Some long-term effects of negative pions in mice exposed to partial body irradiation

The long-term effects of partial body exposure of one-day-old mice given either 60Co gamma rays or negative pions have been studied. Both radiations produced considerable life-shortening; for pions 6.8 +/- 1.5% of life is lost per 100 rad and for gamma rays the value is 5.7 +/- 0.5% per 100 rad. The RBE of pions for ten weeks of life-shortening is about 1.3 compared with 60Co gamma rays, although at lower doses the RBE may be higher reaching about two for six weeks of life shortening. The incidence rate of tumours at any particular age was greater in mice irradiated with pions at the peak and in those given higher doses of gamma rays than in the controls.     

Some in vivo effects of pi mesons in mice

The effect of 70 MeV pi mesons was studied to determine the effectivness of such beams aginst normal tissues in vivo. The end points included thymic weight loss, oocyte and bone marrow CFU-S survival and the induction of macroscopid lens opacities. The results indicate that pi mesons are not signifacantly more effective for these end-points than more conventional radiation sources such as 60Co gama rays, 220 kVp X rays, and 14 MeV X rays and electrons. Nor was there any detectable difference in RBE between the peak and plateau regions of the pi meson beam. The significance of these findings is discussed in relation of the the published pi meson RBE values of between 1 therefore 4 and 5 therefore 0.     

Neutron RBEs for cytopenia and repopulation of stroma and hematopoietic stem cells: mathematical models of marrow cell kinetics

The objectives of this study were to (a) extend previous bone-marrow cell kinetics models that have been published for ionizing photons to include neutron radiations, and (b) provide Relative Biological Effectiveness (RBE) values for time-specific cell killing (cytopenia) and compensatory cellular proliferation (repopulation in response to toxic injury) for neutron doses ranging from 0.01 to 4.5 Gy delivered uniformly over one minute, hour, day, week, and month. RBEs for cytopenia of a cell lineage were based on ratios of protocol-specific doses that determined the same cell population nadir, whereas the RBEs for repopulation of a lineage were based on the ratios of protocol-specific doses that corresponded to the same total number of cells killed over the radiation treatments, and which should be replaced for long-term survival of the animal. Time-dependent RBEs were computed for neutron exposures relative to the effect of 60Co gamma rays given as a prompt dose. By the use of these RBE factors, low or variable dose rates, dose fractionations given over long periods of time, and different protocols involving several radiation qualities can be converted realistically, and by standard convention, into an equivalent dose of a reference radiation comprised of x or gamma rays given either as a pulse or at any other reference dose rate for which risk information based on epidemiological or animal dose-response data are available. For stromal tissues irradiated by fission neutrons, time-dependent RBEs for cytopenia were computed to range from 4.24 to 0.70 and RBEs for repopulation varied from a high of 6.88 to a low of 2.24. For hematopoietic stem cells irradiated by fission neutrons, time-dependent RBEs for cytopenia were computed to range from 5.02 to 0.22 and RBEs for repopulation varied from a high of 5.02 to a low of 1.98. RBEs based on tissue-kerma-free-in-air would be about twofold lower for isotropic cloud or rotational exposure geometries because marrow dose from isotropic neutron fields suffer factor-of-two greater attenuation than the gamma doses from gamma photons. For certain doses and dose rates, the RBE values computed for compensatory cellular proliferation clearly demonstrate the behavior that is commonly referred to as an inverse dose-rate effect, i.e., protraction of exposure may-under certain conditions-increase the magnitude of the dose response. Furthermore, because of non-linear rates for repair and repopulation, the highest RBEs are not necessarily found for the lowest doses nor the lowest RBEs always found at the highest doses.     

