A partial-body irradiation model with approximately 2.5% bone marrow sparing (PBI/BM2.5) was established to determine the radiation dose-response relationships for the prolonged and delayed multi-organ effects of acute radiation exposure. Historically, doses reported to the entire body were assumed to be equal to the prescribed dose at some defined calculation point, and the dose-response relationship for multi-organ injury has been defined relative to the prescribed dose being delivered at this point, e.g., to a point at mid-depth at the level of the xiphoid of the non-human primate (NHP). In this retrospective-dose study, the true distribution of dose within the major organs of the NHP was evaluated, and these doses were related to that at the traditional dose-prescription point. Male rhesus macaques were exposed using the PBI/BM2.5 protocol to a prescribed dose of 10 Gy using 6-MV linear accelerator photons at a rate of 0.80 Gy/min. Point and organ doses were calculated for each NHP from computed tomography (CT) scans using heterogeneous density data. The prescribed dose of 10.0 Gy to a point at midline tissue assuming homogeneous media resulted in 10.28 Gy delivered to the prescription point when calculated using the heterogeneous CT volume of the NHP. Respective mean organ doses to the volumes of nine organs, including the heart, lung, bowel and kidney, were computed. With modern treatment planning systems, utilizing a three-dimensional reconstruction of the NHP's CT images to account for the variations in body shape and size, and using density corrections for each of the tissue types, bone, water, muscle and air, accurate determination of the differences in dose to the NHP can be achieved. Dose and volume statistics can be ascertained for any body structure or organ that has been defined using contouring tools in the planning system. Analysis of the dose delivered to critical organs relative to the total-body target dose will permit a more definitive analysis of organ-specific effects and their respective influence in multiple organ injury.

Conventional daily administration of filgrastim is effective in reducing the duration of severe neutropenia and enhancing survival following lethal radiation, myelosuppressive cytotoxic therapy or myeloablation and stem cell transplantation. A sustained-duration form of filgrastim, pegfilgrastim has significantly simplified scheduling protocols after chemotherapy-induced neutropenia to a single injection while maintaining the therapeutic effectiveness of daily administration of filgrastim. We examined the ability of a single or double (weekly) administration of pegfilgrastim to significantly improve neutrophil recovery in a rhesus macaque model of severe radiation-induced myelosuppression. Animals were exposed to potentially lethal 6 Gy total-body X radiation. After irradiation all animals received supportive care and were administered either pegfilgrastim at 300 μg/kg on day 1 or day 1 and day 7 post exposure, or filgrastim at 10 μg/kg/day initiated on day 1 post exposure and continued daily through neutrophil recovery. Pharmacokinetic parameters and neutrophil-related values for duration of neutropenia, neutrophil nadir, time to recovery to an absolute neutrophil count ≥500/μL or ≥2000/μL, and days of antibiotic support were determined. Effective plasma concentrations of pegfilgrastim were maintained in neutropenic animals until after the onset of hematopoietic recovery, which is consistent with neutrophil-dependent properties of elimination. Administration of pegfilgrastim at day 1 and day 7 was most effective at improving neutrophil recovery compared to daily administration of filgrastim or a single injection of pegfilgrastim on day 1, after severe, radiation-induced myelosuppression in rhesus macaques.

