Stephen A. Marino

RBE as a function of neutron energy. I. Experimental observations

The survival of Chinese hamster cells in culture was used as a test system to determine the RBE of neutrons over a wide energy range. The Radiological Research Accelerator Facility (RARAF) at Brookhaven National Laboratory was used for experiments involving nine neutron energies between 110 keV and 15 MeV; for all but the lowest energy the beams were essentially monoenergetic. Additional experiments were performed with high energy cyclotron-produced neutrons at the Naval Research Laboratory, Washington, D. C., and at the Texas A&M Variable Energy Cyclotron (TAMVEC). In both cases broad neutron energy spectra were involved. In each experiment, survival curves were obtained for one neutron energy and compared with 250 kVp X rays, using cells from the same suspension and common controls. In this way a detailed study was made of the relation between RBE and neutron absorbed dose for each neutron energy. At any given cell survival level, RBE varies with neutron energy. Neutrons at 350 keV are biologically the ...

The biological effectiveness of radon-progeny alpha particles. II: Oncogenic transformation as a function of linear energy transfer

Epidemiological studies have established an association between exposure to radon and carcinoma of the lung. However, based on data for either lung cancer in uranium miners exposed to radon or bronchial epithelial carcinomas in Japanese A-bomb survivors, it has not been possible to assign estimates of risk of lung cancer for the general population exposed to radon in their homes. Based on past success with the excellent quantitative properties of the C3H 10T1/2 in vitro oncogenic transformation assay system, the relative biological effectiveness (RBE) for radiation-induced transformation for charged particles of defined LET has been determined. As the LET of the radiation was increased, the rate of induction of oncogenic transformation increased and the RBEm approached 20. At higher LETs, RBE dropped precipitously. The rapid drop in effectiveness for alpha particles with LETs between 120 and 265 keV/microns implies a lower quality factor than the 20-25 currently considered appropriate when estimating lung cancer mortality.

Inactivation of synchronized Chinese hamster V79 cells with charged-particle track segments

BIRD, R. P., ROHRIG, N., COLVETT, R. D., GEARD, C. R., AND MARINO, S. A., Inactivation of Synchronized Chinese Hamster V79 Cells with Charged-Particle Track Segments. Radiat. Res. 82, 277-289 (1980). Synchronized Chinese hamster V79 cells were irradiated with protons, deuterons, and helium-3 ions in a series of cell inactivation experiments. The charged particles were accelerated at the Radiological Research Accelerator Facility at Brookhaven National Laboratory and provided a range of LET values from 10 to 170 keV/C/m. Two parts of the cell cycle were irradiated to determine clonogenic survival, the G,/S transition and late-S phase, both obtained by hydroxyurea-induced synchrony. The difference in radiation sensitivity between these two synchronized cell populations decreased with increased values of LET, but a 10-fold difference persisted with the highest LET values. The inactivation cross sections calculated from initial slopes of the survival curves increased to plateau values of about 19 and 38 C/m2 for the late-S and G,/S cells, respectively. In contrast, the respective cross sections of cell nuclei were measured and had mean values of 222 and 165 /Lm2.

Radiobiological studies with monoenergetic neutrons

The Radiological Research Accelerator Facility (RARAF) has the capability of producing essentially monoenergetic neutron beams, ranging in energy from 16.4 MeV down to 220 keV. In addition, two lower energy neutron beams are available which consist of a wide spectrum of energies and are described as the 110 keV and 60 keV spectra. Seedlings of Vicia faba have been used to measure the oxygen enhancement ratio (OER) and the relative biological effectiveness (RBE) of each of these neutron beams. The OER decreases as the neutron energy is reduced between 15.4 MeV and 220 keV, but does not appear to decrease further for lower energy neutrons. RBE increases as the neutron energy is reduced from 15.4 AleV to 440 keV; the curve then goes through a maximum at around 350 keV, and for lower energies the RBE falls again.

The sequential irradiation of mammalian cells with x rays and charged particles of high LET

Chinese hamster V79 cells, synchronized in late-S phase, were irradiated with high-LET charged particles or X rays, or exposed sequentially to a single dose of charged particles followed by graded doses of X rays. The charged-particle irradiations consisted of deuterons (LET, 50 keV/microns) or 3He ions (96 or 160 keV/microns). The survival data obtained following the sequential irradiations show a synergistic effect compared with exposure to the low-LET radiation. An enhancement ratio is defined in order to quantify this effect. On the basis of this concept the present data show an enhanced interaction effect as the LET of the primary dose is increased or the size of this dose is increased. The data are also discussed in terms of a recent theoretical formulation which predicts synergism as a result of the interaction between the sublethal damage produced by the two radiations.