Chromosome aberrations in human fibroblasts induced by monoenergetic neutrons. I. Relative biological effectiveness

The relative biological effectiveness (RBE) of neutrons for many biological end points varies with neutron energy. To test the hypothesis that the RBE of neutrons varies with respect to their energy for chromosome aberrations in a cell system that does not face interphase death, we studied the yield of chromosome aberrations induced by monoenergetic neutrons in normal human fibroblasts at the first mitosis postirradiation. Monoenergetic neutrons at 0.22, 0.34, 0.43, 1, 5.9 and 13.6 MeV were generated at the Accelerator Facility of the Center for Radiological Research, Columbia University, and were used to irradiate plateau-phase fibroblasts at low absorbed doses from 0.3 to 1.2 Gy at a low dose rate. The reference low-LET, low-dose-rate radiation was 137Cs-gamma rays (0.66 MeV). A linear dose response (Y = alphaD) for chromosome aberrations was obtained for all monoenergetic neutrons and for the gamma rays. The yield of chromosome aberrations per unit dose was high at low neutron energies (0.22, 0.34 and 0.43 MeV) with a gradual decline with the increase in neutron energy. Maximum RBE (RBEm) values varied for the different types of chromosome aberrations. The highest RBE (24.3) for 0.22 and 0.43 MeV neutrons was observed for intrachromosomal deletions, a category of chromosomal change common in solid tumors. Even for the 13.6 MeV neutrons the RBEm (11.1) exceeded 10. These results show that the RBE of neutrons varies with neutron energy and that RBEs are dissimilar between different types of asymmetric chromosome aberrations and suggest that the radiation weighting factors applicable to low-energy neutrons need firmer delineation. This latter may best be attained with neutrons of well-defined energies. This would enable integrations of appropriate quality factors with measured radiation fields, such as those in high-altitude Earth atmosphere. The introduction of commercial flights at high altitude could result in many more individuals being exposed to neutrons than occurs in terrestrial workers, emphasizing the necessity for better-defined estimates of risk.     

0.3 keV carbon K ultrasoft X-rays are four times more effective than gamma-rays when inducing oncogenic cell transformation at low doses

Oncogenic transformation and inactivation were investigated in C3H10T1/2 mouse embryo fibroblasts exposed to proton-induced 0.28 keV carbon K (CK)-characteristic X-rays and 60Co gamma-rays as reference radiation at high dose-rate (2-3 and 0.7 Gy/min respectively). Both oncogenic cell transformation and cell inactivation followed a linear-quadratic relationship with dose. At low doses where the linear component dominates CK ultrasoft X-rays were more effective, by a factor of 4, at inducing oncogenic cell transformation and cell inactivation compared with 60Co gamma-rays. For both endpoints the RBE of CK ultrasoft X-rays gradually decreased with increasing dose mainly due to the greater quadratic component for 60Co gamma-rays compared with CK ultrasoft X-rays. Our experimental data are in agreement with the hypothesis that single DNA double-strand breaks (dsbs), which are induced by 0.28-keV ultrasoft CK X-rays, may lead to oncogenic cell transformation. With increasing absorbed dose, i.e. with decreasing mean distance between dsbs induced by 0.28-keV ultrasoft X-rays, oncogenic cell transformation and cell inactivation may also be induced by interaction between those dsbs.     

Changes in RBE of 14-MeV (d + T) neutrons for V79 cells irradiated in air and in a phantom: is RBE enhanced near the surface?

