Neutron RBE for induction of tumors with high lethality in Sprague-Dawley rats

Assessing the Relative Biological Effectiveness of Neutrons across Organs of Varying Depth among the Atomic Bomb Survivors

When assessing radiation-related risk among the atomic bomb survivors, choices in modeling approach can have an important impact on the results, which are then used to inform radiation protection standards throughout the world. The atomic bombings of Hiroshima and Nagasaki produced a mixed-field radiation exposure from two sources: neutrons and gamma rays. Neutrons are more densely ionizing and cause greater biological damage per unit absorbed dose, resulting in greater relative biological effectiveness (RBE) than gamma rays. To account for this, a combined weighted dose is typically calculated as the sum of the gamma-ray dose and 10 times the neutron dose in the Radiation Effects Research Foundation's reports of mortality, solid cancer incidence and other outcomes. In addition, the colon, which is often chosen as the whole-body representative organ in these analyses, is relatively deep in the body and therefore its dose calculation involves heavy body shielding of neutrons and a low neutron/gamma-ray ratio. With added follow-up and recently updated doses, we used a data-driven approach to determine the best-fitting neutron RBE for a range of organs of varying depth. Aggregated person-year tables of solid cancer incidence (1958-2009) from the Life Span Study were created with separate neutron and gamma-ray DS02R1 doses for several organs including breast, brain, thyroid, bone marrow, lung, liver and colon. Typical excess relative risk models estimating the linear effect of radiation dose were fitted using a range of neutron weights (1-250) to calculate combined dose for each organ, and model deviances were compared to assess fit. Furthermore, models using separate terms for gamma-ray and neutron dose were also examined, wherein the ratio of the neutron/gamma-ray linear terms indicated the best estimate of the RBE. The best-fitting RBE value for the traditional weighted colon dose was 80 [95% confidence interval (CI): 20-190], while the RBEs for other organs using weighted doses ranged from 25 to 60, with the best-fitting weights and confidence interval widths both incrementally increasing with greater depth of organ. Models using separate neutron- and gamma-ray-dose terms gave similar results to weighted linear combinations, with a neutron/gamma-ray term ratio of 79.9 (95% CI: 18.8-192.3) for colon. These results indicated that the traditionally modeled RBE of 10 may underestimate the effect of neutrons across the full dose range, although these updated estimates still have fairly wide confidence bounds. Furthermore, the colon is among the deepest of organs and may not be the best choice as a single surrogate organ dose, as it may minimize the role of the neutrons. Future work with more refined organ doses could shed more light on RBE-related information available in the Life Span Study data.

ANALYTICAL MODEL TO ESTIMATE EQUIVALENT DOSE FROM INTERNAL NEUTRONS IN PROTON THERAPY OF CHILDREN WITH INTRACRANIAL TUMORS

This study developed a computationally efficient and easy-to-implement analytical model to estimate the equivalent dose from secondary neutrons originating in the bodies ('internal neutrons') of children receiving intracranial proton radiotherapy. A two-term double-Gaussian mathematical model was fit to previously published internal neutron equivalent dose per therapeutic absorbed dose versus distance from the field edge calculated using Monte Carlo simulations. The model was trained using three intracranial proton fields of a 9-year-old girl. The resulting model was tested against two intracranial fields of a 10-year-old boy by comparing the mean doses in organs at risk of a radiogenic cancer estimated by the model versus those previously calculated by Monte Carlo. On average, the model reproduced the internal neutron organ doses in the 10-year-old boy within 13.5% of the Monte Carlo at 3-10 cm from the field edge and within a factor of 2 of the Monte Carlo at 10-20 cm from the field edge. Beyond 20 cm, the model poorly estimated H/DRx, however, the values were very small, at <0.03 mSv Gy-1.

