Purpose/Objective(s) Radicals generated by ionizing radiation (IR) can drive specific chemical reactions. This phenomenon underpins radiation-dependent prodrugs (RPDs), which are specifically activated by IR. RPDs thus represent a novel approach to enhance the therapeutic index of antineoplastic drugs by harnessing the spatial targeting potential of radiotherapy. Independent research groups have shown the effectiveness of RPDs in various animal tumor models, highlighting their translational potential. Despite the expanding array of IR-sensitive linkers, critical questions remain. In this work, we investigate the impact of clinically relevant factors including radiation energy and dose rate on cleavage efficiency across distinct linker chemistries. Materials/Methods We confirmed the chemical structures of radio-conversion substrates via NMR and mass spectrometry. Radiation-induced activation was examined across four linker chemistries: dimethoxy benzyl alcohol (DMBA), phenyl azide (Az), amine oxide (NO), and quaternary ammonium (QA), all integrated with a common quenched fluorochrome core structure for standardized comparison and sensitive detection. Gentisic acid served as the reducing radical quencher, with tert-butanol and sulfuric acid as the oxidizing counterpart. We evaluated radiation-induced conversion efficiency by quantifying the dose-dependent release of probes across various radiation doses and conditions. Radiation was applied using a 320kV X-ray irradiator, and a 6MV clinical beam, with dose rates ranging from 10 to 600 cGy/min. Solid water phantoms (5 and 10 cm) simulated treatment depth. Payload release was measured by calibrated LC/MS analysis of product mixtures post-irradiation. Results Using radical species quenchers, we determined the specific radical species dependence for each probe's radioconversion. The DMBA conversion was driven by oxidizing radicals, whereas the Az, NO, and QA linkers primarily depended on reducing radicals, aligning with both existing literature and predicted reaction mechanisms. Our evaluation of conversion efficiencies revealed two key insights: first, linkers reliant on reducing radicals demonstrated superior conversion efficiencies (69.6 nM/Gy for DMBA vs 131-183 nM/Gy for Az, NO and QA); second, across a spectrum of radiation dose rates, energies, and treatment depths, conversion efficiencies remained comparable for each linker chemistry. Importantly, unirradiated linker stability was variable across linker moieties, highlighting the potential need to balance sensitivity and specificity in RPD activation. Conclusion Our results highlight linker chemistry and radical-species dependence as particularly important factors in determining conversion efficiency. Moreover, we note comparable conversion efficiencies across a range of radiation conditions extending to those of clinical relevance. This work offers insight into the design of future RPDs and their translation into clinical studies.

Purpose/Objective(s) Healthy bone is commonly irradiated during radiotherapy for many cancers, and bone fractures can be a late complication. A recent study revealed a higher incidence of rib fractures after breast irradiation with protons (7%) compared to photons (1%) at the same dose, but the underlying biological mechanisms are unknown. Here we used in vitro experiments, supported by Monte Carlo track structure simulations, to investigate differences in bone cell responses to proton and photon radiation, and to identify potential therapeutic interventions. Materials/Methods We used TOPAS-nBio, a radiobiology simulation toolkit, to simulate a segment of bone using geometric models of bone cells (osteoblasts, osteoclasts, and osteocytes). Photon and proton tracks were simulated through bone matrix to investigate energy distributions received by each cell. For in vitro experiments, we utilized the murine osteocytic cell line Ocy454. Cells were irradiated with different doses of protons or photons. Surviving fraction as a function of dose was determined using colony formation assays. DNA double-strand breaks were assessed by 53BP1 foci assay. For PTH treatments, cells were treated with 10 nM human PTH(1-34) for 24 hours prior to irradiation. All experiments were repeated independently three or more times. Statistical analysis was performed by unpaired t-test with Bonferroni correction. Results Monte Carlo track simulations showed osteocytes are the bone cell most likely to receive radiation damage, particularly with protons. In the case of large multi-nucleated osteoclasts, both photons and protons spared some of the nuclei, suggesting functional sparing of osteoclasts from radiation damage. We therefore focused our in vitro experiments on Ocy454 osteocytes. Interestingly, the surviving fraction of osteocytes was significantly lower after protons compared to photons for all investigated doses of 1 to 8 Gy. The dose for 10% surviving fraction was 5.38 Gy for protons and 7.46 Gy for photons (Relative Biological Effect 1.39). Although protons and photons induced similar numbers of 53BP1 foci at 1 hour (79% vs. 72% cells with foci), a significantly higher number of foci was observed at 24 hours after protons compared to photons (22% vs. 6.5%, p=0.0078), suggestive of persistent DNA damage after protons. Remarkably, we found that pre-treatment with parathyroid hormone (PTH) decreased 53BP1 foci at 24 hours in proton-irradiated cells to levels comparable to that of non-irradiated cells (22% without PTH vs. 6.3% with PTH, p=0.014). Conclusion Surviving fraction of osteocytes was lower after proton compared to photon irradiation, suggesting a greater sensitivity of osteocytes to protons. Protons result in greater DNA damage in osteocytes, but this damage may be mitigated with the addition of PTH. Further study is needed to investigate the potential of PTH to reduce bone damage induced by proton irradiation.

