Dose-response curves for late functional changes in the normal rat brain after single carbon-on doses evaluated by magnetic resonance imaging: influence of follow-up time and calculation of relative biological effectiveness

This study investigated late effects in the brain after irradiation with carbon ions using a rat model. Thirty-six animals were irradiated stereotactically at the right frontal lobe using an extended Bragg peak with maximum doses between 15.2 and 29.2 Gy. Dose-response curves for late changes in the normal brain were measured using T1- and T2-weighted magnetic resonance imaging (MRI). Tolerance doses were calculated at several effect probability levels and times after irradiation. The MRI changes were progressive in time up to 17 months and remained stationary after that time. At 20 months the tolerance doses at the 50% effect probability level were 20.3 +/- 2.0 Gy and 22.6 +/- 2.0 Gy for changes in T1- and T2-weighted MRI, respectively. The relative biological effectiveness (RBE) was calculated on the basis of a previous animal study with photons. Using tolerance doses at the 50% effect probability level, RBE values of 1.95 +/- 0.20 and 1.88 +/- 0.18 were obtained for T1- and T2-weighted MRI. A comparison with data in the literature for the spinal cord yielded good agreement, indicating that the RBE values for single-dose irradiations of the brain and the spinal cord are the same within the experimental uncertainty.     

Evaluation of the relative biological effectiveness of a clinical epithermal neutron beam using dog brain

This investigation was designed to determine the relative biological effectiveness (RBE) of an epithermal neutron beam (FiR 1 beam) using the brains of dogs. The FiR 1 beam was developed for the treatment of patients with glioma using boron neutron capture therapy. Comparisons were made between the effects of whole-brain irradiation with epithermal neutrons and 6 MV photons. For irradiations with epithermal neutrons, three dose groups were used, 9.4 +/- 0.1, 10.2 +/- 0.1 and 11.5 +/- 0.2 Gy. These physical doses were given as a single exposure and are quoted at the 90% isodose. Four groups of five dogs were irradiated with single doses of 10, 12, 14 or 16 Gy of 6 MV photons to the 100% isodose. Different reference isodoses were used to obtain the most comparable dose distribution in the brain for the two different irradiation modalities. Sequential magnetic resonance images (MRI) were taken for 77-115 weeks after irradiation to detect changes in the brain. Dose-effect relationships were established for changes in the brain as detected either by MRI or by subsequent gross morphology and histology. The doses that caused a specified response in 50% of the animals (ED(50)) were calculated from these dose-effect curves for each end point, and these values were used to calculate the RBE values for the different end points. The RBE values for the FiR 1 beam, based on changes observed on MRI, were in the range 1.2-1.3. For microscopic and gross pathological lesions, the values were in the range 1.2-1.4. The corresponding RBE values for the MRI and pathological end points for the high-LET components (protons from nitrogen capture and recoil protons from fast neutrons) were in the ranges 3.5-4.0 and 3.4-4.4, respectively. This assumed a dose-rate reduction factor of 0.6 for the low-dose-rate gamma-ray component of this beam. Finally, a comparison was made between experimentally derived photon doses, for a specified end point, with calculated photon equivalent doses, which were obtained using the weighting factors for clinical studies on the epithermal neutron beam on the Brookhaven Medical Research Reactor (BNL) in New York. This indicated that the radiation-induced lesions seen in the present study were, on average, detected at a 12% lower photon dose than predicted by the use of the BNL clinical weighting factors. This indicates the need for caution in the extrapolation of results from one reactor-based epithermal neutron beam to another.     