Independent application of an analytical model for secondary neutron equivalent dose produced in a passive-scattering proton therapy treatment unit

The purpose of this study was to independently apply an analytical model for equivalent dose from neutrons produced in a passive-scattering proton therapy treatment unit, H. To accomplish this objective, we applied the previously-published model to treatment plans of two pediatric patients. Their model accounted for neutrons generated by mono-energetic proton beams stopping in a closed aperture. To implement their model to a clinical setting, we adjusted it to account for the area of a collimating aperture, energy modulation, air gap between the treatment unit and patient, and radiation weighting factor. We used the adjusted model to estimate H per prescribed proton absorbed dose, D _Rx , for the passive-scattering proton therapy beams of two children, a 9-year-old girl and 10-year-old boy, who each received intracranial boost fields as part of their treatment. In organs and tissues at risk for radiation-induced subsequent malignant neoplasms, T, we calculated the mass-averaged H, H _T , per D _Rx . Finally, we compared H _T /D _Rx values to those of previously-published Monte Carlo (MC) simulations of these patients' fields. H _T /D _Rx values of the adjusted model deviated from the MC result for each organ on average by 20.8  ±  10.0% and 44.2  ±  17.6% for the girl and boy, respectively. The adjusted model underestimated the MC result in all T of each patient, with the exception of the girl's bladder, for which the adjusted model overestimated H _T /D _Rx by 3.1%. The adjusted model provided a better estimate of H _T /D _Rx than the unadjusted model. That is, between the two models, the adjusted model reduced the deviation from the MC result by approximately 37.0% and 46.7% for the girl and boy, respectively. We found that the previously-published analytical model, combined with adjustment factors to enhance its clinical applicability, predicted H _T /D _Rx in out-of-field organs and tissues at risk for subsequent malignant neoplasms with acceptable accuracy. This independent application demonstrated that the analytical model may be useful broadly for clinicians and researchers to calculate equivalent dose from neutrons produced externally to the patient in passive-scattering proton therapy.

Special radiobiological features of second cancer risk after particle radiotherapy

In absolute terms: second cancer risks from radiotherapy of first cancers in adults are small compared to the benefits from radiotherapy but this is not so for radiotherapy of childhood cancers. Moreover, the radiation dose dependence of cancer induction differs between organs and tissues. The organ-specific dose dependence of second cancer risks may indicate the existence of different radiobiological mechanisms. As an inevitable consequence of the age dependence of organ sensitivity to second cancer induction, the organ/tissue weighting factors which have been proposed by ICRP for calculating effective dose (the dose unit Sv) and for risk estimation in the general population should not be used in medical radiation exposures. In adult cancer radiotherapy, the most common unwanted effect is local tumour recurrence whereas both, severe late normal tissue damage and radiation-induced second cancers are rare, around 1% of locally controlled cancer patients. In childhood cancers, local failures are rare (<10% in some cancers) yet second cancers are more common than uncontrolled primaries. The main reason for considering particle radiotherapy for childhood cancers is the possibility to exploit their physical characteristics to reduce the radiation exposure to organs and tissues close to and distant from the primary cancer which is to be targeted. However, the relative biological effectiveness of the radiation doses within the proton beam is not a constant and the relative biological effectiveness of the neutrons is not known as far as the mechanisms of late normal tissue damage and second cancer risk are concerned. In view of the highly charged discussions of the potential risks of treatment-induced seecond cancers from the neutron contamination of exposure doses in out-of-PTV critical organs a comprehensive European project called ANDANTE was performed which integrated the disciplines of radiation physics, molecular biology, systems biology modelling and epidemiology in order to investigate the RBE of induction of cancer from exposure to neutrons compared to photons. Since out-of-field "effective" neutron doses from proton therapy are smaller than the photon stray doses whichever reasonable RBE is chosen for comparison, and since the absolute risk of radiation-induced second cancer rates are in the order of 1% in the cohorts of adult patients who have been treated in the past with methods which caused relatively high out-of-field doses to large body volumes, it is highly unlikely that such patients treated in future with highly conformal particle therapy are at a higher radiation-induced second cancer risk than those patients treated with photons and described before. Still, the potential risks of second cancers from scattered proton radiotherapy for childhood cancers may cause concern. Yet, the overall risk of undesired consequences of radiation exposure of children which are more complex and manifold than in adult patients (including developmental, neurocognitive, hormonal and growth impairment effects) are likely to be very much reduced by the better focussing of the radiation dose in the target offered by particle radioherapy. This benefit may far outweigh the still hypothetical second cancer risk from particle radiotherapy in pediatric radiotherapy.

