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

ICRP, 123. Assessment of radiation exposure of astronauts in space. ICRP Publication 123

During their occupational activities in space, astronauts are exposed to ionising radiation from natural radiation sources present in this environment. They are, however, not usually classified as being occupationally exposed in the sense of the general ICRP system for radiation protection of workers applied on Earth. The exposure assessment and risk-related approach described in this report is clearly restricted to the special situation in space, and should not be applied to any other exposure situation on Earth. The report describes the terms and methods used to assess the radiation exposure of astronauts, and provides data for the assessment of organ doses. Chapter 1 describes the specific situation of astronauts in space, and the differences in the radiation fields compared with those on Earth. In Chapter 2, the radiation fields in space are described in detail, including galactic cosmic radiation, radiation from the Sun and its special solar particle events, and the radiation belts surrounding the Earth. Chapter 3 deals with the quantities used in radiological protection, describing the Publication 103 (ICRP, 2007) system of dose quantities, and subsequently presenting the special approach for applications in space; due to the strong contribution of heavy ions in the radiation field, radiation weighting is based on the radiation quality factor, Q, instead of the radiation weighting factor, wR. In Chapter 4, the methods of fluence and dose measurement in space are described, including instrumentation for fluence measurements, radiation spectrometry, and area and individual monitoring. The use of biomarkers for the assessment of mission doses is also described. The methods of determining quantities describing the radiation fields within a spacecraft are given in Chapter 5. Radiation transport calculations are the most important tool. Some physical data used in radiation transport codes are presented, and the various codes used for calculations in high-energy radiation fields in space are described. Results of calculations and measurements of radiation fields in spacecraft are given. Some data for shielding possibilities are also presented. Chapter 6 addresses methods of determining mean absorbed doses and dose equivalents in organs and tissues of the human body. Calculated conversion coefficients of fluence to mean absorbed dose in an organ or tissue are given for heavy ions up to Z=28 for energies from 10 MeV/u to 100 GeV/u. For the same set of ions and ion energies, mean quality factors in organs and tissues are presented using, on the one hand, the Q(L) function defined in Publication 60 (ICRP, 1991), and, on the other hand, a Q function proposed by the National Aeronautics and Space Administration. Doses in the body obtained by measurements are compared with results from calculations, and biodosimetric measurements for the assessment of mission doses are also presented. In Chapter 7, operational measures are considered for assessment of the exposure of astronauts during space missions. This includes preflight mission design, area and individual monitoring during flights in space, and dose recording. The importance of the magnitude of uncertainties in dose assessment is considered. Annex A shows conversion coefficients and mean quality factors for protons, charged pions, neutrons, alpha particles, and heavy ions(2 < Z ≤2 8), and particle energies up to 100 GeV/u.

High-energy particle-induced tumorigenesis throughout the gastrointestinal tract

Epidemiological data reveals the gastrointestinal (GI) tract as one of the main sites for low-LET radiation-induced cancers. Importantly, the use of particle therapy is increasing, but cancer risk by high-LET particles is still poorly understood. This gap in our knowledge also remains a major limiting factor in planning long-term space missions. Therefore, assessing risks and identifying predisposing factors for carcinogenesis induced by particle radiation is crucial for both astronauts and cancer survivors. We have previously shown that exposure to relatively high doses of high-energy (56)Fe ions induced higher intestinal tumor frequency and grade in the small intestine of Apc(Min/+) mice than γ rays. However, due to the high number of spontaneous lesions (∼30) that develop in Apc(Min/+) animals, this Apc mutant model is not suitable to investigate effects of cumulative doses <1 Gy, which are relevant for risk assessment in astronauts and particle radiotherapy patients. However, Apc(1638N/+) mice develop a relatively small number of spontaneous lesions (∼3 per animal) in both small intestine and colon, and thus we propose a better model for studies on radiation-induced carcinogenesis. Here, we investigated model particle radiation increases tumor frequency and grade in the entire gastrointestinal tract (stomach and more distal intestine) after high- and low-radiation doses whether in the Apc(1638N/+). We have previously reported that an increase in small intestinal tumor multiplicity after exposure to γ rays was dependent on gender in Apc(1638N/+) mice, and here we investigated responses to particle radiation in the same model. Phenotypical and histopathological observations were accompanied by late changes in number and position of mitotic cells in intestinal crypts from animals exposed to different radiation types.

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.

