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.     

The effectiveness of monoenergetic neutrons at 565 keV in producing dicentric chromosomes in human lymphocytes at low doses

The induction of dicentric chromosomes in human lymphocytes from one individual irradiated in vitro with monoenergetic neutrons at 565 keV was examined to provide additional data for an improved evaluation of neutrons with respect to radiation risk in radioprotection. The resulting linear dose-response relationship obtained (0.813 +/- 0.052 dicentrics per cell per gray) over the dose range of 0.0213-0.167 Gy is consistent with published results obtained for irradiation with neutrons from different sources and with different spectra at energies lower than 1000 keV. Comparing this value to previously published "average" dose-response curves obtained by different laboratories for (60)Co gamma rays and orthovoltage X rays resulted in maximum RBEs (RBE(m)) of about 37 +/- 8 and 16 +/- 4, respectively. However, when our neutron data were matched to low-LET dose responses that were constructed several years earlier for lymphocytes from the same individual, higher values of RBE(m) resulted: 76.0 +/- 29.5 for (60)Co gamma rays and 54.2 +/- 18.4 for (137)Cs gamma rays; differentially filtered 220 kV X rays produced values of RBE(m) between 20.3 +/- 2.0 or 37.0 +/- 7. 1. The results highlight the dependence of RBE(m) on the choice of low-LET reference radiation and raise the possibility that differential individual response to low-LET radiations may need to be examined more fully in this context.     

A radiobiological model for the relative biological effectiveness of high-dose-rate 252Cf brachytherapy

While there is significant clinical experience using both low- and high-dose-rate 252Cf brachytherapy, there are minimal data regarding values for the neutron relative biological effectiveness (RBE) with both modalities. The aim of this research was to derive a radiobiological model for 252Cf neutron RBE and to compare these results with neutron RBE values used clinically in Russia. The linear-quadratic (LQ) model was used as the basis to characterize cell survival after irradiation, with identical cell killing rates (S(N) = S(gamma)) between 252Cf neutrons and photons used for derivation of RBE. Using this equality, a relationship among neutron dose and LQ radiobiological parameter (i.e., alpha(N), beta(N), alpha(gamma), beta(gamma)) was obtained without the need to specify the photon dose. These results were used to derive the 252Cf neutron RBE, which was then compared with Russian neutron RBE values. The 252Cf neutron RBE was determined after incorporating the LQ radiobiological parameters obtained from cell survival studies with fast neutrons and teletherapy photons. For single-fraction high-dose-rate neutron doses of 0.5, 1.0, 1.5 and 2.0 Gy, the total biologically equivalent doses were 1.8, 3.4, 4.7 and 6.0 RBE Gy with 252Cf neutron RBE values of 3.2, 2.9, 2.7 and 2.5, respectively. Using clinical data for late-responding reactions from 252Cf, Russian investigators created an empirical model that predicted high-dose-rate 252Cf neutron RBE values ranging from 3.6 to 2.9 for similar doses and fractionation schemes and observed that 252Cf neutron RBE increases with the number of treatment fractions. Using these relationships, our results were in general concordance with high-dose-rate 252Cf RBE values obtained from Russian clinical experience.     

Renal damage in the mouse: the effect of d(4)-Be neutrons

A further study on the response of the mouse kidney to d(4)-Be neutrons (EN = 2.3 MeV) is described. The results confirm and augment the work published previously by Stewart et al. [Br. J. Radiol. 57, 1009-1021 (1984)]; the present paper includes the data from a "top-up" design of experiment which extends the measurements of neutron RBE (relative to 240 kVp X rays) down to X-ray doses of 0.75 Gy per fraction. The mean RBE for these neutrons increases from 5.8 to 7.3 as X-ray dose per fraction decreases from 3.0 to 1.5 Gy in the kidney. This agrees with the predictions from the linear quadratic (LQ) model, based on the renal response to X-ray doses above 4 Gy per fraction. The mean RBE estimate from a single dose group at 0.75 Gy per fraction of X rays is, however, 3.9. This is below the LQ prediction and may indicate increasing X-ray sensitivity at low doses. Data from this study and from those published previously have been used to determine more accurately the shape of the underlying response to d(4)-Be neutrons; an alpha/beta ratio of 20.5 +/- 3.7 Gy was found. The best value of alpha/beta for X rays determined from these experiments was 3.04 +/- 0.35 Gy, in agreement with previous values.     