Review of NASA approach to space radiation risk assessments for Mars exploration

Long duration space missions present unique radiation protection challenges due to the complexity of the space radiation environment, which includes high charge and energy particles and other highly ionizing radiation such as neutrons. Based on a recommendation by the National Council on Radiation Protection and Measurements, a 3% lifetime risk of exposure-induced death for cancer has been used as a basis for risk limitation by the National Aeronautics and Space Administration (NASA) for low-Earth orbit missions. NASA has developed a risk-based approach to radiation exposure limits that accounts for individual factors (age, gender, and smoking history) and assesses the uncertainties in risk estimates. New radiation quality factors with associated probability distribution functions to represent the quality factor's uncertainty have been developed based on track structure models and recent radiobiology data for high charge and energy particles. The current radiation dose limits are reviewed for spaceflight and the various qualitative and quantitative uncertainties that impact the risk of exposure-induced death estimates using the NASA Space Cancer Risk (NSCR) model. NSCR estimates of the number of "safe days" in deep space to be within exposure limits and risk estimates for a Mars exploration mission are described.

Lifetime increased cancer risk in mice following exposure to clinical proton beam-generated neutrons

PURPOSE

To evaluate the life span and risk of cancer following whole-body exposure of mice to neutrons generated by a passively scattered clinical spread-out Bragg peak (SOBP) proton beam.

METHODS AND MATERIALS

Three hundred young adult female FVB/N mice, 152 test and 148 control, were entered into the experiment. Mice were placed in an annular cassette around a cylindrical phantom, which was positioned lateral to the mid-SOBP of a 165-MeV, clinical proton beam. The average distance from the edge of the mid-SOBP to the conscious active mice was 21.5 cm. The phantom was irradiated with once-daily fractions of 25 Gy, 4 days per week, for 6 weeks. The age at death and cause of death (ie, cancer and type vs noncancer causes) were assessed over the life span of the mice.

RESULTS

Exposure of mice to a dose of 600 Gy of proton beam-generated neutrons, reduced the median life span of the mice by 4.2% (Kaplan-Meier cumulative survival, P=.053). The relative risk of death from cancer in neutron exposed versus control mice was 1.40 for cancer of all types (P=.0006) and 1.22 for solid cancers (P=.09). For a typical 60 Gy dose of clinical protons, the observed 22% increased risk of solid cancer would be expected to decrease by a factor of 10.

CONCLUSIONS

Exposure of mice to neutrons generated by a proton dose that exceeds a typical course of radiation therapy by a factor of 10, resulted in a statistically significant increase in the background incidence of leukemia and a marginally significant increase in solid cancer. The results indicate that the risk of out-of-field second solid cancers from SOBP proton-generated neutrons and typical treatment schedules, is 6 to 10 times less than is suggested by current neutron risk estimates.

Space radiation risks for astronauts on multiple International Space Station missions

Mortality and morbidity risks from space radiation exposure are an important concern for astronauts participating in International Space Station (ISS) missions. NASA's radiation limits set a 3% cancer fatality probability as the upper bound of acceptable risk and considers uncertainties in risk predictions using the upper 95% confidence level (CL) of the assessment. In addition to risk limitation, an important question arises as to the likelihood of a causal association between a crew-members' radiation exposure in the past and a diagnosis of cancer. For the first time, we report on predictions of age and sex specific cancer risks, expected years of life-loss for specific diseases, and probability of causation (PC) at different post-mission times for participants in 1-year or multiple ISS missions. Risk projections with uncertainty estimates are within NASA acceptable radiation standards for mission lengths of 1-year or less for likely crew demographics. However, for solar minimum conditions upper 95% CL exceed 3% risk of exposure induced death (REID) by 18 months or 24 months for females and males, respectively. Median PC and upper 95%-confidence intervals are found to exceed 50% for several cancers for participation in two or more ISS missions of 18 months or longer total duration near solar minimum, or for longer ISS missions at other phases of the solar cycle. However, current risk models only consider estimates of quantitative differences between high and low linear energy transfer (LET) radiation. We also make predictions of risk and uncertainties that would result from an increase in tumor lethality for highly ionizing radiation reported in animal studies, and the additional risks from circulatory diseases. These additional concerns could further reduce the maximum duration of ISS missions within acceptable risk levels, and will require new knowledge to properly evaluate.

