The relationship between o.e.r., r.b.e. and number of fractions for X-ray- or neutron-induced skin damage.

The skin reactions in aerated and hypoxic mouse tails after single or fractionated doses of 250 kV X-rays or fast neutrons (6 MeV deuterons on beryllium) have been measured. The o.e.r. for one to sixteen fractions of X-rays remains constant, while that for one to ten fractions of neutrons decreases with increasing neutron fractionation and decreasing neutron dose/fraction. The o.e.r. for X-rays was 1.7, for single-neutron doses 1.4, and for ten fractions of neutrons 1.25. It was anticipated that the o.e.r. for neutron-induced damage would decrease further as neutron fractionation is increased because the contribution to damage from the highest LET components of dose, the alpha and heavy recoil particles, would increase relative to the lowe LET components. The r.b.e. values obtained for skin damage were higher at all neutron doses/fraction examined in this study on tails than all those previously obtained in studies on skin at other sites on four species. This may be due to the influence of hypoxia on the r.b.e. measurements in the mouse tail.     

Calculation of clinical r.b.e. values for neutrons

A method is described for calculating r.b.e. values for normal tissues at risk in clinical neutron beam therapy. This is based on the assumption that with high l.e.t. radiations the slope of the cell survival curve is steeper, mainly in the initial or low-dose region. This effect is quantified by using two coefficients, one (epsilon) to produce a proportionate increase in the initial slope, and a second (eta) determining the change in the terminal slope (D0) of the survival curve. Analysis of published experimental data shows epsilon to be a variable quantity, different for different tissues; epsilon is larger when the survival curve has a large shoulder or slope ratio (rho). By contrast, eta is relatively constant (for a given beam) and less dependent on the tissue or end-point studied. For low doses, the r.b.e. approaches epsilon, which can be calculated given eta (characteristic of the beam) and rho (characteristic of the relevant tissue) [epsilon = eta + rho(eta - 1)]. This provides a useful approximation to the clinical r.b.e. for specific tissues relative to conventionally fractionated low-l.e.t. photons.     

The r.b.e. of different energy neutrons as measured by the heamatopoietic spleen-colony technique.

The spleen-colony technique has been used for determining the relative biological effectiveness (r.b.e.) for several energies of neutron radiation. Donor mice were exposed for fission and accelerator-generated neutrons at a variety of doses and energies. Immediately after exposure, donor bone-marrow was removed from the hind legs, and standard amounts were injected intravenously into lethally X-irradiated recipients. After 7 days the recipients spleens were evaluated for surface colonies. Dose-response curves were obtained for each type of radiation and the Do was determined. The neutron r.b.e. values from the Do compared with 250kVp X-rays were: reactor 1.58, 252Cf 1:59, and accelerator varied from 2.85 at 1.0 Mev to 0.85 at 13.4 MeV.     

The r.b.e. of fast fission neutrons (2 MeV) for chronic radiation damage of the large bowel of rats after single dose and fractionated irradiation

Late damage in the rectum of rats was studied after local irradiation with 2 MeV fast fission neutrons and the results were compared with those for 300 kV X-rays. A similarity between latency times, and between clinical, gross pathological and histological appearances of the rectal obstruction after the two types of radiation was observed. The r.b.e. evaluated from ED 50/200 days values was 1.8 for single doses, 2.1 for two fractions and 3 for five fractions and these are similar to r.b.e.s from other investigations of chronic intestinal radiation damage. Using linear quadratic analysis, a limiting r.b.e. of 5 was obtained. The beta-values are similar for both radiation qualities. Because of the high gamma-contamination (25 per cent) in the RENT I beam a possible correction of the r.b.e. for neutrons only, assuming interaction, is discussed.     

Chromosome aberrations induced in human lymphocytes by neutron irradiation.

