Relative biological effectiveness of neutrons for cancer induction and other late effects: a review of radiobiological data

The risk of secondary cancer induction after a therapeutic irradiation with conventional photon beams is well recognised and documented. However, in general, it is totally overwhelmed by the benefit of the treatment. The same is true to a large extent for the combinations of radiation and drug therapy. After fast neutron therapy, the risk of secondary cancer induction is greater than after photon therapy. This can be expected from the whole set of radiobiological data, accumulated so far, which shows systematically a greater relative biological effectiveness (RBE) for neutrons for all the biological systems which have been investigated. Furthermore, the neutron RBE increases with decreasing dose and there is extensive evidence that neutron RBE is greater for cancer induction and for other late effects relevant in radiation protection than for cell killing at high doses as used in therapy. Almost no reliable human epidemiological data are available so far, and the aim of this work is to derive the best risks estimate for cancer induction after neutron irradiation and in particular fast neutron therapy. Animal data on RBE for tumour induction are analysed. In addition, other biological effects are reviewed, such as life shortening, malignant cell transformation in vitro, chromosome aberrations, genetic effects. These effects can be related, directly or indirectly, to cancer induction to the extent that they express a "genomic" lesion. Since neutron RBE depends on the energy spectrum, the radiation quality has to be carefully specified. Therefore, the microdosimetric spectra are reported each time they are available. Lastly, since heavy-ion beam therapy is being developed at several centres worldwide, the available data on RBE at low doses are reviewed. It can be concluded from this review that the risk of induction of a secondary cancer after fast neutron therapy should not be greater than 10-20 times the risk after photon beam therapy. For heavy ions, and in particular for carbon ions, the risk estimate should be divided by a factor of about 3 due to the reduced integral dose. The risk has to be balanced against the expected improvement in cure rate when the indication for high-LET therapy has been correctly evaluated in well-selected patient groups.

Transformation of C3H 10T1/2 cells by low doses of ionising radiation: a collaborative study by six European laboratories strongly supporting a linear dose-response relationship

For the assessment of radiation risk at low doses, it is presumed that the shape of the low-dose-response curve in humans for cancer induction is linear. Epidemiological data alone are unlikely to ever have the statistical power needed to confirm this assumption. Another approach is to use oncogenic transformation in vitro as a surrogate for carcinogenesis in vivo. In mid-1990, six European laboratories initiated such an approach using C3H 10T1/2 mouse cells. Rigid standardisation procedures were established followed by collaborative measurements of transformation down to absorbed doses of 0.25 Gy of x-radiation resulting in a total of 759 transformed foci. The results clearly support a linear dose-response relationship for cell transformation in vitro with no evidence for a threshold dose or for an enhanced, supralinear response at doses approximately 200-300 mGy. For radiological protection this represents a large dose, and the limitations of this approach are apparent. Only by understanding the fundamental mechanisms involved in radiation carcinogenesis will further knowledge concerning the effects of low doses become available. These results will, however, help validate new biologically based models of radiation cancer risk thus providing increased confidence in the estimation of cancer risk at low doses.

Radiation-induced neoplastic transformation of stationary phase C3H/10T1/2 cells in response to different dose rates and to post-irradiation treatment with tumor promoter

The induction of neoplastic transformation by exposure to high (HDR, 0.66 Gy/min) or very low (LDR, 4.8 x 10(-4) Gy/min) dose rates of 137Cs gamma-rays was studied in C3H/10T1/2 mouse embryo fibroblasts. Cells in stationary phase were exposed in the dose range 1-6 Gy in combination with a post-irradiation treatment with the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA). The post-irradiation treatment with TPA during 6 weeks of transformation assay did not induce any notable increase in the slope of the dose response curves for transformation frequency, compared to the conditions without TPA treatment. The lack of an enhancing TPA effect at both dose rates applied in this study may be related to the fact that the cells were irradiated in the stationary growth phase. Thus, the results differ from those generally obtained when exponentially growing cells are exposed to gamma-rays and afterwards treated with TPA in the transformation assay. Earlier studies of exponentially growing C3H/10T1/2 cells exposed to different dose rates show a significantly higher transformation frequency for high dose rate. This study, using stationary phase cells, also shows that the slopes of dose response curves for transformed foci were somewhat higher (about 1.5-fold) for HDR exposure compared with LDR exposure. However, the difference was not statistically significant.

Extension of a generalized state-vector model of radiation carcinogenesis to consideration of dose rate

Mathematical models for radiation carcinogenesis typically employ transition rates that either are a function of the dose to specific cells or are purely empirical constructs unrelated to biophysical theory. These functions either ignore or do not explicitly model interactions between the fates of cells in a community. This paper extends a model of mitosis, cell transformation, promotion, and progression to cases in which interacting cellular communities are irradiated at specific dose rates. The model predicts that lower dose rates are less effective at producing cancer when irradiation is by X- or gamma rays but are generally more effective in instances of irradiation by alpha particles up to a dose rate in excess of 0.01 Gy/day. The resulting predictions are compared with existing experimental data.

From energy deposition to cancer

Recent progress in molecular biology, genetics and microdosimetry has considerably increased our knowledge of the mechanisms of radiation-induced carcinogenesis. However, as a result of the complexities involved in the many genetic and epigenetic changes in cells leading to the expression of malignancy only years or even decades after radiation exposure, risk coefficients for the quantification of health detriment still have to be derived largely from epidemiological data and animal studies. On the other hand, improved understanding of molecular and cellular mechanisms is increasingly important in testing and refuting hypotheses about the relative carcinogenic potential of different radiation qualities and dose rates, and of low-level exposures.