Assessing the risk of second malignancies after modern radiotherapy

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

Reducing stray radiation dose to patients receiving passively scattered proton radiotherapy for prostate cancer

Proton beam radiotherapy exposes healthy tissue to stray radiation emanating from the treatment unit and secondary radiation produced within the patient. These exposures provide no known benefit and may increase a patient's risk of developing a radiogenic second cancer. The aim of this study was to explore strategies to reduce stray radiation dose to a patient receiving a 76 Gy proton beam treatment for cancer of the prostate. The whole-body effective dose from stray radiation, E, was estimated using detailed Monte Carlo simulations of a passively scattered proton treatment unit and an anthropomorphic phantom. The predicted value of E was 567 mSv, of which 320 mSv was attributed to leakage from the treatment unit; the remainder arose from scattered radiation that originated within the patient. Modest modifications of the treatment unit reduced E by 212 mSv. Surprisingly, E from a modified passive-scattering device was only slightly higher (109 mSv) than from a nozzle with no leakage, e.g., that which may be approached with a spot-scanning technique. These results add to the body of evidence supporting the suitability of passively scattered proton beams for the treatment of prostate cancer, confirm that the effective dose from stray radiation was not excessive, and, importantly, show that it can be substantially reduced by modest enhancements to the treatment unit.

Indications of the neutron effect contribution in the solid cancer data of the A-bomb survivors

Risk estimates for radiation-induced cancer are primarily based on the follow-up of the Japanese A-bomb survivors. Their exposures were due to gamma rays and neutrons, and, currently--with the assumed low RBE = 10 of neutrons and reference to the colon dose--the late radiation effects are almost fully attributed to the gamma rays. Solid cancer risk estimates for different organ sites are assessed here, and an inconspicuous but statistically highly significant trend of larger values is found for the organs closer to the body surface; i.e., the organs with less body shielding and, therefore, with larger neutron dose-fractions. Underestimation of the RBE of neutrons can explain this apparent correlation. The trend of ERR/Gy vs. depth ceases to be statistically significant for RBE values close to 100. The suggestion of high RBE values and the corresponding reduction of gamma-ray risk estimates is found to be in line with log-likelihood computations in terms of AMFIT, which provide for the solid cancer mortality of the A-bomb survivors the minimum deviance for RBE = 100 with a 95% confidence lower limit of 25. The present assessment had to use the data made publicly available by RERF. In this form they contain city-, sex-, age-, and dose-categories, but--instead of a separate neutron-dose category--only the mean neutron dose for each data cell. The tentative conclusions that are here obtained should, therefore, be examined by a more definitive analysis, either in terms of grouped data with a separate classification of neutron doses or, ideally, in terms of person by person calculations to be performed at RERF with individually estimated neutron doses.

Measurements of fast neutrons in Hiroshima by use of (39)Ar

The survivors of the A-bomb explosions over Hiroshima and Nagasaki were exposed to a mixed neutron and gamma radiation field. To validate the high-energy portion of the neutron field and thus the neutron dose to the survivors, a method is described that allows retrospective assessment of the fast neutrons from the A-bombs. This is accomplished by the extraction of the noble gas argon from biotites separated from Hiroshima granite samples, and then the detection of the (39)Ar activity that was produced by the capture of the fast neutrons on potassium. Adjusted to the year 1945, activities measured in the first samples taken at distances of 94, 818, 992, and 1,173 m from the hypocenter were 6.9+/-0.2, 0.32+/-0.01, 0.14+/-0.02, and 0.09+/-0.01 mBq/g K, respectively. All signals were significantly above detector background and show low uncertainties. Considering their uncertainties they agree with the calculated (39)Ar activation in the samples, based on the most recent dosimetry system DS02. It is concluded that this method can be used to investigate samples obtained from large distances in Hiroshima, where previous data on fast neutrons are characterized by considerable uncertainties. Additionally, the method can be used to reconstruct the fast neutron fluence in Nagasaki, where no experimental data exist.