A review of dosimetry studies on external-beam radiation treatment with respect to second cancer induction

It has been long known that patients treated with ionizing radiation carry a risk of developing a second cancer in their lifetimes. Factors contributing to the recently renewed concern about the second cancer include improved cancer survival rate, younger patient population as well as emerging treatment modalities such as intensity-modulated radiation treatment (IMRT) and proton therapy that can potentially elevate secondary exposures to healthy tissues distant from the target volume. In the past 30 years, external-beam treatment technologies have evolved significantly, and a large amount of data exist but appear to be difficult to comprehend and compare. This review article aims to provide readers with an understanding of the principles and methods related to scattered doses in radiation therapy by summarizing a large collection of dosimetry and clinical studies. Basic concepts and terminology are introduced at the beginning. That is followed by a comprehensive review of dosimetry studies for external-beam treatment modalities including classical radiation therapy, 3D-conformal x-ray therapy, intensity-modulated x-ray therapy (IMRT and tomotherapy) and proton therapy. Selected clinical data on second cancer induction among radiotherapy patients are also covered. Problems in past studies and controversial issues are discussed. The needs for future studies are presented at the end.

Secondary carcinogenesis in patients treated with radiation: a review of data on radiation-induced cancers in human, non-human primate, canine and rodent subjects

Concern for risk of radiation-induced cancer is growing with the increasing number of cancer patients surviving long term. This study examined data on radiation transformation of mammalian cells in vitro and on the risk of an increased cancer incidence after irradiation of mice, dogs, monkeys, atomic bomb survivors, occupationally exposed persons, and patients treated with radiation. Transformation of cells lines in vitro increased linearly with dose from approximately 1 to approximately 4-5 Gy. At <0.1 Gy, transformation was not increased in all studies. Dose-response relationships for cancer incidence varied with mouse strain, gender and tissue/organ. Risk of cancer in Macaca mulatta was not raised at 0.25-2.8 Gy. From the atomic bomb survivor study, risk is accepted as increasing linearly to 2 Sv for establishing exposure standards. In irradiated patients, risk of cancer increased significantly from 1 to 45 Gy (a low to a high dose level) for stomach and pancreas, but not for bladder and rectum (1-60 Gy) or kidney (1-15 Gy). Risk for several organs/tissues increased substantially at doses far above 2 Gy. There is great heterogeneity in risk of radiation-associated cancer between species, strains of a species, and organs within a species. At present, the heterogeneity between and within patient populations of virtually every parameter considered in risk estimation results in substantial uncertainty in quantification of a general risk factor. An implication of this review is that reduced risks of secondary cancer should be achieved by any technique that achieved a dose reduction down to approximately [corrected] 0.1 Gy, i.e. dose to tissues distant from the target. The proportionate gain should be greatest for dose decrement to less than 2 Gy.

Adaptive response in embryogenesis: V. Existence of two efficient dose-rate ranges for 0.3 Gy of priming irradiation to adapt mouse fetuses

The adaptive response is an important phenomenon in radiobiology. A study of the conditions essential for the induction of an adaptive response is of critical importance to understanding the novel biological defense mechanisms against the hazardous effects of radiation. In our previous studies, the specific dose and timing of radiation for induction of an adaptive response were studied in ICR mouse fetuses. We found that exposure of the fetuses on embryonic day 11 to a priming dose of 0.3 Gy significantly suppressed prenatal death and malformation induced by a challenging dose of radiation on embryonic day 12. Since a significant dose-rate effect has been observed in a variety of radiobiological phenomena, the effect of dose rate on the effectiveness of induction of an adaptive response by a priming dose of 0.3 Gy administered to fetuses on embryonic day 11 was investigated over the range from 0.06 to 5.0 Gy/min. The occurrence of apoptosis in limb buds, incidences of prenatal death and digital defects, and postnatal mortality induced by a challenging dose of 3.5 Gy given at 1.8 Gy/min to the fetuses on embryonic day 12 were the biological end points examined. Unexpectedly, effective induction of an adaptive response was observed within two dose-rate ranges for the same dose of priming radiation, from 0.18 to 0.98 Gy/ min and from 3.5 to 4.6 Gy/min, for reduction of the detrimental effect induced by a challenging dose of 3.5 Gy. In contrast, when the priming irradiation was delivered at a dose rate outside these two ranges, no protective effect was observed, and at some dose rates elevation of detrimental effects was observed. In general, neither a normal nor a reverse dose- rate effect was found in the dose-rate range tested. These results clearly indicated that the dose rate at which the priming irradiation was delivered played a crucial role in the induction of an adaptive response. This paper provides the first evidence for the existence of two dose-rate ranges for the same dose of priming radiation to successfully induce an adaptive response in mouse fetuses.