PURPOSE: The relative biological effectiveness (RBE) for inactivation of V79 cells was determined as function of dose at the Heidelberg 14-MeV (d + T) neutron therapy facility after irradiation with single doses in air and at different depths in a therapy phantom. Furthermore, to assess the reproducibility of RBE determinations in different experiments we examined the relationship between the interexperimental variation in radiosensitivity towards neutrons with that towards low LET 60Co photons. METHODS: Clonogenic survival of V79 cells was determined using the colony formation assay. The cells were irradiated in suspension in small volumes (1.2 ml) free in air or at defined positions in the perspex phantom. Neutron doses were in the range, Dt = 0.5-4 Gy. 60Co photons were used as reference radiation. RESULTS: The radiosensitivity towards neutrons varied considerably less between individual experiments than that towards photons and also less than RBE. However, the mean sensitivity of different series was relatively constant. RBE increased with decreasing dose per fraction from RBE = 2.3 at 4 Gy to RBE = 3.1 at 0.5 Gy. No significant difference in RBE could be detected between irradiation at 1.6 cm and 9.4 cm depth in the phantom. However, an approximately 20% higher RBE was found for irradiation free in air compared with inside the phantom. Combining the two effects, irradiation with 0.5 Gy free in air yielded an approximately 40% higher RBE than a dose of 2 Gy inside the phantom. CONCLUSION: The measured values of RBE as function of dose per fraction within the phantom is consistent with the energy of the neutron beam. The increased RBE free in air, however, is greater than expected from microdosimetric parameters of the beam and may be due to slow recoil protons produced by interaction of multiply scattered neutrons or to an increased contribution of alpha particles from C(n, alpha) reactions near the surface. An enhanced RBE in subcutaneous layers of skin combined with an increase in RBE at low doses per fraction outside the target volume could potentially have significant consequences for normal tissue reactions in radiotherapy patients treated with fast neutrons.     

Scanning electron microscopy of mouse intestinal mucosa after cobalt 60 and D-T neutron irradiation

The stem-cell population of the intestinal crypt is an important model system in experimental radiobiology. Standardized techniques have been developed to allow quantitation of the response of crypt cells to radiation injury following doses of 0-2 krad of D-T neutrons or 60Co gamma rays. These techniques rely on the identification of regenerating crypt cells three-and-a-half days after irradiation. The results are expressed as the number of regenerating crypts per circumference of small intestine, as determined by conventional histological examination; the more profound the injury, the smaller the crypt count. The practical relevance of crypt-counting techniques to clinical radiotherapy is limited by their relative insensitivity; the dose levels commonly used in fractionated radiotherapy produce no detectable response. Scanning electron microscopy of the mucosal surface provides a more sensitive measure of radiation injury. The earliest detectable changes occur at the level of 300 rad of gamma radiation, well below the threshold of the crypt-counting technique. At around 1,000 rad, where the first drop in crypt counts occurs, there are well-marked morphological changes which become more severe with increasing dose levels. Some differences have been observed between the morphological effects of gamma and neutron irradiation at points of radiobiological equivalence in terms of crypt counts (using an RBE value of about 2). The changes observed may reflect more than the disruption of epithelial cell kinetics. Mucosal morphology is the total expression of many different biological parameters of which the regenerative ability of the crypt cells is only one. The surface microanatomy of the gut may be the most sensitive indicator of radiation injury which is conveniently available for study.     

Comparative evaluation of markers of bone resorption in patients with breast cancer-induced osteolysis before and after bisphosphonate therapy

The induction of the his(-)-->his+ mutants in vegetative and spores of Bacillus subtilis wild-type cells irradiated with gamma rays and helium ions (LET = 20-80 keV/micron) has been investigated. It was shown that the dose dependence of the mutation induction in vegetative cells is described by a linear-quadratic function of dose in case of both gamma-rays and helium ions. RBE (LET) dependencies on the lethal and mutagenic effect of irradiation have a local maximum. The maximum of RBE (LET) dependence on the mutagenic assay is shifted at the low region of LET in comparison with the lethal effect of irradiation.     

Response of HeLa cells to irradiation with pi- mesons

HeLa cell suspensions were irradiated at various depths of Perspex in a beam of 70 MeV pi- mesons. The dose-rate in the Bragg peak varied between 40 rad hour-1 and 150 rad hour-1. The cells were assayed in vitro for loss of reproductive integrity. The results for irradiation in the peak indicated an RBE of about 2-1 when compared to the response obtained using 60Co gamma-rays at comparable dose-rates. At dose-rates of 100-150 rad hour-1, the RBE peak-to-plateau was about 1-4, while at lower dose-rates, the RBE peak-to-plateau was about 1.     