In vitro dose--response curves of unstable chromosome aberrations in human lymphocytes have been obtained for neutron spectra of mean energies 0-7, 0-9, 7-6 and 14-7 MeV. The aberration yields have been fitted to the quadratic function Y = alphaD + betaD2, which is consistent with the single-track and two-track model of aberration formation. However with high-LET radiation, the linear component of yield, corresponding to damage caused by single tracks, predominants, and this term becomes more dominant with increasing LET, so that for fission spectrum neutrons the relationship is linear, Y = alphaD. At low doses, such as those recieved by radiation workers, limiting r.b.e. values between 13 and 47 are obtained relative to 60Co gamma-radiation. At higher doses, as used in radiotherapy, the values are much lower; ranging from 2-7 to 8 at 200 rad of equivalent gamma-radiation. Both sets of r.b.e. values correlate well with track-averaged LET but not with dose-averaged LET. When the numbers of cells without aberrations are plotted against radiation dose, curves are obtained which are similar in shape to those for conventional cell-survival experiments with comparable neutron spectra. The Do values obtained in the present study are close to those from other cell system.     

The r.b.e. of different-energy neutrons as determined by human bone-marrow cell-culture techniques.

The effect of X-rays and different-energy neutrons on human bone-marrow cells was studied using two different cell-culture techniques--diffusion chamber (DC) growth and colony formation in vitro (CFU-C). Based on the survival of proliferative granulocytes in DC on day 13, the D0 value was 80 rad with X-rays, and 117 rad as measured by the CFU-C assay. The D0 values for neutrons depended on the radiation source and the energy level. The r.b.e. values, which dropped with increasing energy levels of mono-energetic neutrons, were (i) 0.44 MeV; DC 3.7, CFU-C 4.1; (ii) 6 MeV; DC 1.8, CFU-C 2.0; (iii) 15 MeV; DC 1.6, CFU-C 1.6; (iv) fission neutrons; DC 2.6, CFU-C 2.4.     

R.b.e. and o.e.r. of extended-Bragg-peak helium ions: survival and development of rat embryos.

Rats were exposed under aerobic or hypoxic conditions to 200-1200 rads of 60Co gamma-rays or extended-Bragg-peak helium ions on the eighth day of gestation. Uterine contents were examined on the twentieth day of gestation. At the 50 per cent embryonic survival level, helium ion r.b.e. was 1(.0) (aerobic) and 1(.2) (hypoxic). Maximum attainable gamma-ray and helium-ion o.e.r.s. were 2(.2) and 1(.7) respectively, indicating an oxygen-effect gain (o.e.g.) of 1(.2). At the 10 per cent survival level helium ion r.b.e. was 1(.1) (aerobic) and 1(.4) (hypoxic). Gamma-ray and helium-ion 0.e.r.s. were 2(.0) and 1(.5) respectively, indicating a helium ion o.e.g. of 1(.3). These data demonstrate that the small fraction of high-LET radiation present in this helium ion beam has a neglible effect on the aerobic r.b.e., but lowers the effective o.e.r. of the beam approximately 25 per cent relative to that of gamma-rays. Helium ions were significantly more effective than gamma-rays in killing embryos under hypoxic conditions, in producing congenital abnormalities under aerobic conditions, and in stunting foetal growth under both conditions.     

Fast neutron r.b.e. for lethality and genotoxicity in a wild-type and a repair-deficient strain of yeast

The relative biological effectiveness (r.b.e.) of cyclotron-produced fast neutrons (11 MeV) in relation to 60Co gamma-rays, was studied in a wild-type and a DNA repair-deficient yeast strain for cell killing and genotoxicity. In the wild-type (D7) strain the r.b.e. varied from 2.7 to 4.1 for lethality, 2.8 to 7.1 for reverse mutation and 3.5 to 7.8 for mitotic gene conversion. At different survival levels, the repair deficient strain (D7 rad 52/rad 52) generally showed a lower r.b.e. for both cell killing and genotoxicity (25.2 to 37.2 per cent reduction for the cell death and 24.8 to 70.6 per cent for mutation and gene conversion) compared to the wild type. Except at very low dose levels, the r.b.e. values for cell killing and genotoxicity were similar within a given strain. At similar survival levels, neutrons were no more genotoxic than gamma-rays.     