Radiation effectiveness factors for use in calculating probability of causation of radiogenic cancers

This paper presents so-called radiation effectiveness factors that are intended to represent the biological effectiveness of different radiation types, relative to high-energy Co gamma rays, for the purpose of estimating cancer risks and probability of causation of radiogenic cancers in identified individuals. Radiation effectiveness factors are expressed as subjective probability distributions to represent uncertainty that arises from uncertainties in estimates of relative biological effectiveness obtained from radiobiological studies of stochastic endpoints, limited data on biological effectiveness obtained from human epidemiological studies, and other judgments involved in evaluating the applicability of available information to induction of cancers in humans. Primarily on the basis of reviews and evaluations of available data by experts, probability distributions of radiation effectiveness factors are developed for the following radiation types: neutrons of energy less than 10 keV, 10-100 keV, 0.1-2 MeV (including fission neutrons), 2-20 MeV, and greater than 20 MeV; alpha particles of any energy emitted by radionuclides; photons of energy 30-250 keV and less than 30 keV; and electrons of energy less than 15 keV. Photons of energy greater than 250 keV and electrons of energy greater than 15 keV are assumed to have the same biological effectiveness as reference Co gamma rays and are assigned a radiation effectiveness factor of unity, without uncertainty. For neutrons and alpha particles, separate probability distributions of radiation effectiveness factors are developed for solid tumors and leukemias, and small corrections to represent an inverse dose-rate effect are applied to those distributions in cases of chronic exposure. A radiation effectiveness factor different from unity for 15-60 keV electrons is discussed but is not adopted due to a lack of relevant radiobiological data. Radiation effectiveness factors presented in this paper are incorporated in the Interactive RadioEpidemiological Program and were developed for use by The National Institute for Occupational Safety and Health and U.S. Department of Labor in evaluating claims for compensation for radiogenic cancers by workers at U.S. Department of Energy facilities.

Patient exposure in medical X-ray imaging in Europe

Patients are exposed to X rays when undergoing medical examinations in diagnostic radiology. Exposure data acquired and assessed in Germany for the year 1997 resulted in a mean annual effective dose of 2 +/- 0.5 mSv per head of the population, thereby reaching or exceeding the average level of environmental radiation in many cases. The underlying frequency of medical X-ray examinations was approximately 136 million, i.e. approximately 1.7 examinations annually per head of the population. For comparison, corresponding data of other countries were extracted from the UNSCEAR 2000 report or originate from the literature. Data analysis shows significant differences in national radiological practices and a very uneven distribution of patient doses amongst the world population. The mean annual effective dose per head of the population varies by up to a factor of 60 between health care level I and IV countries, and still by a factor of approximately 6 within health care level I countries. While projection radiography has succeeded in reducing dose consumption, computed tomography and radiological interventions have given rise to a significant growth of patient exposure, and interventional radiology can even exceed thresholds for deterministic radiation effects. Patient exposure is further shown to result from misadministration and retakes of X-ray examinations, usually not registered, as well as from technical failures of X-ray facilities, which can cause significantly enhanced exposure times. Corresponding data are presented and comments are made on the international situation of non-harmonised data collection on patient exposure as well as of parameters affecting the assessment of exposure and risk.

Relative biological effectiveness (RBE), quality factor (Q), and radiation weighting factor (w(R)). A report of the International Commission on Radiological Protection

The effect of ionising radiation is influenced by the dose, the dose rate, and the quality of the radiation. Before 1990, dose-equivalent quantities were defined in terms of a quality factor, Q(L), that was applied to the absorbed dose at a point in order to take into account the differences in the effects of different types of radiation. In its 1990 recommendations, the ICRP introduced a modified concept. For radiological protection purposes, the absorbed dose is averaged over an organ or tissue, T, and this absorbed dose average is weighted for the radiation quality in terms of the radiation weighting factor, w(R), for the type and energy of radiation incident on the body. The resulting weighted dose is designated as the organ- or tissue-equivalent dose, H(T). The sum of the organ-equivalent doses weighted by the ICRP organ-weighting factors, w(T), is termed the effective dose, E. Measurements can be performed in terms of the operational quantities, ambient dose equivalent, and personal dose equivalent. These quantities continue to be defined in terms of the absorbed dose at the reference point weighted by Q(L). The values for w(R) and Q(L) in the 1990 recommendations were based on a review of the biological and other information available, but the underlying relative biological effectiveness (RBE) values and the choice of w(R) values were not elaborated in detail. Since 1990, there have been substantial developments in biological and dosimetric knowledge that justify a re-appraisal of w(R) values and how they may be derived. This re-appraisal is the principal objective of the present report. The report discusses in some detail the values of RBE with regard to stochastic effects, which are central to the selection of w(R) and Q(L). Those factors and the dose-equivalent quantities are restricted to the dose range of interest to radiation protection, i.e. to the general magnitude of the dose limits. In special circumstances where one deals with higher doses that can cause deterministic effects, the relevant RBE values are applied to obtain a weighted dose. The question of RBE values for deterministic effects and how they should be used is also treated in the report, but it is an issue that will demand further investigations. This report is one of a set of documents being developed by ICRP Committees in order to advise the ICRP on the formulation of its next Recommendations for Radiological Protection. Thus, while the report suggests some future modifications, the w(R) values given in the 1990 recommendations are still valid at this time. The report provides a scientific background and suggests how the ICRP might proceed with the derivation of w(R) values ahead of its forthcoming recommendations.