Acute lethality syndromes produced by the accidental exposure of humans to mixed neutron and γ radiation from external sources can be related to acute lethality from photon irradiation using the relative biological effectiveness (RBE) for common end points. We used the canine as a model to study injury following exposure to mixed neutron and γ radiation from the AFRRI TRIGA reactor. Exposures from the reactor were steady-state mode (40 cGy/min, bilateral) with an average neutron energy of 0.85 MeV; tissue-air ratio = 0.59 at midline abdominal. Healthy male and female canines were irradiated free-in-air behind a 6-in. lead wall; the neutron-γ ratio was 5.4:1 at the entrance skin surface; exposures are reported as midline tissue doses. Bilateral exposure resulted in an ${\rm LD}_{50/30}$ of 153 cGy without therapeutic clinical support. Addition of clinical support consisting of fluids, antibiotics, and fresh irradiated platelets/whole blood increased the bilateral ${\rm LD}_{50/30}$ to 185 cGy, a dose modifying factor (DMF) of 1.21. This corresponds to respective ${\rm LD}_{50/30}$ values for bilateral 60 Co γ exposures of 260 and 338 cGy for nonsupported and clinically supported animals, and a DMF of 1.30. The RBE based on the values determined at midline tissue is approximately 1.69. Clinical support after bilateral irradiation produced a similar DMF to those of mixed fission neutrons and γ rays and 60 Co γ rays alone. The RBE of 1.69 for midline tissue bilateral exposures is higher than 1, an RBE often cited for large animals. Therapeutic support administered to lethally irradiated canines significantly improved survival and increased the ${\rm LD}_{50/30}$ independent of radiation quality.

An experimental model has been developed for quantitative studies of radiobiological damage to the canine small intestine following partial-body nonuniform irradiation. Animals were irradiated with 60 Co γ rays to simulate the nonuniform irradiation which do occur in victims of radiation accidents. The model used a short source-to-surface distance for unilateral irradiations to produce a dose gradient of a factor of two laterally across the canine intestinal region. The remainder of the animal's body was shielded to prevent lethal damage to the bone marrow. In situ dosimetry measurements were made using thermoluminescent dosimeters to determine the radiation dose delivered as a function of position along a segment of the small intestine. This system made it possible to correlate the radiation dose delivered at a specific point along the small intestine with the macroscopic and microscopic appearance of the intestinal mucosa at that point, as determined by direct observation and biopsy using a fiberoptic endoscope. A key feature of this model is that dosimetry data for multiple sites, which receive a graded range of radiation doses, can be correlated with biological measurements to obtain a dose-response curve. This model is being used to evaluate the efficacy of new therapeutic procedures to improve survival following nonuniform irradiation.

To investigate the effects of nonuniform irradiation on the small intestine, we prepared 24 dogs for continent isoperistaltic ileostomies under aseptic surgical conditions and general anesthesia. After a 3-week recovery period, the ileum was catheterized with a fiberoptic endoscope to observe the intestinal mucosa and to harvest mucosal biopsies. The baseline macroscopic and microscopic appearance of the intestinal mucosa was determined. Two weeks later, the ileum was catheterized with a 100-cm soft tube containing 40 groups of three thermoluminescent dosimeters placed at equally spaced intervals, and a dose of either 4.5, 8, 10, 11, or 15 Gy 60 Co γ rays was delivered to the right abdomen (nonuniform exposure). This method allowed a direct and precise assessment of the dose received at 40 sites located in the 100-cm intestinal segment. The intestinal mucosa was again evaluated 1, 4, and 6 days after irradiation. All animals exposed to 4.5 and 8 Gy survived, whereas none survived after 11 and 15 Gy. After exposure to 10 Gy, 60% of the animals died within 4-6 days and 40% survived with symptoms associated with both the intestinal and the hematopoietic syndromes. Crypt cell necrosis, blunting of villi, and reduction of the mucosal lining increased between 1 and 4 days after irradiation, and mucosal damage was correlated with intraintestinal dosimetry at Day 6. The granulocyte counts at Day 4 were significantly lower than baseline level in animals that died within 4-6 days but not in survivors. The present model appears to be realistic and clinically relevant, allowing the concurrent study of the intestinal and hematopoietic effects of high-dose nonuniform irradiation similar to that received by patients during radiation therapy as well as by radiation accident victims.