Neutron-energy-dependent oncogenic transformation of C3H 10T1/2 mouse cells

The relative biological effectiveness (RBE) of a range of neutron energies relative to 250-kVp X rays has been determined for oncogenic transformation and cell survival in the mouse C3H 10T 1/2 cell line. Monoenergetic neutrons at 0.23, 0.35, 0.45, 0.70, 0.96, 1.96, 5.90, and 13.7 MeV were generated at the Radiological Research Accelerator Facility of the Radiological Research Laboratories, Columbia University, and were used to irradiate asynchronous cells at low absorbed doses from 0.05 to 1.47 Gy. X irradiations covered the range 0.5 to 8 Gy. Over the more than 2-year period of this study, the 31 experiments provided comprehensive information, indicating minimal variability in control material, assuring the validity of comparisons over time. For both survival and transformation, a curvilinear dose response for X rays was contrasted with linear or nearly linear dose responses for the various neutron energies. RBE increased as dose decreased for both end points. Maximal RBE values for transformation ranged from 13 for cells exposed to 5.9-MeV neutrons to 35 for 0.35-MeV neutrons. This study clearly shows that over the range of neutron energies typically seen by nuclear power plant workers and individuals exposed to the atomic bombs in Japan, a wide range of RBE values needs to be considered when evaluating the neutron component of the effective dose. These results are in concordance with the recent proposals in ICRU 40 both to change upward and to vary the quality factor for neutron irradiations.

microRNAome changes in bystander three-dimensional human tissue models suggest priming of apoptotic pathways

The radiation-induced bystander effect (RIBE) is a phenomenon whereby unexposed cells exhibit molecular symptoms of stress exposure when adjacent or nearby cells are traversed by ionizing radiation (IR). Recent data suggest that RIBE may be epigenetically mediated by microRNAs (miRNAs), which are small regulatory molecules that target messenger RNA transcripts for translational inhibition. Here, we analyzed microRNAome changes in bystander tissues after α-particle microbeam irradiation of three-dimensional artificial human tissues using miRNA microarrays. Our results indicate that IR leads to a deregulation of miRNA expression in bystander tissues. We report that major bystander end points, including apoptosis, cell cycle deregulation and DNA hypomethylation, may be mediated by altered expression of miRNAs. Specifically, c-MYC-mediated upregulation of the miR-17 family was associated with decreased levels of E2F1 and RB1, suggesting a switch to a proliferative state in bystander tissues, while priming these cells for impending death signals. Upregulation of the miR-29 family resulted in decreased levels of its targets DNMT3a and MCL1, consequently affecting DNA methylation and apoptosis. Altered expression of miR-16 led to changes in expression of BCL2, suggesting modulation of apoptosis. Thus, our data clearly show that miRNAs play a profound role in the manifestation of late RIBE end points. In summary, this study creates a roadmap for understanding the role of microRNAome in RIBE and for developing novel RIBE biomarkers.

Characterization of radiation-induced Apoptosis in rodent cell lines

For REC:myc(ch1), Rat1 and Rat1:myc(b) cells, we determined the events in the development of radiation-induced apoptosis to be in the following order: cell division followed by chromatin condensation, membrane blebbing, loss of adhesion and the uptake of vital dye. Experimental data which were obtained using 4He ions of well-defined energies and which compared the dependence of apoptosis and clonogenic survival on 4He range strongly suggested that in our cells both apoptosis and loss of clonogenic survival resulted from radiation damage to the cell nucleus. Corroboratory evidence was that BrdU incorporation sensitized these cells to radiation-induced apoptosis. Comparing the dose response for apoptosis and the clonogenic survival curves for Rat1 and Rat1:myc(b) cells, we concluded that radiation-induced apoptosis contributed to the overall radiation-induced cell inactivation as assayed by clonogenic survival, and that a modified linear-quadratic model, proposed previously, modeled such a contribution effectively. In the same context, the selective increase in radiation-induced apoptosis during late S and G2 phases reduced the relative radioresistance observed for clonogenic survival during late S and G2 phases.

Oncogenic Transformation by Fractionated Doses of Neutrons

Oncogenic transformation was assayed after C3H 10T1/2 cells were irradiated with monoenergetic neutrons; cells were exposed to 0.23-, 0.35-, 0.45-, 5.9-, and 13.7-MeV neutrons given singly or in five equal fractions over 8 h. At the biologically effective neutron energy of 0.45 MeV, enhancement of transformation was evident with some small fractionated doses (below 1 Gy). When transformation was examined as a function of neutron energy at 0.5 Gy, enhancement was seen for cells exposed to three of the five energies (0.35, 0.45, and 5.9 MeV). Enhancement was greatest for cells irradiated with 5.9-MeV neutrons. Of the neutron energies examined, 5.9-MeV neutrons had the lowest dose-averaged lineal energy and linear energy transfer. This suggests that enhancement of transformation by fractionated low doses of neutrons may be radiation-quality dependent.

The Inverse Dose-Rate Effect for Oncogenic Transformation by Charged Particles Is Dependent on Linear Energy Transfer

Mouse C3H 10T1/2 cells were exposed to single or fractionated doses of charged particles of defined linear energy transfer (LET) from 25 to 200 keV/microns. Dose fractionation with prolonged time intervals enhanced the yield of transformed foci compared with a single acute dose for a range of LET values between 40 and 120 keV/microns. Radiations of lower or higher LET did not show the enhancement that is commonly referred to as the inverse dose-rate effect. The fractionation scheme that was used consisted of three dose fractions; the maximum enhancement of transformation occurred with an interval of 150 min between dose fractions. This inverse dose-rate effect, demonstrated for cycling cells in log phase, was not seen for cells in plateau phase.