Integrated interdisciplinary training in the radiological sciences

The radiation sciences are increasingly interdisciplinary, both from the research and the clinical perspectives. Beyond clinical and research issues, there are very real issues of communication between scientists from different disciplines. It follows that there is an increasing need for interdisciplinary training courses in the radiological sciences. Training courses are common in biomedical academic and clinical environments, but are typically targeted to scientists in specific technical fields. In the era of multidisciplinary biomedical science, there is a need for highly integrated multidisciplinary training courses that are designed for, and are useful to, scientists who are from a mix of very different academic fields and backgrounds. We briefly describe our experiences running such an integrated training course for researchers in the field of biomedical radiation microbeams, and draw some conclusions about how such interdisciplinary training courses can best function. These conclusions should be applicable to many other areas of the radiological sciences. In summary, we found that it is highly beneficial to keep the scientists from the different disciplines together. In practice, this means not segregating the training course into sections specifically for biologists and sections specifically for physicists and engineers, but rather keeping the students together to attend the same lectures and hands-on studies throughout the course. This structure added value to the learning experience not only in terms of the cross fertilization of information and ideas between scientists from the different disciplines, but also in terms of reinforcing some basic concepts for scientists in their own discipline.

Bystander effect and adaptive response in C3H 10T½ cells

PURPOSE

To address the relationship between the bystander effect and the adaptive response that can compete to impact on the dose-response curve at low doses.

MATERIALS AND METHODS

A novel radiation apparatus, where targeted and non-targeted cells were grown in close proximity, was used to investigate these phenomena in C3H 10T(1/2) cells. It was further examined whether a bystander effect or an adaptive response could be induced by a factor(s) present in the supernatants of cells exposed to a high or low dose of X-rays, respectively.

RESULTS

When non-hit cells were co-cultured for 24 h with cells irradiated with 5 Gy alpha-particles, a significant increase in both cell killing and oncogenic transformation frequency was observed. If these cells were treated with 2 cGy X-rays 5 h before co-culture with irradiated cells, approximately 95% of the bystander effect was cancelled out. A 2.5-fold decrease in the oncogenic transformation frequency was also observed. When cells were cultured in medium donated from cells exposed to 5 Gy X-rays, a significant bystander effect was observed for clonogenic survival. When cells were cultured for 5 h with supernatant from donor cells exposed to 2 cGy and were then irradiated with 4 Gy X-rays, they failed to show an increase in survival compared with cells directly irradiated with 4 Gy. However, a twofold reduction in the oncogenic transformation frequency was seen.

CONCLUSIONS

An adaptive dose of X-rays cancelled out the majority of the bystander effect produced by alpha-particles. For oncogenic transformation, but not cell survival, radioadaption can occur in unirradiated cells via a transmissible factor(s).

Routine screening mammography: how important is the radiation-risk side of the benefit-risk equation?

The potential radiation hazards associated with routine screening mammography, in terms of breast cancer induction, are discussed in the context of the potential benefits. The very low energy X-rays used in screening mammography (26-30 kVp) are expected to be more hazardous, per unit dose, than high-energy X- or gamma-rays, such as those to which A-bomb survivors (from which radiation risk estimates are derived) were exposed. Based on in vitro studies using oncogenic transformation and chromosome aberration end-points, as well as theoretical estimates, it seems likely that low doses of low-energy X-rays produce an increased risk per unit dose (compared with high energy photons) of about a factor of 2. Because of the low doses involved in screening mammography, the benefit-risk ratio for older women would still be expected to be large, though for younger women the increase in the estimated radiation risk suggests a somewhat later age than currently recommended--by about 5-10 years--at which to commence routine breast screening.

A microdosimetry chamber for low-energy X rays

A wall-less proportional counter designed to measure single event spectra produced by low energy X rays is described. The sensitive volume of the counter and the housing are made entirely of non-metallic materials to minimise distortions in the secondary electron spectrum.