Compared to saline-injected mice 9 days after 6.5 Gy irradiation, there were twofold more Day 8 spleen colony-forming units (CFU-S) per femur and per spleen from B6D2F1 mice administered a radioprotective dose of human recombinant interleukin-1-α (rIL-1) 20 h prior to their irradiation. Studies in the present report compared the numbers of CFU-S in nonirradiated mice 20 h after saline or rIL-1 injection. Prior to irradiation, the number of Day 8 CFU-S was not significantly different in the bone marrow or spleens from saline-injected mice and rIL-1-injected mice. Also, in the bone marrow, the number of Day 12 CFU-S was similar for both groups of mice. Similar seeding efficiencies for CFU-S and percentage of CFU-S in S phase of the cell cycle provided further evidence that rIL-1 injection did not increase the number of CFU-S prior to irradiation. In a marrow repopulation assay, cellularity as well as the number of erythroid colony-forming units, erythroid burst-forming units, and granulocyte-macrophage colony-forming cells per femur of lethally irradiated mice were not increased in recipient mice of donor cells from rIL-1-injected mice. These results demonstrated that a twofold increase in the number of CFU-S at the time of irradiation was not necessary for the earlier recovery of CFU-S observed in mice irradiated with sublethal doses of radiation 20 h after rIL-1 injection.

Based on murine survival studies, endogenous hemopoietic spleen colony formation (E-CFU), and recovery of bone marrow and splenic granulocyte-macrophage colony-forming cells (GM-CFC), it was demonstrated that the postirradiation administration of glucan, an immunomodulator and hemopoietic stimulant, enhances the radioprotective effects of WR-2721. <tex-math>${\rm LD}_{50/30}$</tex-math> dose reduction factors for mice treated with WR-2721 (200 mg/kg ∼30 min before irradiation), glucan (250 mg/kg ∼1 h after irradiation), or both agents were 1.37, 1.08, and 1.52, respectively. Enhanced survival in mice treated with both agents appeared to be due in part to glucan's ability to accelerate hemopoietic regeneration from stem cells initially protected from radiation-induced lethality by WR-2721. Following a 10-Gy radiation exposure, E-CFU numbers in mice treated with saline, WR-2721, glucan, or both WR-2721 and glucan were 0.05 ± 0.03, 6.70 ± 1.05, 0.95 ± 0.24, and 33.90 ± 2.96, respectively. Similarly, bone marrow and splenic GM-CFC numbers were greater in mice treated with both WR-2721 and glucan than in mice treated with either agent alone. These results demonstrated at least additive radioprotective effects when mice were given WR-2721 prior to irradiation and glucan following irradiation. These effects appeared to depend on the sequential cell protection mediated by WR-2721 and hemopoietic repopulation mediated by glucan.

Conditioned media (CM) from allogeneic stimulated cultures of light density cells ( $<1.08\ {\rm g}/{\rm cm}^{3}$ ) from the peripheral blood of normal dogs were used to stimulate the growth of erythroid burst-forming units (BFU-E) in bone marrow from normal dogs. Maximum numbers of BFU-E were obtained when 5% (vol/vol) 3× CM and 2 U/ml erythropoietin were added to plasma clot cultures of bone marrow cells. In addition, the radiation sensitivity (D 0 value) was determined for CFU-E and for BFU-E in bone marrow cells exposed in vitro to 1 MeV fission neutron radiation or 250 kVp X rays. BFU-E were more sensitive than CFU-E to the lethal effects of both types of radiation. For bone marrow cells exposed to 1 MeV neutron radiation, the D 0 for CFU-E was 0.27 ± 0.01 Gy, and the D 0 for BFU-E was 0.16 ± 0.03 Gy. D 0 values for CFU-E and BFU-E were, respectively, 0.61 ± 0.05 Gy and 0.26 ± 0.09 Gy for cells exposed to X rays. The neutron RBE values for the culture conditions described were 2.3 ± 0.01 for CFU-E and 1.6 ± 0.40 for BFU-E.