The characterization and transmissibility of chromosome aberrations induced in peripheral blood lymphocytes by in vitro alpha-particle radiation

Peripheral blood lymphocytes were irradiated in vitro with (213)Bi alpha particles at doses of 0, 10, 20, 50, 100, 200 and 500 mGy. Chromosome analysis was performed on 47-h cultures using single-color fluorescence in situ hybridization (FISH) to paint chromosomes 1, 3 and 5. The whole genome was analyzed for unstable aberrations to derive aberration frequencies and determine cell stability. The dose response for dicentrics was 33.60 +/- 0.47 x 10(-2) per Gy. A more detailed analysis revealed that the majority of aberrations scored as dicentrics were part of complex/multiple aberrations, with the proportion of cells containing complexes increasing with dose. Cells containing aberrations involving painted chromosomes (FISH aberrations) were further classified according to cell stability and complexity. The majority of cells with FISH aberrations were unstable. The proportion of aberrant FISH cells with complex/multiple aberrations ranged from 56% at 10 mGy to 89% at 500 mGy. A linear dose response for genomic frequencies of translocations in stable cells fitted the data from 0 to 200 mGy with a dose response of 7.90 +/- 0.98 x 10(-2) per Gy, thus indicating that they are likely to be observed in peripheral blood lymphocytes from individuals with past or chronic exposure to high-LET radiation. Comparisons with the dose response for low-LET radiation suggest an RBE of 13.6 for dicentrics in all cells and 3.2 for translocations in stable cells. Since stochastic effects of radiation are attributable to genetic changes in viable cells, translocations in stable cells may be a better measure when considering the comparative risks of different qualities of radiation.     

Ex vivo determination of the effect of whole-body exposure to fast neutrons on murine spleen cell viability and apoptosis

The effects of high-linear energy transfer (LET) radiations on lymphoid tissues and lymphocytes are not well understood. As a first approach to delineate these effects, the present work was conducted to assess the effects of high-LET radiations on murine spleen cells ex vivo and in vitro. BALB/c mice were irradiated whole-body with 65 MeV neutrons or 15 MV X rays at doses ranging from 0.2 to 3 Gy. Spleens were removed 1 day postirradiation and weighed, and single cell suspensions were prepared and cultured for several days. Apoptosis occurring in vitro was determined at different times by flow cytometry analysis of cells labeled with propidium iodide. It was found that irradiation with fast neutrons reduced spleen weight and cellularity to a greater extent than photons. Considering the spleen cellularity as end point, the relative biological effectiveness (RBE) of fast neutrons was 2. However, for both modes of irradiation, apoptosis of recovered spleen cells in vitro increased as a function of dose and the duration of culture. The level of apoptosis occurring at various times postirradiation was found to be identical for high- and low-LET radiations. Taken together, these results suggest that external as well as cellular factors might differentially modulate the sensitivity of lymphocytes to fast neutrons and photons.     

Relative biological effectiveness and tolerance dose of fission neutrons in canine skin for a potential combination of neutron capture therapy and fast-neutron therapy

To investigate the potential efficacy of fission neutrons from a fast-neutron reactor for the treatment of radioresistant tumors, the relative biological effectiveness (RBE) and tolerance dose of fission neutrons in canine skin were determined. The forelimbs of 34 healthy mongrel dogs received a single dose of fission neutrons (5.6, 6.8, 8.2, 9.6 or 11 Gy) or 137Cs gamma rays (10, 15, 20, 25 or 30 Gy). Based on observations of radiodermatitis for each radiation, the single-fraction RBE of fission neutrons in the sixth month was calculated as approximately 3. The tolerance doses of fission neutrons and gamma rays, defined as the highest doses giving no moist desquamation on the irradiated skin in the recovery phase, were estimated as 7.6 Gy and 20 Gy, respectively. The tolerance dose of 7.6 Gy of fission neutrons included 5.0 Gy of fast neutrons possessing high anti-tumor effects and 1.4 x 10(12) n/cm2 of thermal neutrons, which could be applicable to neutron capture therapy (NCT). The combination of fast-neutron therapy and NCT using a fast-neutron reactor might be useful for the treatment of radioresistant tumors.     