Radioprotective effects of diltiazem on cytogenetic damage and survival in gamma ray exposed mice

Diltiazem, a calcium ion channel blocker, already in use in cardiovascular therapeutics, has been observed to protect against bone marrow damage (cytogenetic damage, cell death) and mortality in whole body irradiated mice. The micronuclei fraction in bone marrow cells of whole body irradiated (60Co gamma rays, 2.0 Gy) mice was reduced from 2.24 +/- 0.23% to about 0.74 +/- 0.33% by preirradiation administration (-20 min) of 110 mg/kg body wt. diltiazem (ip). Endogenous colony forming unit counts in spleen of mice administered 110 mg/kg body wt. (-20 min) of diltiazem before 10 Gy whole body irradiation were 6 times more than untreated irradiated controls. Pretreatment with diltiazem accelerated the recovery of radiation induced weight loss also. Diltiazem (110 mg/kg body wt, -20 min) enhanced 30 day survival to about 95% and 85% after lethal whole body absorbed dose of 9 and 10 Gy respectively and also mitigated radiation induced life- span shortening. Post-irradiation (10 Gy) administration of diltiazem (+20 to 30 min) enhanced survival from about 2 to 15% only but was highly significant (P < 0.001). Possible modes of radioprotective action of diltiazem have been discussed.     

Postnatal survival and growth of mouse irradiated in utero with low dose

In order to investigate radiation risks associated with low dose and low dose-rates, pregnant Swiss albino mice were exposed to gamma rays, 0.80 Gy from a cobalt-60 source at two different dose-rates (0.0795 and 0.0012 Gy/min) on 18 day post conception. In females exposed to lower dose-rate (0.0012 Gy/min), litter size was found to be decreased, while those exposed to higher dose-rate (0.0795 Gy/min), it remained unaltered. In both groups, appearance of fur and development of complete fur were delayed, whereas gait was delayed only in higher dose-rate group. Male offspring exhibited a biphasic mode of weight loss, while female offspring after an initial weight loss at 1 week, displayed a continuous recovery, but could not attain the normal weight till 12 weeks of age. It appears that higher dose-rate is more effective in delaying the appearance of physiological markers and weight loss, while in terms of litter size lower dose-rate (0.0012 Gy/min) is more effective.     

Characteristics of neutron beam generated by 500 MeV proton beam

A neutron irradiation facility was constructed at PARMS, University of Tsukuba to produce an ultrahigh energy neutron beam with a depth dose distribution superior to an x-ray beam generated by a modern linac. This neutron beam was produced from the reaction on a thick uranium target struck by a 500 MeV proton beam from the booster synchrotron of the High Energy Physics Laboratory. The percentage depth dose of this neutron beam was nearly equivalent to that of x-rays around 20 MV and the dose rate was 15 cGy per minute. The relative biological effectiveness (RBE) of this neutron beam has been estimated using the cell inactivation effect and the HMV-I cell line. The survival curve of cells after neutron irradiation has a shoulder with n and Dq of 8 and 2.3 Gy, respectively. The RBE value at the 10(-2) survival level for the present neutron beam as compared with 137Cs gamma rays was 1.24. The results suggest that the biological effects of ultrahigh energy neutrons are not large enough to be useful, although the depth dose distribution of neutrons can be superior to that of high energy linac x-rays.     

Synthesis of a potent wide-spectrum serotonin-, norepinephrine-, dopamine-reuptake inhibitor (SNDRI) and a species-selective dopamine-reuptake inhibitor based on the gamma-amino alcohol functional group

Mortality data for B6CF1 mice exposed to 60Co gamma rays for the duration of life were used to make quantitative predictions of age-specific mortality observed in comparably exposed beagles. Simple Kaplan-Meier survival curves for the beagles and their 95% confidence intervals were computed for each dose-rate group observed. A dose-response equation was estimated from the mortality data for mice using a proportional hazard model. The dose-response model for mice was then used to generate predicted survivorship curves at dose rates that would recreate the dose burdens observed in the beagle at comparable points within the life span of the two organisms. When these predicted survivorship curves were scaled to adjust for species differences in the life span of control animals, the predictions for the mouse fell within the confidence intervals observed for the beagle. The successful interspecies extrapolation of age-specific mortality risks for species as different as the mouse and dog enhances both the value of studies involving laboratory animals and the potential relevance of the animal studies to the prediction of health effects in humans.     