A generalized state-vector model for radiation-induced cellular transformation

A mathematical model is developed, detailing the manner in which radiation brings about the transformation of cells to a state of uncontrolled growth. The model is based on the concepts of initiation and promotion, with the irradiation acting both to damage intracellular structures and to change the state of cells surrounding a damaged (initiated) cell. The complete model requires that the radiation produce two forms of damage within a cell, with at least one of the forms requiring an interaction which is a function of time since irradiation. Some form of contact inhibition must be removed, with this step being a function of the probability that a cell in an initiated state will be surrounded by n dead cells. The cell then must divide, with the probability of moving the cell to the final transformed state being a function of the number of cellular divisions. Prior to irradiation, it is assumed that cells may be characterized by an initial state vector describing the probability that any given cell is in one of the states specified by the model. The resulting model then is used to explain data concerning in vitro irradiation of cells by acute doses of X-rays, alpha particles and neutrons. Limited tests of the theory under conditions of fractionated irradiation are also provided. A controlling factor in such studies is the number of cells already in intermediate states prior to the irradiation.

Cell density dependence of transformation frequencies in C3H10T1/2 cells exposed to X-rays

The effects of cell density on transformation frequencies were studied in C3H10T1/2 cells exposed to 0.5 and 7 Gy of 200 kVp X-rays. Initial cell density strongly influenced transformation frequency; this decreased by a factor of between 4 and 10 when the initial seeding density was changed from 50 to 2500 cells/10 cm diameter Petri dish. The data were fitted with two equations: (a) an allometric function represented on a log-log scale by a straight line and (b) a sigmoidal function with plateaux between 50 and 250 cells/dish and above 600. The two curves are compared and their probabilities discussed. Our data indicate that the region between 50 and 250 cells/dish would be the most suitable region for dose-effect measurements. A study of the growth curves at 0.5 and 8.5 Gy shows that cell growth rates are not influenced by initial cell density.

Lack of inverse dose-rate effect on fission neutron induced transformation of C3H/10T1/2 cells

Exponential and density-inhibited cultures of C3H/10T1/2 cells were exposed to a single dose of 0.3 Gy of fission neutrons delivered at rates ranging from 0.005 to 0.1 Gy/min. No discernible effect upon cell survival or transformation was observed by a lowering of the fission neutron dose rate in either exponential or plateau cultures. At the level of 2.3 x 10(-4) transformants per surviving cell, the RBE for neoplastic transformation was three at acute dose rates and ten at the lowest dose rate studied (0.005 Gy/min for neutrons and 0.01 Gy/min for X-rays).

Reversed dose-rate effect and RBE of 252-californium radiation in the induction of 6-thioguanine-resistant mutations in mouse L5178Y cells

The effects of californium-252 radiation (average neutron energy E = 2.13 MeV) were investigated using mouse leukemia L5178Y cells. No dose-rate effect was detected for cell killing, but a 'reversed' dose-rate effect was observed for mutation induction. The frequency of 6-thioguanine-resistant mutations increased linearly up to 100 cGy (1 Gy = 100 rad), then began to level off at a dose rate of 1.2 cGy/min, while it increased continuously up to 200 cGy at a reduced dose rate of 0.16 cGy/min. Compared with results obtained using 60Co gamma-rays, the ratio of the initial slope of each dose-response curve was 4-5 for cell killing, and more than 11 for mutagenesis. Since one-third of 252Cf radiation consists of gamma-rays, the relative biological effectiveness (RBE) of 252Cf neutrons would be even greater, 16 or more, for mutation induction in the present assay.

Repair time for oncogenic transformation in C3H/10T1/2 cells subjected to protracted X-irradiation

With exponential cultures of C3H/10T1/2 cells, we have investigated the effect of X-ray dose protraction on oncogenic cell transformation in the dose range 0.25-2 Gy. Within a particular experiment a constant exposure time was used. In different experiments exposure time varied between 1 and 5h. Cell transformation was analysed using the linear-quadratic relation, gamma (D) = alpha 1D + alpha 2D2, between transformation frequency per surviving cell and X-ray dose. Based on values of the linear coefficients, we developed an empirical formula for relating slopes of dose induction curves obtained at high or reduced dose rate condition. Our estimate of repair half-time for cell transformation with 95 per cent confidence limits is 2.4 (1.8, 3.0) h.

Single, split and fractionated dose X-radiation-induced malignant transformation in A31-11 mouse BALB/3T3 cells

Studies were conducted to determine the effects of a single, split and fractionated doses (separated by 1-h intervals) of 100 rad of X-irradiation on the morphological transformation of BALB/c 3T3 clone A31-11 mouse fibroblasts grown in media containing calf serum. Both spontaneous and radiation-induced transformation levels were lower for these cells grown in the cell-serum containing media than previously reported for these cells grown in fetal calf serum containing media. In the studies reported here, cells were irradiated either as density-inhibited plateau phase cultures or as low density cultures at 10-14 h after being reseeded from confluent dishes. We observed that a 4-fraction 100-rad dose resulted in a reduced yield of transformants compared to a single dose of 100 rad when plateau phase cultures were utilized for the radiation exposures, but not in low density cultures in which the cells were allowed to proliferate during the radiation exposures, these results suggest that the growth phase of the cells can play a major role in determining the yield of transformants induced by fractionated doses of radiation. It is noteworthy that, for the other data obtained in these studies, in none of 12 different experimental points (involving 5 separate experiments) did a fractionated dose protocol result in a reduced yield of transformants when compared to a single dose protocol.