Induction of minisatellite mutations in the mouse germline by low-dose chronic exposure to gamma-radiation and fission neutrons

Germline mutation induction at mouse minisatellite loci by paternal low-dose (0.125-1 Gy) exposure to chronic (1.66 x 10(-4) Gy min(-1)) low-linear energy transfer (low-LET) gamma-irradiation and high-LET fission neutrons (0.003 Gy min(-1)) was studied at pre-meiotic stages of spermatogenesis. Both types of radiation produced linear dose-response curves for mutation of the paternal allele. In contrast to previous results using higher doses, the pattern of induction of minisatellite mutation after chronic gamma-irradiation was similar to acute (0.5 Gy min(-1)) exposure to X-rays, indicating that the elevated mutation rate was independent of the ability of the cell to repair damage induced immediately or over a period of up to 100 h. Chronic exposure to fission neutrons was more effective than acute or chronic low-LET exposure (relative biological effectiveness, RBE=3.36). The data also provide strong support for the previous conclusion that increases in minisatellite mutation rate are not caused by radiation-induced DNA damage at minisatellite loci themselves, but rather from damage induced by ionising radiation elsewhere in the genome/cell.     

On the question of RBE reversal at high doses.

We present theoretical arguments to explain observations of a "reversal" of the RBE at relatively large doses; that is, the RBE of high-LET vs low-LET radiation is less than one. Numerical examples are given and the results of Bogo et al. (Radiat. Res. 118, 341-352, 1989) are discussed qualitatively.     

High yields of isochromatid breaks and successive formation of chromosome exchanges may lead to reproductive cell death following high-LET irradiation

To clarify the relationship between cell death and chromosomal aberrations following exposure to heavy-charged ion particles beams, exponentially growing Human Salivary Gland Tumor cells (HSG cells) were irradiated with various kinds of high energy heavy ions; 13 keV/μm carbon ions as a low-LET charged particle radiation source, 120 keV/μm carbon ions and 440 keV/μm iron ions as high-LET charged particle radiation sources. X-rays (200 kVp) were used as a reference. Reproductive cell death was evaluated by clonogenic assays, and the chromatid aberrations in G2/M phase and their repairing kinetics were analyzed by the calyculin A induced premature chromosome condensation (PCC) method. High-LET heavy-ion beams introduced much more severe and un-repairable chromatid breaks and isochromatid breaks in HSG cells than low-LET irradiation. In addition, the continuous increase of exchange aberrations after irradiation occurred in the high-LET irradiated cells. The cell death, initial production of isochromatid breaks and subsequent formation of chromosome exchange seemed to be depend similarly on LET with a maximum RBE peak around 100–200 keV/μm of LET value. Conversely, un-rejoined isochromatid breaks or chromatid breaks/gaps seemed to be less effective in reproductive cell death. These results suggest that the continuous yield of chromosome exchange aberrations induced by high-LET ionizing particles is a possible reason for the high RBE for cell death following high-LET irradiation, alongside other chromosomal aberrations additively or synergistically.     