Lung tumour induction in mice after X-rays and neutrons

Dose-response curves were determined for pulmonary adenomas and adenocarcinomas in mice after single acute doses of 200 kVp X-rays and cyclotron neutrons (E = 7.5 MeV). A serial-killing experiment established that the radiation induces the tumours and does not merely accelerate the appearance of spontanoeus cancers [corrected]. The dose versus incidence (I) of tumours in male and female mice for X-ray doses between 0.25 and 7.5 Gy is 'bell-shaped' and best fitted with a purely quadratic induction and exponential inactivation terms, i.e. I = A + BD2e-alpha D. In contrast, the tumour dose-response after 0.1-4.0 Gy of neutrons is best fitted by I = A + BDe-alpha D and is steeply linear less than or equal to 1 Gy, peaks between 1 and 3 Gy and sharply declines at 4.0 Gy. The data for the female mice less than or equal to 1 Gy neutrons are best fitted to the square root of the dose. A major objective of the experiments was to derive neutron RBE values. Because of the differences between the X-ray (quadratic) and neutron (linear) curves, the RBEn will vary inversely with decreasing X-ray dose. The RBE values at 1 Gy of X-rays derived from the B coefficients in the above equations are 7.4 +/- 3.2 (male and female); 8.6 +/- 3.6 (female) and 4.7 +/- 1.8 (male). These are high values and imply even higher values at the doses of interest to radiation protection. If, however, one restricts the analysis to the initial, induction side of the response (less than or equal to 1 Gy neutrons, less than or equal to 3 Gy X-rays) then good linear fits are obtainable for both radiations and indicate neutron RBE values of 7.4 +/- 2.3 for female mice and 4.5 +/- 1.8 for males, and these are independent of dose level.     

Relative cataractogenic effects of X rays, fission-spectrum neutrons, and 56Fe particles: a comparison with mitotic effects

The eyes of Sprague-Dawley rats were irradiated with doses of 2.5-10 Gy 250-kVp X rays, 1.25-2.25 Gy fission-spectrum neutrons (approximately 0.85 MeV), or 0.1-2.0 Gy 600-MeV/A 56Fe particles. Lens opacifications were evaluated for 51-61 weeks following X and neutron irradiations and for 87 weeks following X and 56Fe-particle irradiations. Average stage of opacification was determined relative to time after irradiation, and the time required for 50% of the irradiated lenses to achieve various stages (T50) was determined as a function of radiation dose. Data from two experiments were combined in dose-effect curves as T50 experimental values taken as percentages of the respective T50 control values (T50-% control). Simple exponential curves best describe dose responsiveness for both high-LET radiations. For X rays, a shallow dose-effect relationship (shoulder) up to 4.5 Gy was followed at higher doses by a steeper exponential dose-effect relationship. As a consequence, RBE values for the high-LET radiations are dose dependent. Dose-effect curves for cataracts were compared to those for mitotic abnormalities observed when quiescent lens epithelial cells were stimulated mechanically to proliferate at various intervals after irradiation. Neutrons were about 1.6-1.8 times more effective than 56Fe particles for inducing both cataracts and mitotic abnormalities. For stage 1 and 2 cataracts, the X-ray Dq was 10-fold greater and the D0 was similar to those for mitotic abnormalities initially expressed after irradiation.     

Incidence of radiation-induced skin tumours in mice and variations with dose rate

Mice were exposed to weakly penetrating beta-particles from an external source, using 12 different surface doses ranging from 5.4 to 260 Gy and given at four different dose rates from 200 to 1.7 cGy/min. As in previous investigations, both epidermal and dermal tumours occurred with the latter predominating. The lowest surface dose to produce a statistically significant increase in skin tumours was 21.7 Gy, no effect being detected with doses of 5.4-16.3 Gy. The dose-response curves rose steeply when obvious increases occurred. Consideration of these findings and the fact that radiation-induced skin tumours can have an exceptionally long latent period leads to the suggestion that there is some relatively radioresistant factor which normally restrains potential radiation-induced cancer cells in the skin from becoming tumours until the skin is subjected to high local doses. Tumour-induction was unaffected by reducing the highest dose rate by a factor of 10 and the dose-response curves were almost identical. Further reductions of dose rate, encompassing a further factor of 10, in general resulted in fewer tumours.     