Chromosome aberrations induced in human lymphocytes by radiation from 252Cf.

In vitro dose--response curves of unstable chromosome aberrations in human lymphocytes have been obtained for 252Cf neutron radiation. The aberration yields fitted best to the linear function Y=aD, which is consistent with the single-track model of aberration formation for high LET radiation. The curves have been compared with others previously produced in this laboratory for several energies of neutrons and for 60Co gamma radiation. The r.b.e. for 252Cf with respect to 60Co is 27 at very low doses, decreasing to 6 at an aberration yield equivalent to 400 rad of 18 rad/hour gamma radiation. A profile of chromosome-aberration induction with depth in a perspex phantom was obtained by placing blood samples at several distances over the range 0.65-2.0 cm from the californium source. This profile was compared with depth-damage calculations for a radium needle. The r.b.e. of 252Cf radiation relative to 226Ra gamma radiation increased with the distance from the source, implying that californium is more effective at greater distances in destroying the ability of cells to divide, which may be an advantage in the treatment of large tumours.     

Micronucleus induction in Vicia faba roots. Part 2. Biological effects of neutrons below 1 cGy

A dose-effect relationship has been established for high-energy neutrons (maximum energy 600 MeV) within a dose range of 0.2 to 80 cGy and for low-energy neutrons produced by a 252Cf source (mean energy 2.35 MeV) for doses between 0.2 and 5 cGy. The frequency of micronuclei was found to increase linearly with dose. The relative biological effectiveness (r.b.e) values calculated using 60Co radiation as a reference were, in the high-dose region, 4.7 +/- 0.4 and 11.8 +/- 1.3 for the high- and low-energy neutrons, respectively. At doses below 1 cGy constant values of 25.4 +/- 4.4 and 63.7 +/- 12 were reached for the respective neutron energies.     

Chromosome aberrations in human lymphocytes induced by fission neutrons

The dose-response relationships of dicentrics and excess acentrics were analysed after exposure of human lymphocytes to a mixed fission neutron-gamma-ray beam. From the analysis of exclusively first division cells a linear-quadratic relation was obtained for dicentrics with the ratio of linear and quadratic components, zeta, equal to 2.76 Gy. Over the range of doses studied (0.04-1.97 Gy) intratrack events therefore predominated. This also applied to acentrics which were linearly related to dose. At the lowest level of observed effect and dose, r.b.e. values with respect to 60Co gamma-rays of up to about 11 were derived for dicentrics and acentrics. With increasing neutron dose the r.b.e. decreased.     

The survival of mice and the metallothionein content of their livers and kidneys as criteria for assessing exposure to pulsed neutron radiation

The effect of pulsed neutron radiation was studied in comparison with continuous neutron radiation and continuous gamma-radiation. Animal survival and induction of metallothionein (MT) synthesis in liver and kidney of mice exposed to equivalent doses were chosen as criteria for evaluation of radiation effects. It was found that the level of MT in liver and kidney of mice exposed to neutron radiation decreased 24 hours after irradiation and then continued decreasing in kidney for 48 hours after irradiation. This is evidence of more intensive free-radical processes initiated by pulsed neutron radiation. At the same time, RBE values of pulsed neutrons did not differ significantly from that of continuous neutron radiation.     

Influence of radiation quality on the effectiveness of small doses for induction of reproductive death and chromosome aberrations in mammalian cells.