Characterization of neutron beams for boron neutron capture therapy: in-air radiobiological dosimetry

The survival curves and the RBE for the dose components generated in boron neutron capture therapy (BNCT) were determined separately in neutron beams at Japan Research Reactor No. 4. The surviving fractions of V79 Chinese hamster cells with or without 10B were obtained using an epithermal neutron beam (ENB), a mixed thermal-epithermal neutron beam (TNB-1), and a thermal (TNB-2) neutron beam; these beams were used or are planned for use in BNCT clinical trials. The cell killing effect of the neutron beam in the presence or absence of 10B was highly dependent on the neutron beam used and depended on the epithermal and fast-neutron content of the beam. The RBEs of the boron capture reaction for ENB, TNB-1 and TNB-2 were 4.07 +/- 0.22, 2.98 +/- 0.16 and 1.42 +/- 0.07, respectively. The RBEs of the high-LET dose components based on the hydrogen recoils and the nitrogen capture reaction were 2.50 +/- 0.32, 2.34 +/- 0.30 and 2.17 +/- 0.28 for ENB, TNB-1 and TNB-2, respectively. The RBEs of the neutron and photon components were 1.22 +/- 0.16, 1.23 +/- 0.16, and 1.21 +/- 0.16 for ENB, TNB-1 and TNB-2, respectively. The approach to the experimental determination of RBEs outlined in this paper allows the RBE-weighted dose calculation for each dose component of the neutron beams and contributes to an accurate inter-beam comparison of the neutron beams at the different facilities employed in ongoing and planned BNCT clinical trials.     

Relative biological effectiveness of 144 keV neutrons in producing dicentric chromosomes in human lymphocytes compared with 60Co gamma rays under head-to-head conditions

The RBE for neutrons was assessed in a head-to-head experiment in which cultures of lymphocytes from the same male donor were irradiated simultaneously with 144 keV neutrons and with 60Co gamma rays as the reference radiation and evaluated using matched time, culture conditions, and the end point of chromosomal aberrations to avoid potential confounding factors that would influence the outcome of the experiment. In addition, the irradiation time was held constant at 2 h for the high-dose groups for both radiation types, which resulted in rather low dose rates. For the induction of dicentric chromosomes, the exposure to the 144 keV neutrons was found to be almost equally as effective (yield coefficient alpha(dic) = 0.786 +/- 0.066 dicentrics per cell per gray) as that found previously for irradiation with monoenergetic neutrons at 565 keV (alpha(dic) = 0.813 +/- 0.052 dicentrics per cell per gray) under comparable exposure and culture conditions (Radiat. Res. 154, 307-312, 2000). However, the values of the maximum low-dose RBE (RBE(m)) relative to 60Co gamma rays that were determined in the present and previous studies show an insignificant but conspicuous difference: 57.0 +/- 18.8 and 76.0 +/- 29.5, respectively. This difference is mainly due to the difference in the alpha(dic) value of the 60Co gamma rays, the reference radiation, which was 0.0138 +/- 0.0044 Gy(-1) in the present study and 0.0107 +/- 0.0041 Gy(-1) in the previous study. In the present experiment, irradiations with 144 keV neutrons and 60Co gamma rays were both performed at 21 degrees C, while in the earlier experiment irradiations with 565 keV neutrons were performed at 21 degrees C and the corresponding reference irradiation with gamma rays was performed at 37 degrees C. However, the temperature difference between 21 degrees C and 37 degrees C has a minor influence on the yield of chromosomal alterations and hence RBE values. The large cubic PMMA phantom that was used for the gamma irradiations in the present study results in a larger dose contribution from Compton-scattered photons compared to the mini-phantom used in the earlier experiments. The contribution of these scattered photons may explain the large value of alpha(dic) for gamma irradiation in the present study. These results indicate that the yield coefficient alpha(dic) for 144 keV neutrons is similar to the one for 565 keV neutrons, and that modification of the alpha(dic) value of the low-LET reference radiation, due to changes in the experimental conditions, can influence the RBE(m). Consequently, alpha(dic) values cannot be shared between cytogenetic laboratories for the purpose of assessment of RBM(m) without verification of the comparability of the experimental conditions.     

