Enhanced Sensitivity to Neoplastic Transformation by137Cs γ-rays of Cells in the G2-/M-phase Age Interval

C3H mouse 10T1/2 cells, exposed to low doses of fission-spectrum neutrons, have an enhanced frequency of neoplastic transformation if protracted exposures are used (Hill et al. 1982, 1984a, 1985). To explain this anomaly, a biophysical model was proposed (Elkind 1991a,b) having the following essential features: (1) a narrow age interval exists in the growth cycle of 10T1/2 cells in which cells have high sensitivities to transformation; (2) in the latter age interval, cells are also sensitive to killing; (3) with increasing dose, cells at ages earlier in the growth cycle are progressively delayed from entering the sensitive age window; and (4) with increasing dose, the transformation sensitivity of cells in the sensitive window is not expressed due to increased killing. Protracted low doses result in elevated frequencies because of less killing, and reductions in delays in cell progression. Therefore, transformation-sensitive cells can progress into the sensitive interval to replace those that have progressed out of it. The unique shape and radiobiological properties of cells in and around mitosis, led to the proposal that the sensitive window is mitosis and possible cells just preceding or just following M phase (Elkind 1991a,b). Because of the likelihood that the properties of the cells in a sensitive window would not be evident only when fission-spectrum neutrons are used, this study was undertaken using 137Cs gamma-rays. We have found that late G2- to M-phase 10T1/2 cells have a maximal sensitivity to neoplastic transformation as well as to killing by 137Cs gamma-rays.

The effect of dose rate on radiation-induced neoplastic transformation in vitro by low doses of low-LET radiation

The dependence of the incidence of radiation-induced cancer on the dose rate of the radiation exposure is a question of considerable importance to the estimation of risk of cancer induction by low-dose-rate radiation. Currently a dose and dose-rate effectiveness factor (DDREF) is used to convert high-dose-rate risk estimates to low dose rates. In this study, the end point of neoplastic transformation in vitro has been used to explore this question. It has been shown previously that for low doses of low-LET radiation delivered at high dose rates, there is a suppression of neoplastic transformation frequency at doses less than around 100 mGy. In the present study, dose-response curves up to a total dose of 1000 mGy have been generated for photons from (125)I decay (approximately 30 keV) delivered at doses rates of 0.19, 0.47, 0.91 and 1.9 mGy/min. The results indicate that at dose rates of 1.9 and 0.91 mGy/min the slope of the induction curve is about 1.5 times less than that measured at high dose rate in previous studies with a similar quality of radiation (28 kVp mammographic energy X rays). In the dose region of 0 to 100 mGy, the data were equally well fitted by a threshold or linear no-threshold model. At dose rates of 0.19 and 0.47 mGy/min there was no induction of transformation even at doses up to 1000 mGy, and there was evidence for a possible suppressive effect. These results show that for this in vitro end point the DDREF is very dependent on dose rate and at very low doses and dose rates approaches infinity. The relative risks for the in vitro data compare well with those from epidemiological studies of breast cancer induction by low- and high-dose-rate radiation.

Carcinogenic risk of hot-particle exposures

It has been suggested that spatially non-uniform radiation exposures, such as those from small radioactive particles ('hot particles'), may be very much more carcinogenic than when the same amount of energy is deposited uniformly throughout a tissue volume. This review provides a brief summary of in vivo and in vitro experimental findings, and human epidemiology data, which can be used to evaluate the veracity of this suggestion. Overall, this supports the contrary view and indicates that average dose, as advocated by the ICRP, is likely to provide a reasonable estimate of carcinogenic risk (within a factor of approximately +/- 3). There are few human data with which to address this issue. The limited data on lung cancer mortality following occupational inhalation of plutonium aerosols, and the incidence of liver cancer and leukaemia due to thorotrast administration for clinical diagnosis, do not appear to support a significant enhancement factor. Very few animal studies, including mainly lung and skin exposures, provide any indication of a hot-particle enhancement for carcinogenicity. Some recent in vitro malignant transformation experiments provide evidence foran enhanced cell transformation for hot-particle exposures but, properly interpreted, the effect is modest. Few studies extend below absorbed doses of approximately 0.1 Gy.

Neutron carcinogenesis: past, present, and future

An interest in the possible cancer causing ability of neutrons began soon after their discovery. Early use of neutrons from radioactive sources and from cyclotrons led to a need to define risk for such exposures. This need was soon followed by a more tangible need to define risk to the general population of high LET radiation from nuclear fall out and use of the Atomic bomb and possible use of the H-bomb. Neutrons were soon found to be very effective cell killing agents compared to conventional ionizing radiation. However High LET radiation sources and neutrons in particular, come in many different energies and from many types of sources. I will survey the differences between different energy neutrons and conventional types of radiation, particularly with respect to the dose rate of exposures and the influence of repair or lack thereof and more recently the effect of cell cycle distribution on the carcinogenic outcome. I will illustrate these ideas with examples of carcinogenicity studies and mutation studies from my own laboratory and in some cases from the work of others. Lastly I will introduce some possible avenues for molecular studies of neutron effects that might answer such vexing questions as the real risk at very low doses, is repair error free or error prone, do neutrons cause genetic instability for many cell generations after exposure, and others? There remain many questions about the biology of neutron action that require answers if we are to protect the ever increasing number of people exposed to them because of their growing use in medicine, in the military and in commercial industry.

Induction of micronuclei by acute and chronic exposure in vivo to gamma rays in murine polychromatic erythrocytes

The effect on micronuclei (MN) frequency of in vivo exposure to different dose rates of gamma rays in murine polychromatic erythrocytes (PCE) was studied. Groups of animals were irradiated with 1.0 Gy of gamma rays administered in 10, 100, 1000 or 10,000 min; the micronucleated polychromatic-erythrocytes (MN-PCE) frequency was scored in blood samples obtained from the tail of mice at various times before, during or after irradiation. The time-response curves for the 10, 100, and 1000 min exposure were similar; however, the two first curves showed a peak at 1800 min and the last at 2400 min after exposure. The curve obtained from the 10,000 min exposure showed a plateau for a long period during and after the exposure. However, the integration of the area under the curves indicates that the damage caused by the different radiation protocols was the same, suggesting that throughout time, the lesions were not repaired but rather diluted by cell division.

Cancer genes: current status, future prospects, and applications in radiotherapy/oncology

There is a sense of excitement in contemporary cancer research, generated largely by the discovery, and subsequent characterization, of oncogenes. These genes are part of the normal complement of cells, and become altered in their structure or expression, during the development of the neoplastic phenotype. In this review, I highlight some of the important advances in the field, starting with the relationships between viral oncogenes and their cellular homologs. I illustrate some of the molecular mechanisms whereby a harmless, or quiescent, cellular gene can be converted ("activated") by radiation or by other carcinogens to a full-blown oncogene involved in carcinogenesis. Next, I discuss two areas where oncogene research has specific relevance for professionals working with radiation, namely the question of radiation-induced cancer, and the issue of the radiocurability of tumors. I also assess the important role of tumor-suppressor genes in oncogenesis. I then describe a genetic model, to illustrate the current status of our understanding of carcinogenesis. Finally, I discuss potential applications of specific interest to oncologists: topics such as prognostic indicators, novel therapeutic strategies, and gene-replacement techniques, are critically reviewed.