An analysis and interpretation is presented of published data concerning the dependence of radiobiological effectiveness on the radiation quality of photons, neutrons and heavy ions for the induction of these two effects in different types of mammalian cell. The results of this analysis suggest that chromosome aberrations observable at mitosis show a stronger dependence on YF or LET infinity than cell inactivation. At high YF, observable abberrations provide a major contribution to cell reproductive death induced by small doses. At low YF the effectiveness of small doses for cell death depends mainly on another type of damage, possibly also induced in the chromosomes, but not observable at mitosis. This type of damage depends less of YF or LET infinity than observable aberrations. The implications of these differences in damage in relation to radiation quality for the extrapolation of data on other types of damage to small doses of interest in radiation protection are discussed in relation to maximum r.b.e values observed.     

The effect of X-rays and neutrons on lymphocyte death and transformation.

The effects of X-rays and neutrons on human lymphocytes in vitro has been tested. Radiation sensitivity of untransformed lymphocytes was assessed by the appearance of pyknotic cells, and the response of cells after stimulation by phyto-haemagglutinin was tested (a) morphologically and (b) by changes in DNA synthesis, using a labelled thymidine analogue. The data obtained for interphase cells suggest that lymphocytes are a mixed cell population with an insensitive component forming about 20 per cent of the population. The percentage of normal cells observed after both X-ray and neutron irradiation lie on the same dose--effect curve giving an r.b.e. of one. A biphasic response is seen after PHA stimulation with both tests of damage indicating at least two sub-populations of lymphocytes and these give r.b.e. values in the range 1.95 to 2.45. Providing the in vivo response is similar to that in vitro the r.b.e. for damage to circulating lymphocytes will be small and the reduction in white cell count will not therefore be a major factor limiting dose in neutron therapy.     

Radiation carcinogenesis in experimental animals and its implications for radiation protection

Cancer induction is generally considered to be the most important somatic effect of low doses of ionizing radiation. It is therefore of great concern to assess the quantitative cancer risk of exposure to radiations of different quality and to obtain information on the dose-response relationships for carcinogenesis. Tissues in the human with a high sensitivity for cancer induction include the bone marrow, the lung, the thyroid and the breast in women. If the revised dosimetry estimates for the Japanese survivors of the atomic bomb explosions are correct, there is no useful data base left to derive r.b.e. values for human carcinogenesis. As a consequence, it will be necessary to rely on results obtained in biological systems, including experimental animals, for these estimates. With respect to radiation protection, the following aspects of experimental studies on radiation carcinogenesis are of relevance: Assessment of the nature of dose-response relationships. Determination of the relative biological effectiveness of radiations of different quality. Effects of fractionation or protraction of the dose on tumour development. For the analysis of tumour data in animals, specific approaches have to be applied which correct for competing risks. These methods include actuarial estimates, non-parametric models and analytical models. The dose-response curves for radiation-induced cancers in different tissues vary in shape. This is exemplified by studies on myeloid leukaemia in mice and mammary neoplasms in different rat strains. The results on radiation carcinogenesis in animal models clearly indicate that the highest r.b.e. values are observed for neutrons with energies between 0.5 and 1 MeV. On the basis of such results it might be concluded that the maximum quality factor of 10 for neutrons should be increased. Based on current evidence, an increase by a factor of 2 to 3 seems more realistic than a tenfold rise. The diversity of dose-response relationships point to different mechanisms involved in the induction of different tumours in various species and even in different strains of the same species.     

Neutron RBEs for mouse skin at low doses per fraction

The effect of low doses of 240 kVp X rays or of 3 MeV neutrons has been investigated using skin reactions on mouse feet as the biological system. Eight or nine repeated small doses of radiation were used, followed by graded "top-up" doses to bring the reactions into a detectable range. By comparing dose-response curves, the RBE has been determined for neutron doses per fraction ranging from 0.25-1.0 Gy. The data are consistent with a limiting RBE of between 7 and 10 at very low doses. A review of other published RBE values for low doses per fraction shows a wide range of RBEs . Very few studies show a plateau value for the RBE. These findings are more consistent with dose-response data that fit a linear-quadratic model than with a multitarget single-hit model.     