Radiation tolerance of the rat spinal cord after single and split doses of photons and carbon ions

The sensitivity of the rat spinal cord to single and split doses of radiation and the resulting relative biological effectiveness (RBE) were determined for carbon-ion irradiations (12C) in the plateau and Bragg-peak regions. The cranial part of the cervical and thoracic spinal cords of 180 rats were irradiated with one or two fractions of 12C ions or photons, respectively. Dose-response curves for the end point symptomatic myelopathy were established, and the resulting values for the ED50 (dose for 50% complication probability) were used to determine the RBEs. A median latency for myelopathy of 167 days (range, 121-288 days) was found. The ED50 values were 17.1 +/- 0.8 Gy, 24.9 +/- 0.7 Gy (one and two fractions, 12C plateau) and 13.9 +/- 0.8, 15.8 +/- 0.7 Gy (one and two fractions, 12C Bragg peak), respectively. For photons we obtained ED50 values of 24.5 +/- 0.8 Gy for single doses and 34.2 +/- 0.7 Gy when two fractions were applied. The corresponding RBEs were 1.43 +/- 0.08, 1.37 +/- 0.12 (one and two fractions, 12C plateau) and 1.76 +/- 0.05, 2.16 +/- 0.11 (one and two fractions, 12C Bragg peak), respectively. Hematoxylin and eosin staining revealed necrosis of the white matter in the spinal cord in all symptomatic animals. In summary, from one- and two-fraction photon, 12C plateau and Bragg-peak irradiation of the rat spinal cord, we have established RBEs as well as the individual ED50's. From the latter there is a clear indication of repair processes for fractionated photons and 12C plateau ions which are significantly reduced by using Bragg-peak ions. Additional studies are being carried with 6 and 18 fractions to further refine and define the RBE and ED50 values and estimate the alpha/beta ratios.     

Dose dependency of FISH-detected translocations in stable and unstable cells after Cs gamma irradiation of human lymphocytes in vitro

Human peripheral lymphocytes were exposed to 137Cs gamma-rays (0-4.3 Gy) in order to check the impact of unstable cells on the dose-response curve for translocations. Chromosomes 2, 4 and 8 were FISH-painted. 17,720 first dividing cells were analysed. For the discrimination between stable and unstable cells the painted and the counter-stained chromosomes were analysed at doses of 1 Gy and higher. The cell distribution of translocations follows a Poisson distribution. The data were fitted to the linear-quadratic function, y = c + alphaD + betaD2. As expected, the alpha coefficients of the dose-response curves for translocations in stable cells or in total cells do not differ. However, at doses >1 Gy, the frequency of all translocations in stable cells seems to be lower than the frequency in total cells. For the establishment of calibration curves for past dose assessment purposes, only complete translocations should be scored, in order to estimate reliable doses.     

Choice of model and uncertainties of the gamma-ray and neutron dosimetry in relation to the chromosome aberrations data in Hiroshima and Nagasaki

Chromosome data pertaining to blood samples from 1,703 survivors of the Hiroshima and Nagasaki A-bombs, were utilized and different models for chromosome aberration dose response investigated. Models applied included those linear or linear-quadratic in equivalent dose. Models in which neutron and gamma doses were treated separately (LQ-L model) were also used, which included either the use of a low-dose limiting value for the relative biological effectiveness (RBE) of neutrons of R(0)=70+/-10 or an RBE value of R(1)=15+/-5 at 1 Gy. The use of R(1) incorporates the assumption that it is much better known than R(0), with much less associated uncertainty. In addition, error-reducing transformations were included which were found to result in a 50% reduction of the standard error associated with one of the model fit parameters which is associated with the proportion of cells with at least one aberration, at 1 Gy gamma dose. Several justifiable modifications to the DS86 doses according to recent nuclear retrospective dosimetry measurements were also investigated. Gamma-dose modifications were based on published thermoluminescence measurements of quartz samples from Hiroshima and on a tentative reduction for Nagasaki factory worker candidates by a factor of 0.6. Neutron doses in Hiroshima were modified to become consistent with recent fast neutron activation data based on copper samples. The applied dose modifications result in an increase in non-linearity of the dose-response curve for Hiroshima, and a corresponding decrease in that for Nagasaki, an effect found to be most pronounced for the LQ-L models investigated. As a result the difference in the dose-response curves observed for both cities based on DS86 doses, is somewhat reduced but cannot be entirely explained by the dose modifications applied. The extent to which the neutrons contribute to chromosome aberration induction in Hiroshima depends significantly on the model used. The LQ-L model including an R(1) value of 15 at 1 Gy which is recommended here, would predict between 10% and 20% of the observed chromosome aberrations to be due to neutrons, at all doses. Because of the good agreement between DS86 predictions and the results of retrospective gamma and neutron dosimetry, the modifications applied here to DS86 doses are relatively small. Consequently, the choices of model and RBE values were found to be the major factors dominating the interpretation of the chromosome data for Hiroshima and Nagasaki, with the dose modifications resulting in a smaller influence.     