Novel approaches with track segment alpha particles and cell co-cultures in studies of bystander effects

There is now a significant body of data that indicate that the effects of ionising radiation may extend to more than those cells that directly suffer damage to DNA in the cell nucleus. Cells neighbouring those cells that are irradiated, or even well separated from those that are irradiated demonstrate several responses that are recorded in hit cells as a function of absorbed dose. That is, the responding non-hit cells are bystanders of hit cells. A protocol has been devised which allows for examination of one means of eliciting bystander responses, specifically, effects on non-contacting cells. Cell culture chambers are set up such that a population of cells is physically separate from the energy depositions of track segment charged particles. Absorption of energy in sub-millimetre distances in the cell culture medium ensures that one population of cells can only respond to factors generated in the irradiated medium or in another population of irradiated co-cultured cells, which may be of similar or dissimilar origin. For irradiation of medium alone, enhanced levels of micronuclei, and of delays in cell cycle progression occur in normal human fibroblasts, but not epithelial cells. This procedure allows for a defining of the factors responsible for initiating bystander effects and for determining their quantitative relevance.

Oncogenic transformation in C3H10T1/2 cells by low-energy neutrons

PURPOSE

Occupational exposure to neutrons typically includes significant doses of low-energy neutrons, with energies below 100 keV. In addition, the normal-tissue dose from boron neutron capture therapy will largely be from low-energy neutrons. Microdosimetric theory predicts decreasing biological effectiveness for neutrons with energies below about 350 keV compared with that for higher-energy neutrons; based on such considerations, and limited biological data, the current radiation weighting factor (quality factor) for neutrons with energies from 10 keV to 100 keV is less than that for higher-energy neutrons. By contrast, some reports have suggested that the biological effectiveness of low-energy neutrons is similar to that of fast neutrons. The purpose of the current work is to assess the relative biological effectiveness of low-energy neutrons for an endpoint of relevance to carcinogenesis: in vitro oncogenic transformation.

METHODS

Oncogenic transformation induction frequencies were determined for C3H10T1/2 cells exposed to two low-energy neutron beams, respectively, with dose-averaged energies of 40 and 70 keV, and the results were compared with those for higher-energy neutrons and X-rays.

RESULTS

These results for oncogenic transformation provide evidence for a significant decrease in biological effectiveness for 40 keV neutrons compared with 350 keV neutrons. The 70 keV neutrons were intermediate in effectiveness between the 70 and 350 keV beams.

CONCLUSIONS

A decrease in biological effectiveness for low-energy neutrons is in agreement with most (but not all) earlier biological studies, as well as microdosimetric considerations. The results for oncogenic transformation were consistent with the currently recommended decreased values for low-energy neutron radiation weighting factors compared with fast neutrons.

Neutron-energy-dependent cell survival and oncogenic transformation

Both cell lethality and neoplastic transformation were assessed for C3H10T1/2 cells exposed to neutrons with energies from 0.040 to 13.7 MeV. Monoenergetic neutrons with energies from 0.23 to 13.7 MeV and two neutron energy spectra with average energies of 0.040 and 0.070 MeV were produced with a Van de Graaff accelerator at the Radiological Research Accelerator Facility (RARAF) in the Center for Radiological Research of Columbia University. For determination of relative biological effectiveness (RBE), cells were exposed to 250 kVp X rays. With exposures to 250 kVp X rays, both cell survival and radiation-induced oncogenic transformation were curvilinear. Irradiation of cells with neutrons at all energies resulted in linear responses as a function of dose for both biological endpoints. Results indicate a complex relationship between RBEm and neutron energy. For both survival and transformation, RBEm was greatest for cells exposed to 0.35 MeV neutrons. RBEm was significantly less at energies above or below 0.35 MeV. These results are consistent with microdosimetric expectation. These results are also compatible with current assessments of neutron radiation weighting factors for radiation protection purposes. Based on calculations of dose-averaged LET, 0.35 MeV neutrons have the greatest LET and therefore would be expected to be more biologically effective than neutrons of greater or lesser energies.