DNA strand breakage in Chinese hamster V79 cells caused by low levels of incorporated [3H] and [14C]thymidine

A sensitive alkali-unwinding assay was used to measure DNA strand breakage in Chinese hamster V79 cells caused by low-level incorporation of methyl-labelled [3H] and [14C] thymidine, and to estimate the effective dose per disintegration relative to low doses of gamma-irradiation. Damage equivalent to 0.0035 +/- 0.0006 and 0.0014 +/- 0.0005 Gy was observed for each 3H and 14C disintegration respectively. These values agree well with those expected from the estimated nuclear radiation dose delivered by the beta particles if a relative biological effect (r.b.e.) of 1.0 is assumed, and suggest that strand-breakage produced by these isotopes is determined by the nuclear radiation dose delivered by the beta particles.     

Mutation and inactivation of cultured mammalian cells exposed to beams of accelerated heavy ions. III. Human diploid fibroblasts.

The induction of inactivation and mutation to thioguanine-resistance in cultured human diploid fibroblasts was studied after exposure to ionising radiations with LET's in the range 20--470 keV micrometer-1. Unique r.b.e. values were obtained for inactivation and mutation induction with nine different qualities of radiation. The plot of r.b.e. verus LET gave humped curves for both endpoints; r.b.e. maxima were in the LET range 90--200 keV micrometer-1 but the maximum r.b.e. value for mutation induction was almost twice that for inactivation. The accuracy of estimates of mutation induction are discussed with regard to possible selective effects against mutants during post-irradiation growth.     

Cell inactivation and DNA single- and double-strand breaks in cultured mammalian cells irradiated by a thermal neutron beam

The effects on the cellular viability and induction and repair kinetics of DNA strand breaks in HeLa cells were examined after exposure to a thermal neutron beam and compared with those after gamma-irradiation. The thermal neutron survival curve had no initial shoulder. The relative biological effectiveness (r.b.e.) value of the neutron beam was determined to be 2.2 for cell killing (ratio of D0 values), 1.8 and 0.89 for single strand breakage (ssb) by alkaline sedimentation and alkaline elution respectively, and for double strand breakage (dsb) 2.6 by neutral elution. No difference was observed between thermal neutrons and gamma-rays in the repair kinetics of ssb and dsb. It is suggested that the effect induced by the intracellular nuclear reaction, 14N(n,p)14C is mainly responsible for the high r.b.e. values observed.     

Relative Biological Effectiveness of HZE Particles for Chromosomal Exchanges and Other Surrogate Cancer Risk Endpoints

The biological effects of high charge and energy (HZE) particle exposures are of interest in space radiation protection of astronauts and cosmonauts, and estimating secondary cancer risks for patients undergoing Hadron therapy for primary cancers. The large number of particles types and energies that makeup primary or secondary radiation in HZE particle exposures precludes tumor induction studies in animal models for all but a few particle types and energies, thus leading to the use of surrogate endpoints to investigate the details of the radiation quality dependence of relative biological effectiveness (RBE) factors. In this report we make detailed RBE predictions of the charge number and energy dependence of RBE’s using a parametric track structure model to represent experimental results for the low dose response for chromosomal exchanges in normal human lymphocyte and fibroblast cells with comparison to published data for neoplastic transformation and gene mutation. RBE’s are evaluated against acute doses of γ-rays for doses near 1 Gy. Models that assume linear or non-targeted effects at low dose are considered. Modest values of RBE (<10) are found for simple exchanges using a linear dose response model, however in the non-targeted effects model for fibroblast cells large RBE values (>10) are predicted at low doses <0.1 Gy. The radiation quality dependence of RBE’s against the effects of acute doses γ-rays found for neoplastic transformation and gene mutation studies are similar to those found for simple exchanges if a linear response is assumed at low HZE particle doses. Comparisons of the resulting model parameters to those used in the NASA radiation quality factor function are discussed.