Magnetic resonance imaging and spectroscopy of the rat hippocampus 1 month after exposure to 56Fe-particle radiation

The response of the central nervous system to space radiation is largely unknown. The hippocampus, which is known for its critical role in learning and memory, was evaluated for its response to heavy-ion radiation. At 1 month, animals exposed to brain-only 56Fe-particle irradiation (0-4 Gy) were examined using contrast-enhanced T1 imaging (CET1), T2-weighted imaging (T2WI), diffusion weighted imaging (DWI), and (1)H-magnetic resonance spectroscopy (MRS). Correlative histology was performed after imaging. The T2WI, DWI and CET1 images revealed no overt anatomical changes after irradiation. Quantitative analysis demonstrated a significant increase in T2 at 2 Gy compared to 0 Gy. The apparent diffusion coefficient (ADC) revealed an inverse dose-dependent quantitative change in water mobility. Compared to 0 Gy, the ADC increased 122% at 1 Gy and declined to 44% above control levels at 4 Gy. MRS showed a significant increase in the N-acetylaspartate/choline ratio at 4 Gy and a lactate peak. Histology demonstrated no overt pathological changes in neuronal and astrocyte populations. However, a significant inverse dose-dependent morphological change in the microglial population was detected in irradiated animals. Our results suggest that early tissue matrix modifications induced by 56Fe-particle radiation can be detected by MRI in the absence of evident histopathology. These changes may indicate fundamental changes in the structure and function of the hippocampus.     

Functional and histological changes in rat lung after boron neutron capture therapy

The motivation for this work was an unexpected occurrence of lung side effects in two human subjects undergoing cranial boron neutron capture therapy (BNCT). The objectives were to determine experimentally the biological weighting factors in rat lung for the high-LET dose components for a retrospective assessment of the dose to human lung during cranial BNCT. Lung damage after whole-thorax irradiation was assessed by serial measurement of breathing rate and evaluation of terminal lung histology. A positive response was defined as a breathing rate 20% above the control group mean and categorized as occurring either early (<110 days) or late (>110 days). The ED(50) values derived from probit analyses of the early breathing rate dose-response data for X rays and neutrons were 11.4+/-0.4 and 9.2+/-0.6 Gy, respectively, and were similar for the other end points. The ED(50) values for irradiation with neutrons plus p-boronophenylalanine were 8.7+/-1.0 and 6.7+/-0.4 for the early and late breathing rate responses, respectively, and 7.0+/-0.5 Gy for the histological response. The RBEs for thermal neutrons ranged between 2.9+/-0.7 and 3.1+/-1.2 for all end points. The weighting factors for the boron component of the dose differed significantly between the early (1.4+/-0.3) and late (2.3+/-0.3) breathing rate end points. A reassessment of doses in patients during cranial BNCT confirmed that the maximum weighted doses were well below the threshold for the onset of pneumonitis in healthy human lung.     