The in vivo diffusion chamber (DC) method of marrow culture was used to determine if the injection of host mice with cyclophosphamide (CY) caused, through its cytotoxic action the release of a humoral factor(s) capable of initiating stem cell ( ${\rm CFU}\text{-}{\rm s}$ ) and granulocyte-macrophage progenitor cell ( ${\rm CFU}\text{-}{\rm c}$ ) proliferation. Host mice were injected with CY 1-4 days prior to 800 rad of 60 Co WBI and implantation of DCs containing normal or 400 rad sublethally irradiated (SLI) marrow cells. The greatest proliferative response within ${\rm CFU}\text{-}{\rm s}$ and ${\rm CFU}\text{-}{\rm c}$ populations occurred in those mice injected with CY 3 days prior to implant. The marked ${\rm CFU}\text{-}{\rm s}$ and ${\rm CFU}\text{-}{\rm c}$ regeneration was initiated during the initial 24 hr of culture in both normal and SLI marrow cells. Thereafter growth rates were approximately the same. SLI marrow, however, showed a greater response to the humoral effects of CY injection than did normal marrow. These data provided evidence that CY induced the release of a diffusible factor(s) capable of accelerating regeneration of normal and sublethally irradiated ${\rm CFU}\text{-}{\rm s}$ and ${\rm CFU}\text{-}{\rm c}$ , the magnitude of which was dependent upon the time elapsed between CY injected and implantation of DCs. The marked proliferative response of the SLI stem and progenitor cells to the humoral stimulation may be indicative of the heterogeneity of both ${\rm CFU}\text{-}{\rm s}$ and ${\rm CFU}\text{-}{\rm c}$ populations surviving sublethal radiation exposure. The target cells may have possessed a differential sensitivity to the factor(s) initiating cell proliferation.

A linear relationship exists between the <tex-math>${\rm LD}_{90}$</tex-math> of bacteria and the binding of p-hydroxymercuribenzoate (HMB). The radiation response of twelve strains representing four families and eight genera exhibited a range of <tex-math>${\rm LD}_{90}$</tex-math> from 4.5 to 450 kR with a variety of shapes. HMB binding varied from <tex-math>$2.7\times 10^{-19}$</tex-math> to <tex-math>$4.6\times 10^{-17}$</tex-math> mole per colony-forming unit (CFU). A similar relationship exists between the D 0 and the HMB binding per cell. This agent saturates its binding sites (sulfhydryl groups) in the intact cell in about 30 minutes and is located in the cell surface and in soluble intracellular fractions of both high and low molecular weight. Exceptions to the relationship include Escherichia coli <tex-math>${\rm Bs}\text{-}_{1}$</tex-math> and mutants of Micrococcus radiodurans, which have a lower survival than their HMB binding would predict. Of the four strains of E. coli tested, all bind approximately the same amount of HMB; while their <tex-math>${\rm LD}_{90}\text{'}{\rm s}$</tex-math> vary from 4.5 to 37 kR. Similarly, the mutants of M. radiodurans all contain the same sulfhydryl content, while their <tex-math>${\rm LD}_{90}\text{'}{\rm s}$</tex-math> vary from 105 to 735 kR (12). These data suggest that radioresistance is determined primarily by the presence of a functional repair system in the cells. The radioresistance of this repair system is in turn determined by the sulfhydryl content of the cell.

A method was introduced to document the progressive changes in the ability of rat skeletal marrow to respond to bleeding over a span of 60 weeks following a single sublethal whole-body x-ray dose. Rats were exposed to a 170-R (200-kVp x-ray) whole-body dose. At intervals of 3, 6, 7, 10, 25, 33, 40, and 60 weeks postirradiation, half of each of the x-irradiated and control groups was bled one-third or two-thirds of the calculated blood volume (cbv) each, and their overall skeletal marrow content of nucleated RBC was determined. The remainder of the x-irradiated and control animals were measured for total marrow content of nucleated RBC without bleeding. The increase in RBC precursors on bleeding in the x-irradiated animal was compared with that found in the control animal on bleeding. This ratio (x-irradiated RBC precursor increase ÷ control RBC precursor increase) constitutes the "erythroid response index" (ERI) and is expressed as a percentage. An ERI value of 100% is taken to mean a lack of residual injury. Any value of ERI less than 100% would then be the percentage of residual injury (residual injury (%) = 100% - ERI). When the residual injury was calculated for the various times studied and plotted, a simple negative exponential function was found between 7 and 60 weeks postirradiation. The loss of residual injury measured in this manner was found to have a half-time of 29.2 weeks.