Expression and rapid purification of recombinant rat calretinin: similarity to native rat calretinin

Therapeutic fast neutrons are densely ionizing particles, with a high relative biological effectiveness relative to 60Co gamma rays (RBE) and a low oxygen enhancement ratio (OER). The molecular basis of their properties is not yet entirely understood. In a previous work, we have shown that neutrons induce a different number of DNA frank strand breaks as compared to gamma photons, and we have revealed the presence of breaks due to the direct effects of neutrons. In the present work, we searched for eventual differences in the chemical nature of the attacked sites in DNA irradiated in oxygenated diluted solution. We compare our results with neutrons to those previously reported by other authors using gamma- or X-rays. Using sequencing gel electrophoresis of short natural DNA restriction fragments, or synthetic oligonucleotides, we have shown that, in the case of neutrons, the attack occurs with almost the same probability, at each nucleotide, as reported for gamma- and X-rays. The doubling of bands in the bottom of gels shows the presence of two types of termini, the 3'-phosphate and the 3'-phosphoglycolate. Upon neutron irradiation, the 3'-phosphate end appears with a higher yield than the 3'-phosphoglycolate, whereas equal amounts were obtained with gamma- or X-rays.     

RBE of neutrons generated by 50 MeV deuterons on beryllium for control of artificial pulmonary metastases of a mouse fibrosarcoma

Artificial pulmonary metastases of a mouse fibrosarcoma were produced by the intravenous injection of 10(4) cells admixed with 2 X 10(6) plastic microspheres into mice preconditioned with 600 rad whole-body irradiation 24 hours earlier. Four days after injection of tumour cells, mice were irradiated with neutrons generated by 50 MeV deuterons on Be at the Texas A & M Variable Energy Cyclotron or with 137Cs gamma rays. One, three or six fractions of radiation were delivered on a three-hour fractionation schedule. Surviving lung metastases were scored macroscopically 16 days after irradiation. The data indicate that: (1) the RBE (n/gamma) was in the range 1.6-2.6 depending on the size of dose per fraction; (2) the slopes of the gamma-ray curves decreased with increasing fraction number (i.e. decreasing fraction size); (3) the slopes of the neutron curves decreased only slightly with increasing fraction number (and decreasing fraction size); (4) no additional sparing was achieved by further fractionating doses of neutrons of 300 rad or less.     

A comparative study of dosimetric properties of Plastic Water and Solid Water in brachytherapy applications

In this study the dosimetric properties of Plastic Water and Solid Water phantom materials are evaluated using Monte Carlo photon transport simulations. In particular, their water-equivalence with respect to absorption and attenuation of photons in the brachytherapy energy range are examined. For the given chemical compositions of the materials, the linear attenuation coefficients were calculated for photons of 1 keV-2 MeV. Moreover, absorbed doses to water in each phantom material were calculated at distances of 0.5-12.0 cm from point sources of 20 keV to 60Co gamma rays. These results show that at low photon energies (below 100 keV), there are substantial differences (up to a factor of 5) between the absorbed dose in Plastic Water and that in liquid water. The differences decrease as photon energy increases, and they become insignificant at 60Co gamma rays, as claimed by the manufacturer. In contrast, calculations show that the difference in absorbed dose in Solid Water from that in liquid water, over the entire range of photon energies employed in this study, is less than 25%. The results of this study demonstrate the necessity of careful dosimetric evaluation of a new phantom material, before its clinical application, particularly in energy ranges outside those referred to by the manufacturer.     