Relative biological effectiveness of high-energy iron ions for micronucleus formation at low doses

Dose-response curves for micronucleus (MN) formation were measured in Chinese hamster V79 and xrs6 (Ku80(-)) cells and in human mammary epithelial MCF10A cells in the dose range of 0.05-1 Gy. The Chinese hamster cells were exposed to 1 GeV/nucleon iron ions, 600 MeV/nucleon iron ions, and 300 MeV/nucleon iron ions (LETs of 151, 176 and 235 keV/microm, respectively) as well as with 320 kVp X rays as reference. Second-order polynomials were fitted to the induction curves, and the initial slopes (the alpha values) were used to calculate RBE. For the repair-proficient V79 cells, the RBE at these low doses increased with LET. The values obtained were 3.1 +/- 0.8 (LET = 151 keV/microm), 4.3 +/- 0.5 (LET = 176 keV/microm), and 5.7 +/- 0.6 (LET = 235 keV/microm), while the RBE was close to 1 for the repair-deficient xrs6 cells regardless of LET. For the MCF10A cells, the RBE was determined for 1 GeV/nucleon iron ions and was found to be 5.5 +/- 0.9, slightly higher than for V79 cells. To test the effect of shielding, the 1 GeV/nucleon iron-ion beam was intercepted by various thicknesses of high-density polyethylene plastic absorbers, which resulted in energy loss and fragmentation. It was found that the MN yield for V79 cells placed behind the absorbers decreased in proportion to the decrease in dose both before and after the iron-ion Bragg peak, indicating that RBE did not change significantly due to shielding except in the Bragg peak region. At the Bragg peak itself with an entrance dose of 0.5 Gy, where the LET is very high from stopping low-energy iron ions, the effectiveness for MN formation per unit dose was decreased compared to non-Bragg peak areas.     

Induction of preneoplastic alterations by X rays and neutrons in exposed rat tracheas and isolated tracheal epithelial cells

The relative potential of high- and low-LET radiation to induce preneoplastic alterations in rat tracheal epithelial cells was evaluated using a combined in vivo-cell culture model. The capacity of X rays and high- and low-dose-rate neutrons to induce preneoplastic changes in isolated rat tracheal epithelial cells and in the intact tissue was compared. The presence of altered populations was determined in culture in terms of the frequency of tracheal epithelial cell populations which exhibit enhanced growth capacity in culture and in terms of the induction of persistent morphological alterations in exposed transplanted tracheas. Prior to assaying for altered cells, tracheal epithelial cells were irradiated as part of the intact tissue or as single cells. Low- and high-LET radiation induced similar maximum frequencies of altered cells when cultures were exposed as single cells, although high-LET radiation was more radiobiologically effective (RBE = 20) than low-LET radiation. The most marked difference between high- and low-LET radiation was observed after irradiation of the intact tissue. Damage induced by low-LET radiation, giving rise to altered populations, was modified in the intact tissue, whereas similar damage induced by high-LET radiation was apparently not.     

RBE of densely ionizing radiation for wild-type and radiosensitive mutants of yeast

Survival curves were obtained for haploid and diploid yeasts, Saccharmyces cerevisiae, of wild-type strains and radiosensitive mutants exposed to γ-rays and α-particles. A correlation between the values of the relative biological effectiveness (RBE) of high-LET radiation and cell-repair capacity was found. The difference in radiosensitivities of the wild-type diploid strain and homozygous rad mutants incapable of recovery was significantly higher after low-LET radiation than after high-LET radiation. Possible reasons for the observed radiation responses to low- and high-LET exposure of yeast cells with various genotype are discussed.     