Expression of cytoskeletal and matrix genes following exposure to ionizing radiation: dose-rate effects and protein synthesis requirements

Experiments were designed to examine the effects of radiation dose-rate and cycloheximide on expression of cytoskeletal and matrix elements in Syrian hamster embryo cells. Results here demonstrated little effect of dose-rate for fission-spectrum neutrons when comparing expression of alpha-tubulin or fibronectin genes. Effects of cycloheximide, however, revealed several findings: (a) Cycloheximide repressed accumulation of alpha-tubulin following exposure to high dose-rate neutrons or gamma-rays. (b) Cycloheximide did not affect accumulation of mRNA for actin genes. (c) Cycloheximide abrogated the moderate induction of fibronectin-mRNA which occurred following exposure to gamma-rays and high dose-rate neutrons. These results suggest a role for labile proteins in the maintenance of alpha-tubulin and fibronectin mRNA accumulation following exposure to ionizing radiation.

Application of the constant exposure time technique to transformation experiments with fission neutrons: failure to demonstrate dose-rate dependence

A direct comparison of the effectiveness of fission neutrons at high (11.0-31.3 cGy/min) or several low dose-rates (0.14-3.2 cGy/min) was carried out under identical conditions. Monolayers of exponentially growing C3H/10T1/2 cells were exposed at 37 degrees C to reactor-produced neutrons (fluence-mean energy En = 0.68 MeV, < or = 5% gamma component, frequency mean linear energy yF = 21 keV/micron, dose mean lineal energy yD = 42 keV/micron in an 8-micron spherical cavity). Survival or transformation induction were studied at five doses from 10.5 to 94 cGy. In low dose-rate irradiations, these doses were protracted over 0.5, 1, 3 or 4.5 h, resulting in 17 different dose-rates. Up to six experiments were performed at each of five exposure times. Concurrently with transformation we studied cell proliferation in control versus cells irradiated at 40 cGy (acute and a 4.5-h protraction) and found no evidence of a shift in the cell cycle distribution among these cells. At a given dose and dose-rate, the effect of dose protraction on survival or transformation was assessed by the dose-rate modifying factor (DRMF), defined as the low:high dose-rate effect ratio at the same dose. Survival or transformation induction curves were nearly linear with initial slopes, respectively, of about 6.5 x 10(-3) or 6.2 x 10(-6) cGy-1. Consistent with dose-response curves, DRMFs were independent of the dose and dose-rate. The mean values of the DRMF with their uncertainties and 99% confidence intervals, based on measurements in individual doses and dose-rates for survival or transformation were, respectively: 1.01 +/- 0.03 (0.92, 1.09) or 0.98 +/- 0.04 (0.83, 1.08) indicating a similar precision in determining DRMF for survival or transformation, and no dose or dose-rate influence on these end points.

Dose-rate effects of neutrons and gamma-rays on the induction of mutation and oncogenic transformation in plateau-phase mouse m5S cells

The dose-rate effect of 252-californium neutrons was investigated using confluent cultures of mouse m5S cells. The relative biological effectiveness (RBE) of neutrons for oncogenic transformation was increased from 3.3 to 5.1 when the dose-rate was reduced from 1.8 to 0.12 cGy/min. Similarly, neutron RBE values for HPRT- mutation were 4.9 and 7.4 at dose-rates of 1.8 and 0.12 cGy/min, respectively. The increases in RBE as dose-rate was reduced were due mainly to diminished transformation- and mutation-induction by gamma-rays (the standard radiation). The yields of neutron-induced oncogenic transformation as well as neutron-induced mutation were constant for both dose rates. Our observation contrasts with reports by others using proliferating cells where both oncogenic transformation and mutation were enhanced with neutron exposure at a reduced dose-rate, the so-called inverse dose-rate effect. Since m5S cells are sensitive to postconfluent inhibition of cell division, this observation could be ascribed to cell growth conditions used in these experiments. The mechanism of the inverse dose-rate effect of neutrons suggests that the enhancement of neutron-induced mutation and oncogenic transformation at a reduced dose-rate is strongly associated with cell proliferation during exposure.