The frequency distribution of the number of ion pairs in irradiated tissue

The statistical distribution of the number of ion pairs per ionizing event in a small volume simulating a tissue sphere was obtained by applying the Expectation-Maximization (EM) algorithm to experimental spectra measured by exposing a Rossi-type spherical proportional counter to gamma radiation. The normalized experimental spectrum, r(x), which is the distribution of the number of ion pairs per event from both the primary track and the subsequent electron multiplication, can be represented as Sum(n) p(n) x f(n,x), where the f(n,x)'s for n = 1, 2, 3, ..., n are the normalized spectra for exactly 1, 2, 3, ..., n primary ion pairs and are calculated by convoluting the single-electron spectrum. The coefficients pn represent the mixing proportions of the spectra corresponding to 1, 2, 3, ..., n ion pairs in forming the experimental spectrum. The single-electron spectrum used in our calculations is the distribution of the number of ion pairs due to the multiplication process, and it is represented in analytical form by the Gamma distribution f(1,x) = a x x(b) x e(-cx), where x is energy, usually in eV, and a, b and c are constants. The EM algorithm is an iterative procedure for computing the maximum likelihood or maximum a posteriori estimates of the mixing proportions p(n), which we also refer to as the primary distribution of ion pairs in a microscopic spherical tissue-equivalent volume. The experimental and primary spectra are presented for simulated tissue spheres ranging from 0.25 to 8 microm in diameter exposed to 60Co gamma radiation.

Effect of track structure and radioprotectors on the induction of oncogenic transformation in murine fibroblasts by heavy ions

The oncogenic potential of high-energy 56Fe particles (1 GeV/nucleon) accelerated with the Alternating Gradient Synchrotron at the Brookhaven National Laboratory was examined utilizing the mouse C3H 10T1/2 cell model. The dose-averaged LET for high-energy 56Fe is estimated to be 143 keV/micrometer with the exposure conditions used in this study. For 56Fe ions, the maximum relative biological effectiveness (RBEmax) values for cell survival and oncogenic transformation were 7.71 and 16.5 respectively. Compared to 150 keV/micrometer 4He nuclei, high-energy 56Fe nuclei were significantly less effective in cell killing and oncogenic induction. The prostaglandin E1 analog misoprostol, an effective oncoprotector of C3H 10T1/2 cells exposed to X rays, was evaluated for its potential as a radioprotector of oncogenic transformation with high-energy 56Fe. Exposure of cells to misoprostol did not alter 56Fe cytotoxicity or the rate of 56Fe-induced oncogenic transformation.

Microdosimetry of monoenergetic neutrons

Tissue spheres 0.25, 0.5, 1.0, 2.0, 4.0 and 8.0 micron in diameter were simulated using a wall-less spherical counter filled with propane-based tissue-equivalent gas. Microdosimetric spectra corresponding to these site sizes were measured for five neutron energies (0.22, 0.44, 1.5, 6 and 14 MeV) and the related mean values /yF and /yD were calculated for several site sizes and neutron energies. An elaborate calibration technique combining soft X rays, an 55Fe photon source and a collimated 244Cm alpha-particle source was used throughout the measurements. The spectra and their mean values are compared with theoretically calculated values for ICRU standard tissue. The agreement between the calculated and the measured data is good in spite of a systematic discrepancy, which could be attributed, in part, to the difference in elemental composition between the tissue-equivalent gas and plastic used in the counter and the ICRU standard tissue used in the calculations.

Quantitative assessment of the cataractogenic potential of very low doses of neutrons

We report on the prevalence and relative biological effectiveness (RBE) for various stages of lens opacification in rats induced by very low doses (2 to 250 mGy) of medium-energy (440 keV) neutrons, compared to those for X rays. Neutron doses were delivered either in a single fraction or in four separate fractions and the irradiated animals were followed for over 100 weeks. At the highest observed dose (250 mGy) and at early observation times, there was evidence of an inverse dose-rate effect; i.e., a fractionated exposure was more potent than a single exposure. Neutron RBEs relative to X rays were estimated using a non-parametric technique. The results were only weakly dependent on time postirradiation. At 30 weeks, for example, 80% confidence intervals for the RBE of acutely delivered neutrons relative to X rays were 8-16 at 250 mGy, 10-20 at 50 mGy, 50-100 at 10 mGy and 250-500 at 2 mGy. The results are consistent with the estimated neutron RBEs in Japanese A-bomb survivors, though broad confidence bounds are present in the Japanese results. Our findings are also consistent with data reported earlier for cataractogenesis induced by heavy ions in rats, mice, and rabbits. We conclude from these results that, at very low doses (<10 mGy), the RBE for neutron-induced cataractogenesis is considerably larger than the RBE of 20 commonly used, and use of a significantly larger value for calculating equivalent dose would be prudent.