Relative biological effectiveness (RBE) of low-dose californium-252

Relative biological effectiveness (RBE) of 252Cf, with respect to 192Ir, has been determined at the low dose rates commonly used in interstitial and intracavitary therapy. The biological criterion was growth reduction in Vicia faba bean roots. Two varieties of Vicia faba were used. For Vicia faba Sutton's seeds, an RBE of 5.7 to 6.6 was obtained for 252Cf Dn + gamma doses of 0.5 to 0.2 Gy respectively and at a Dn + gamma dose rate of 0.11 Gy-1. The gamma contribution D gamma/Dn + gamma at the level of the root tipes was 0.35 and the derived RBE of the neutron emission of 252Cf was then 8.2 to 9.7. For Vicia faba Be1B and in the same irradiation conditions, an RBE of 5.1 to 6.2 was obtained for the total (n + gamma) 252Cf emission and for Dn + gamma doses of 0.4 to 0.2 Gy respectively. These values lead to an RBE of 7.4 to 9.0 for the neutron emission of 252Cf. For Vicia faba BelB, but for another source arrangement (Dn + gamma dose rate of 0.13 Gy . h-1 for 252Cf), an RBE of 5.6 to 7.5 was obtained for the total (n + gamma) emission of 252Cf and for Dn + gamma doses of 0.4 to 0.1 Gy respectively. The gamma contribution (D gamma/Dn + gamma) at the level of the root tips was 0.42, and the derived RBE of the neutron emission of 252Cf was then 8.9-12.3.     

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.     

Sequential exposures of mammalian cells to low- and high-LET radiations. II. As a function of cell-cycle stages

The synergistic effects of low- and high-LET radiations were further studied with partially synchronized Chinese hamster V79 cells. Principally, nearly monoenergetic 425 MeV/u neon ions and 570 MeV/u argon ions produced near the Bragg peak were employed as the high-LET radiations and 225 kVp X rays as the low-LET counterpart. It was found that the killing effect due to damage interaction after sequential irradiations with the particle beam and X rays varies throughout the cell cycle. The greatest effect was observed in late-S phase which was most resistant to either of the radiations. The effect was quantitatively less in the G1/S border and in G2. Effects on pure mitotic cells have not been investigated in this study. For all cell stages studied, a dose of high-LET particles modified the shape of the X-ray survival curve in a way similar to the modification predicted by an appropriately selected X-ray dose. This finding suggests that the mechanism for the synergistic effects is similar to that operating for sequential treatments with X rays alone. Experiments with an S population, either incubated at 37 degrees C or room temperature between fractionation of high- and low-LET radiation treatments further verified that the damage involved is a repairable type. At a certain fractionation interval (6 to 8 h) following a dose of high-LET treatment, initially asynchronous cells were found to be very sensitive to X-irradiation. It is noteworthy that the net killing measured at this "radiosensitive window" was as effective as the killing observed by "immediately" sequential treatments with the same doses of high- and low-LET radiations. Such a time window also existed when the order of the treatment sequence was reversed except that the time of occurrence was earlier and the window was broader. This sensitization effect may be explained by radiation-induced G2 arrest together with an increase of radiosensitivity as the previously irradiated cells progress into S phase. Radiotherapy strategies using combined high-LET and low-LET radiations for rapidly proliferative tumors are presented.     

Relative biological effectiveness of fission neutrons for induction of micronucleus formation in mouse reticulocytes in vivo

Following whole-body irradiation of ICR mice with various doses of fission neutrons or X-rays, the frequency of micronuclei (MNs) in peripheral blood reticulocytes was measured at 12 h intervals beginning immediately after irradiation and ending at 72 h after irradiation. The resulting time-course curve of MN frequency had a clear peak 36 h after irradiation, irrespective of the type of radiation applied and the dose used. The MN frequency, averaged as the unweighted mean over the experimental time course, showed a linear increase with increasing dose of either fission neutrons or X-rays. The linear response to X-rays supports reported conclusion that induction of MN formation in reticulocytes is a dose-rate independent phenomenon. The relative biological effectiveness (RBE) of fission neutrons to X-rays for MN induction was estimated to be 1.9 +/- 0.3. This value is considerably lower than the RBE value of 4.6 +/- 0.5 reported for the same fission neutrons for induction of lymphocyte apoptosis in the thymus of ICR mice that represents dose-rate independent, one-track event. Based on these results, we propose that MNs increased in reticulocytes after irradiation mostly represent acentric fragments caused by single chromosome breaks, and that some confounding factor is operating in erythroblasts for the formation of aberrations from non-rejoining DNA double-strand breaks more severely after high-LET radiation than after low-LET radiation.