Saturation of DNA repair measured by alkaline elution

To determine whether the half-times (T1/2) of the DNA repair processes measured by alkaline elution increased in a dose-dependent manner, exponentially growing 9L/Ro rat brain tumor cells were irradiated with doses of 15-50 Gy, and their DNA repair kinetics was measured by alkaline elution. At 15 Gy, the DNA repair kinetics was biphasic with the fast phase having a T1/2 approximately 6 min and the slow phase having a T1/2 approximately 42 min. As the dose was increased to 50 Gy, the fast-phase T1/2 remained at approximately 6 min, but the slow-phase T1/2 increased to approximately 87 min. Although a dose-dependent increase in the T1/2 of the slow phase of DNA repair (saturation) was measured by alkaline elution, both the absolute value of the slow-phase T1/2 and the dependency of the slow-phase T1/2 on dose were less than those measured by alkaline sucrose gradient sedimentation in zonal rotors with slow reorienting gradient capability. Thus these two techniques appear either to depend on different hydrodynamic properties of the DNA or to have different coefficients of dependency for the same hydrodynamic properties of the DNA. The lower sensitivity for detection of the dose dependency of DNA repair makes it unlikely that the alkaline elution technique will be useful for quantitatively relating the shape of mammalian cell survival curves to the doses at which saturation of a DNA repair process occurs.     

Nuclear scintigraphic assessment of radiation-induced intestinal dysfunction

Radiation-induced damage to the intestine can be measured by abnormalities in the absorption of various nutrients. Changes in intestinal absorption occur after irradiation because of loss of the intestinal absorptive surface and a consequent decrease in active transport. In our study, the jejunal absorption of (99m)Tc-pertechnetate, an actively transported gamma-ray emitter, was assessed in C3H/Kam mice given total-body irradiation with doses of 4, 6, 8 and 12.5 Gy and correlated with morphological changes in the intestinal epithelium. The absorption of (99m)Tc-pertechnetate from the intestinal lumen into the circulation was studied with a dynamic gamma-ray-scintigraphy assay combined with a multichannel analyzer to record the radiometry data automatically in a time-dependent regimen. The resulting radioactivity-time curves obtained for irradiated animals were compared to those for control animals. A dose-dependent decrease in absorptive function was observed 3.5 days after irradiation. The mean absorption rate was reduced to 78.8 +/- 9.3% of control levels in response to 4 Gy total-body irradiation (mean +/- SEM tracer absorption lifetime was 237 +/- 23 s compared to 187 +/- 12 s in nonirradiated controls) and to 28.3 +/- 3.7% in response to 12.5 Gy (660 +/- 76 s). The decrease in absorption of (99m)Tc-pertechnetate at 3.5 days after irradiation correlated strongly (P < 0.001) with TBI dose, with the number of cells per villus, and with the percentage of cells in the crypt compartment that were apoptotic or mitotic. A jejunal microcolony assay showed no loss of crypts and hence no measured dose-response effects after 4, 6 or 8 Gy TBI. These results show that dynamic enteroscintigraphy with sodium (99m)Tc-pertechnetate is a sensitive functional assay for rapid evaluation of radiation-induced intestinal damage in the clinically relevant dose range and has a cellular basis.     

Effects of single gamma-irradiation on the activity of ornithine decarboxylase in the thymus and pulmonary tissue]

In studying the dose (0.1-6 Gy) and time (2 h to 180 days) dependence of ornithine decarboxylase activity, it was found that deviations from the control were more pronounced in the thymus than in the pulmonary tissue. The radiation effect was a function of dose and time after irradiation. A nonmonotonous type of the dose-response curve was observed 7 days after irradiation: the radiation effect with a low dose (0.1 Gy) was opposite to that with sublethal doses (1-6 Gy).     

Neutron RBE for primate spinal cord treated with clinical regimens.