[Acquisition and assessment of patient exposure in diagnostic radiology and nuclear medicine]

The regular annual monitoring of patient exposure in radiation diagnostics, as performed by the Federal Office for Radiation Protection (BfS), plays an important role in evaluating the awareness of radiological quality and safety in Germany and the risk-benefit optimization for patients. For the reporting year 1997, X-ray diagnostics resulted in a mean effective dose of 2 +/- 0.5 mSv per head of population. The underlying frequency of medical X-ray examinations was approximately 136 million, i.e., 1.7 examinations annually per head of population. In terms of nuclear medicine diagnostics, the patients exposure amounted to approximately 0.15 mSv effective dose per head of population. In this case, the number of examinations amounted to approximately 4 million, corresponding to a frequency of approximately 0.05 examinations annually per head of population. The paper discusses factors influencing the calculation of exposure, as well as the lack of an internationally accepted protocol to evaluate patient exposure.

Changes in DNA flexibility after irradiation with gamma rays and neutrons studied with the perturbed angular correlation method

Neutron and gamma irradiation of buffered solutions of calf thymus DNA resulted in changes in the dynamics of the macromolecule. In the low-dose region (0.8-10 cGy of 239Pu-Be neutrons and 0.34-3 Gy of 60Co gamma rays), the flexibility of DNA decreased as indicated by slower rotation of the molecules. Neutrons appeared to be approximately 35 times more effective than 60Co gamma rays. The rotational correlation time, tau C, was measured using the perturbed angular correlation (PAC) method. Its variation appears to follow a linear-exponential behavior. An attempt is made to formulate this behavior as a function of the energy deposited on the macromolecule (radiation dose), the average threshold energy (dose) required to form new lesions, and the available population of intact DNA sites.

Is there reliable experimental evidence for a low-dose RBE of about 4 for mammography X rays relative to 200 kV X rays?

recently reported, on the basis of observations of neoplastic transformation in human hybrid CGL1 cells, a low-dose relative biological effectiveness (RBE(M)) of 4.3 for mammography X rays (29 kV) relative to 200 kV X rays. With reference to data in the literature, they inferred a factor of about 8 relative to 60Co gamma rays and concluded that this result is relevant to risk estimation. However, the conclusions do not appear to be valid. The data from the transformation study exhibit uncertainties in the statistical analysis that preclude any generalization of the inferred RBE(M). The data selected or inferred from the literature are likewise insufficient to support the stated RBEs. Our own uniform data set for the yields of dicentrics was obtained for widely varying photon energies with blood samples from the same donor, and it avoids interindividual variations in sensitivity as well as the differences in methodology that are associated with interlaboratory comparisons. Our data provide RBE(M) values for 29 kV X rays of 1.64 +/- 0.27 relative to 220 kV X rays and 4.75 +/- 1.67 and 6.12 +/- 2.51 relative to 60Co gamma rays.

Radiation risk--historical perspective and current issues

The assessment of radiation risk needs to be seen against the background of a historical development that has reversed the initial belief in a general beneficial effect of radiation to apprehension and fear. Numerical risk estimates are, today, based on large epidemiological studies, and the observations on the A-bomb survivors are outlined as the primary source of information. Since the epidemiological findings are obtained from relatively high radiation exposures, extrapolations are required to the much lower doses that are relevant to radiation protection. The evolution of extrapolation procedures up to current attempts at mechanistic modelling is outlined, and some of the open issues are reviewed.