A comparison between combined neutron- and telecobalt-therapy with telecobalt-therapy alone for cancer of the bronchus

The difference in response of human tumours to high and low LET radiation has been investigated in a series of inoperable, histologically confirmed bronchial carcinomas. One hundred and forty-nine were treated with low LET radiation alone (60Co gamma rays) and 108 with a combination of gamma rays and fast neutrons of mean energy 6 MeV, one-fifth to one-third of the effective dose being from neutrons. The response was analysed by histological examination of the autopsy specimens. Tumour cell destruction was found to be significantly greater in the neutron-treated series. The two series were not strictly randomized but were closely similar to terms of tumour volume, histological grade and total treatment time. The sequence of treatments with neutrons and gamma rays (N-gamma, gamma-N, gamma-N -gamma) was found to have no influence on the results.     

Cytogenetic effects of space radiation in lymphocytes of MIR-18 crews

For assessing health risk, the measurement of physical dose received during a space mission, as well as the LETs, energies and charges of particles is important. It is also important to obtain quantitative information regarding the effectiveness of space radiation in causing damage to critical biological targets, e.g., chromosomes, since at present the estimated uncertainty of biological effects of space radiation is more than a factor of two. Such large uncertainty makes accurate health risk assessment very difficult. For this very reason, a study on cytogenetic effects of space radiation in human lymphocytes was proposed and done for MIR-18 mission. This study used FISH technique to score chromosomal translocations and C-banding method to determine dicentrics. Growth kinetics of cells and SCE were examined to ensure that chromosomal aberrations were scored in first mitosis and were induced not by chemical mutagens. Our results showed that chromosomal aberration frequency of post-flight samples was significantly higher than that of pre-flight ones and that SCE frequency was similar between pre- and post-flight samples. Based on a dose-response curve of preflight samples exposed to gamma rays, the absorbed dose received by crews during the mission was estimated to be about 14.5 cSv. Because the absorbed dose measured by physical dosimeters is 4.16 cGy for the entire mission, the RBE is about 3.5.     

The use of Compton scattered gamma rays for tomography

A method of tomographic imaging of soft tissues by the use of Compton scattering of gamma rays is described. A highly collimated source (800 Ci of 60Co) and collimated detectors are used. Spatial resolutions of the order of 0.5 cm or better are achieved, with a density resoution of 2.0% to 4%. Experimental tests with dogs have shown internal organs in sagittal, coronal and transverse section. No contrast or radioactive material is required. Tests have shown that the regional dose for humans will be about 0.25 to 1 rad for one scan.     

Radiation fields backscattered from material interfaces: I. Biological effectiveness

Confluent cultures of CHO-K1 and CHO-xrs5 cells were irradiated attached to 6 microm Mylar with 137Cs gamma rays and 200 kVp X rays adjacent to scattering materials consisting of polystyrene, glass, aluminum, copper, tin and lead. The absorbed dose in cell nuclei was estimated from measurements of backscattered dose made with a parallel-plate ion chamber with a 5-microm Mylar window and a gas volume whose thickness was equivalent to approximately 2.6 microm of cells or tissue. Cell inactivation after various doses was measured by clonogenic assays after trypsinization and enumeration. Survival curves constructed from data pooled from at least two independent experiments were best fitted to a linear-quadratic (LQ) or a linear equation for CHO-K1 and CHO-xrs5 cells, respectively. An average distance of 9.3+/-1.9 microm from the scattering surfaces to the midline of nuclei for both the cell lines was estimated from electron micrographs of fixed cell sections. The major differences in biological effect observed when the cells were irradiated adjacent to these materials could be largely explained by the differences in the physical dose. Further analyses using the LQ equation suggested additional biological effects with implications for the mechanisms involved. CHO-K1 cells showed a small but consistent increase in the low-dose (alpha-inactivation coefficient) mechanism for both radiations scattered from high-Z material. An increased value of the alpha coefficient suggests an increase in RBE which could be associated with a higher proportion of low-energy and track-end electrons in these fields. The radiation fields which produced maximum single-hit killing in CHO-K1 cells also produced less killing by the quadratic (beta-inactivation coefficient) mechanism. In contrast, when similarly irradiated, CHO-xrs5 cells exhibited significantly lower alpha coefficients of inactivation. The mechanistic basis for this opposite effect of backscattered radiations in these cell lines is as yet unknown.