The RBEs of high-energy neutrons given in 9 or 12 fractions for cervical spinal cord injury in rhesus monkeys was determined using photons at 2.2 Gy per fraction as the reference radiation. Because the dose-response functions were not parallel, the RBE was not constant but rather increased with dose or, equivalently, with the probability of myelopathy. This required the development of a novel method of determining the RBE versus level of response. The RBE is presented as a function of probability of myelopathy from 0.1 to 99%. At a 50% incidence of myelopathy, the RBE (+/- 1 SE) was 5.22 +/- 0.15. A difference in the histopathology of lesions induced by photon and neutron treatments was observed.     

A functional and chemical study of radiation effects on rat parotid and submandibular/sublingual glands.

The aim of this study was to monitor composition and rate of secretion of rat parotid and submandibular/sublingual saliva following local single doses of X-rays ranging from 5 to 20 Gy. Pilocarpine-stimulated samples of parotid and submandibular/sublingual saliva were simultaneously collected with miniaturized Lashley cups before and 1-30 days after irradiation. The lag phase (period between injection of pilocarpine and start of the secretion) and flow rate were recorded and the concentrations of sodium, potassium, calcium, phosphate, and amylase were measured. With increasing dose and time, the salivary flow rate as well as sodium concentration decreased, while potassium concentrations increased throughout the follow-up period. The lag phase and the concentration of amylase reached their maximum at 3 and 10 days after irradiation, respectively. The changes in lag phase and flow rate were most obvious after doses of 15 or 20 Gy and showed a great similarity for parotid and submandibular/sublingual saliva. No dose-response relationship was observed for the changes in concentrations of calcium and phosphate. It is concluded that for radiation doses of 10 Gy and above, irreversible changes (lag phase, flow rate, potassium, sodium) were observed. A saturation of the irradiation effects (lag phase, flow rate) seems to exist at doses larger than 15 Gy. No significant differences were observed between the radiation-induced functional changes in parotid and submandibular/sublingual salivary gland tissue.     

Ionic fluxes induced by topical misoprostol in canine gastric mucosa

We studied the dose response of ionic fluxes in canine chambered gastric segment mucosa to increasing doses of topical misoprostol (0.1, 1, 10, 100, and 1000 micrograms). The fluxes were also correlated with the simultaneous changes in focal gastric mucosal blood flow measured by laser-Doppler flowmetry. After misoprostol administration, there was a dose-dependent increase in focal gastric mucosal blood flow (Emax = 8.23 +/- 3.25 V at 10 micrograms; ED50 = 1.05 micrograms), pH, and the outputs of ions (Na+, K+, Cl-, and HCO3-) and fluid (Emax for pH and fluxes greater than or equal to 1000 micrograms). ED50 values for these outputs ranged from 215.40 to 340 micrograms (mean +/- SE = 279.08 +/- 24.27 micrograms). H+ output showed a dose-dependent decrease to zero at the 10-micrograms dose, the dose at and after which net HCO3- secretion became obvious. The slopes of the dose-response curves for the fluxes of fluid, Na+, K+, Cl-, and HCO3- were significantly different (p less than 0.01) from the slope of the curve for mucosal blood flow changes. There were no correlations between the changes in these fluxes and blood flow changes. Na+ and Cl- were the predominant cation (98.84%) and anion (98.19%), respectively, in the misoprostol-induced secretion. Misoprostol stimulates a composite alkaline gastric nonparietal secretion, predominantly Na+ and Cl-, but also containing K+ and HCO3-. Our results suggest different mechanisms for the effects on nonparietal secretion and focal gastric mucosal blood flow.(ABSTRACT TRUNCATED AT 250 WORDS)     

The question of threshold doses for radiation damage: Malformations induced by radiation exposure of unicellular or multicellular preimplantation stages of the mouse

Mouse embryos of the one-cell stage or the 32- to 64-cell stage were exposed to various X-ray doses (one-cell stage: 0.25-2 Gy; 32- to 64-cell stage: 1–3 Gy). It turned out that the shape of the dose-response curves is statistically compatible with the assumption derived from biological considerations that there is no threshold for radiation-induced malformations in the case of the exposure of one-cell embryos, whereas there is a threshold dose (close to 1 Gy) in the case of the exposure of 32- to 64-cell embryos.