Enhanced neoplastic transformation by mammography X rays relative to 200 kVp X rays: indication for a strong dependence on photon energy of the RBE(M) for various end points

The fundamental assumption implicit in the use of the atomic bomb survivor data to derive risk estimates is that the gamma rays of Hiroshima and Nagasaki are considered to have biological efficiencies equal to those of other low-LET radiations up to 10 keV/microm, including mammography X rays. Microdosimetric and radiobiological data contradict this assumption. It is therefore of scientific and public interest to evaluate the efficiency of mammography X rays (25-30 kVp) to induce cancer. In this study, the efficiency of mammography X rays relative to 200 kVp X rays to induce neoplastic cell transformation was evaluated using cells of a human hybrid cell line (CGL1). For both radiations, a linear-quadratic dose-effect relationship was observed for neoplastic transformation of CGL1 cells; there was a strong linear component for the 29 kVp X rays. The RBE(M) of mammography X rays relative to 200 kVp X rays was determined to be about 4 for doses < or = 0.5 Gy. A comparison of the electron fluences for both X rays provides strong evidence that electrons with energies of < or = 15 keV can induce neoplastic transformation of CGL1 cells. Both the data available in the literature and the results of the present study strongly suggest an increase of RBE(M) for carcinogenesis in animals, neoplastic cell transformation, and clastogenic effects with decreasing photon energy or increasing LET to an RBE(M) approximately 8 for mammography X rays relative to 60Co gamma rays.

Risk estimation for fast neutrons with regard to solid cancer

In the absence of epidemiological information on the effects of neutrons, their cancer mortality risk coefficient is currently taken as the product of two low-dose extrapolations: the nominal risk coefficient for photons and the presumed maximum relative biological effectiveness of neutrons. This approach is unnecessary. Since linearity in dose is assumed for neutrons at low to moderate effect levels, the risk coefficient can be derived in terms of the excess risk from epidemiological observations at an intermediate dose of gamma rays and an assumed value, R(1), of the neutron RBE relative to this reference dose of gamma rays. Application of this procedure to the A-bomb data requires accounting for the effect of the neutron dose component, which, according to the current dosimetry system, DS86, amounts on average to 11 mGy in the two cities at a total dose of 1 Gy. With R(1) tentatively set to 20 or 50, it is concluded that the neutrons have caused 18% or 35%, respectively, of the total effect at 1 Gy. The excess relative risk (ERR) for neutrons then lies between 8 per Gy and 16 per Gy. Translating these values into risk coefficients in terms of the effective dose, E, requires accounting for the gamma-ray component produced by the neutron field in the human body, which will require a separate analysis. The risk estimate for neutrons will remain essentially unaffected by the current reassessment of the neutron doses in Hiroshima, because the doses are unlikely to change much at the reference dose of 1 Gy.

Space radiation cancer risks and uncertainties for Mars missions

Projecting cancer risks from exposure to space radiation is highly uncertain because of the absence of data for humans and because of the limited radiobiology data available for estimating late effects from the high-energy and charge (HZE) ions present in the galactic cosmic rays (GCR). Cancer risk projections involve many biological and physical factors, each of which has a differential range of uncertainty due to the lack of data and knowledge. We discuss an uncertainty assessment within the linear-additivity model using the approach of Monte Carlo sampling from subjective error distributions that represent the lack of knowledge in each factor to quantify the overall uncertainty in risk projections. Calculations are performed using the space radiation environment and transport codes for several Mars mission scenarios. This approach leads to estimates of the uncertainties in cancer risk projections of 400-600% for a Mars mission. The uncertainties in the quality factors are dominant. Using safety standards developed for low-Earth orbit, long-term space missions (>90 days) outside the Earth's magnetic field are currently unacceptable if the confidence levels in risk projections are considered. Because GCR exposures involve multiple particle or delta-ray tracks per cellular array, our results suggest that the shape of the dose response at low dose rates may be an additional uncertainty for estimating space radiation risks.