exhibit a useful penetration of 8 cm or greater that is sufficient to irradiate a lesion seated at the brain midline. The performance of the existing facilities will benefit from the introduction of advanced compounds through improved beam penetrability. This could increase by as much as 2 cm for the purest of beams, although the beam intensities generally need to be increased to between 2–5 × 10⁹ n cm⁻² s⁻¹ to maintain manageable irradiation times. These data provide the first consistent measurement of beam performance at the different centers and will enable a preliminary normalization of the calculated patient dosimetry.

Harling, O. K., Riley, K. J., Binns, P. J., Patel, H. and Coderre, J. A. The MIT User Center for Neutron Capture Therapy Research. Radiat. Res. 164, 221–229 (2005).

Neutron capture therapy (NCT) research encompasses a wide range of preclinical and clinical studies needed to develop this promising but complex cancer treatment. Many specialized facilities and capabilities including thermal and epithermal neutron irradiation facilities, boron analysis, specialized mixed-field dosimetry, animal care facilities and protocols, cell culture laboratories, and, for human clinical studies, licenses and review board approvals are required for NCT research. Such infrastructure is essential, but much of it is not readily available within the community. This is especially true for neutron irradiation facilities, which often require significant development and capital investment too expensive to duplicate at each site performing NCT research. To meet this need, the NCT group at the Massachusetts Institute of Technology (MIT) has established a User Center for NCT researchers that is already being accessed successfully by various groups. This paper describes the facilities, capabilities and other resources available at MIT and how the NCT research community can access them.

Atkinson, M. J., Spanner, M. T., Rosemann, M., Linzner, U., Müller, W. A. and Gössner, W. Intracellular Sequestration of ²²³Ra by the Iron-Storage Protein Ferritin. Radiat. Res. 164, 230–233 (2005).

Incorporation of bone-seeking, α-particle-emitting, heavy-metal radionuclides dramatically increases the incidence of osteosarcoma in humans and experimental animals. The accumulation of these radionuclides within the mineral phase of the bone matrix is believed to result in local irradiation of only those proliferating cells close to the bone surface. We now present evidence for a more general pathway for the irradiation of target cells, mediated through the sequestration of heavy-metal radionuclides by the intracellular iron-storage protein ferritin. In vitro studies reveal the transfer of radionuclide from a ²²³Ra-transferrin complex into immunoprecipitable cytosolic ferritin. In vivo studies confirm the co-localization of incorporated ²²⁴Ra and cellular iron stores. This pathway would result in the highly localized irradiation of ferritin-containing cells. Since osteoblastic cells express large quantities of a ferritin isoform specialized in long-term metal storage, we suggest that this may represent an unrecognized source of intracellular irradiation by α-particle-emitting radionuclides. Such a local concentration within target cells has implications both for cellular dosimetry and for inferences of track length and target cell populations within the skeleton.