[The correlation between morphology, electrolytic content and risk factors in breast cysts]

Many studies on the biochemical composition of the liquid aspirated from breast cysts have identified three types of cysts: type I (apocrine) cysts, with a high concentration of K+ and low levels of Na+ and C1-; type II (transudate) cysts, with an electrolytic content similar to that of plasma and high Na+ levels and, finally, type III cysts, with intermediate characteristics. The literature data appear to indicate that the women with type I cysts are at higher risk for breast cancer. The authors report the results of a study carried out on 143 women from October, 1991, through October, 1994, in the Radiology Department of the University of Bologna, to investigate the correlations between some risk factors for breast cancer, the characteristics of cyst fluid and the morphology of the cysts after pneumocystography. Of 186 cysts, 104 (55.9%) were type I, 49 (26.4%) were type II, and 33 (17.7%) were type III. Among the risk factors we considered, only the premenopausal state (41 to 45 years of age) exhibited a statistically significant correlation with the presence of type I cysts. The morphological study of the cysts after pneumocystography showed a surprisingly high correlation between the honeycomb pattern and type I cysts. The constant correlation between cyst morphology and electrolytic content may allow the easy identification of the subgroups of patients eligible for a closer follow-up.

Neutron-induced cell cycle-dependent oncogenic transformation of C3H 10T1/2 cells

Exposure of synchronized populations of mouse C3H 10T1/2 cells to a single dose (0.6 Gy) of 5.9 MeV neutrons at intervals after mitotic shake-off results in a distinctive variation in the oncogenic transformation frequency through the cell cycle. Previous findings show a sensitive window for X-ray-induced oncogenic transformants at late times after mitotic shake-off (14-16 h). Optimal sensitivity to neutrons was observed for cell populations irradiated soon after mitotic shake-off (4-6 h), where the majority of cells would be in the G1 phase of the cell cycle. Additionally, enhanced sensitivity was also found for that period after shake-off (14-16 h) which was maximally sensitive to X rays corresponding to cell populations with a high proportion of G2-phase cells. That is, low-LET radiation (250 kVp X rays) largely appears to produce oncogenic transformants in G2-phase cells, while intermediate-LET radiation (5.9 MeV neutrons) is effective principally on G1- and, to a somewhat lesser extent, G2-phase cells. Cells irradiated with neutrons showed less variation for lethality through the cell cycle than those irradiated with X rays, in agreement with previous findings. The mechanistic basis for the difference in the response of cells in the different phases of the cell cycle to radiations of different quality is unknown but is suggestive of distinct ("signature") molecular changes leading to the observed oncogenic transformation response.

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.

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.

Oncogenic transformation following sequential irradiations with monoenergetic neutrons and X rays

Mouse C3H 10T1/2 cells were exposed sequentially to low doses (0.1 and 0.3 Gy) of monoenergetic neutrons (0.35, 0.45, 5.9, and 13.7 MeV) and 250-kVp X rays (1 and 3 Gy). The incidences of oncogenic transformation in the cells exposed to neutrons followed by X rays indicated that the effects of the individual radiations were simply additive. This supports the contention that risks associated with the two different radiation modalities may be considered to be additive.

The effects of the temporal distribution of dose on oncogenic transformation by neutrons and charged particles of intermediate LET

The effects of dose rate and dose fractionation on high-LET radiation-induced oncogenic transformation of C3H 10T1/2 cells were examined. Cells were irradiated with graded doses of 5.9-MeV monoenergetic neutrons administered either in single acute exposures (30 mGy/min) or extended over an 8-h period at low dose rates (from 0.21 to 1 mGy/min). Although cell survival studies showed no difference in effect with a change in radiation delivery rate, enhancement of oncogenic transformation occurred when the dose rate was reduced. When the neutron dose was divided into three fractions over 8 h, the biological effect was intermediate between that for the acute and that for the low-dose-rate exposures. Further irradiations were made using deuterons with an LET of 40 keV/microns. The dose-mean lineal energy was comparable to that measured for the 5.9-MeV monoenergetic neutrons. An inverse dose-rate/fractionation effect for the induction of transformation by high-LET deuterons was observed when the time between each of three fractions for a 0.3-Gy total dose was at least 45 min. No further enhancement was seen for longer dose fractionations, suggesting that very long protracted exposures of high-LET radiation would produce no additional enhancement.

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.

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 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.