Radiation epidemiology is the study of human disease following radiation exposure to populations. Epidemiologic studies of radiation-exposed populations have been conducted for nearly 100 years, starting with the radium dial painters in the 1920s and most recently with large-scale studies of radiation workers. As radiation epidemiology has become increasingly sophisticated it is used for setting radiation protection standards as well as to guide the compensation programmes in place for nuclear weapons workers, nuclear weapons test participants, and other occupationally exposed workers in the United States and elsewhere. It is known with high assurance that radiation effects at levels above 100-150mGy can be detected as evidenced in multiple population studies conducted around the world. The challenge for radiation epidemiology is evaluating the effects at low doses, below about 100 mGy of low-linear energy transfer radiation, and assessing the risks following low dose-rate exposures over years. The weakness of radiation epidemiology in directly studying low dose and low dose-rate exposures is that the signal, i.e. the excess numbers of cancers associated with low-level radiation exposure, is so very small that it cannot be seen against the very high background occurrence of cancer in the population, i.e. a lifetime risk of incidence reaching up to about 38% (i.e. 1 in 3 persons will develop a cancer in their lifetime). Thus, extrapolation models are used for the management of risk at low doses and low dose rates, but having adequate information from low dose and low dose-rate studies would be highly desirable. An overview of recently conducted radiation epidemiologic studies which evaluate risk following low-level radiation exposures is presented. Future improvements in risk assessment for radiation protection may come from increasingly informative epidemiologic studies, combined with mechanistic radiobiologic understanding of adverse outcome pathways, with both incorporated into biologically based models.

The TOPAS Monte Carlo (MC) system is used in radiation therapy and medical imaging research, having played a significant role in making Monte Carlo simulations widely available for proton therapy related research. While TOPAS provides detailed simulations of patient scale properties, the fundamental unit of the biological response to radiation is a cell. Thus, our goal was to develop TOPAS-nBio, an extension of TOPAS dedicated to advance understanding of radiobiological effects at the (sub-)cellular, (i.e., the cellular and sub-cellular) scale. TOPAS-nBio was designed as a set of open source classes that extends TOPAS to model radiobiological experiments. TOPAS-nBio is based on and extends Geant4-DNA, which extends the Geant4 toolkit, the basis of TOPAS, to include very low-energy interactions of particles down to vibrational energies, explicitly simulates every particle interaction (i.e., without using condensed histories) and propagates radiolysis products. To further facilitate the use of TOPAS-nBio, a graphical user interface was developed. TOPAS-nBio offers full track-structure Monte Carlo simulations, integration of chemical reactions within the first millisecond, an extensive catalogue of specialized cell geometries as well as sub-cellular structures such as DNA and mitochondria, and interfaces to mechanistic models of DNA repair kinetics. We compared TOPAS-nBio simulations to measured and published data of energy deposition patterns and chemical reaction rates (G values). Our simulations agreed well within the experimental uncertainties. Additionally, we expanded the chemical reactions and species provided in Geant4-DNA and developed a new method based on independent reaction times (IRT), including a total of 72 reactions classified into 6 types between neutral and charged species. Chemical stage simulations using IRT were a factor of 145 faster than with step-by-step tracking. Finally, we applied the geometric/chemical modeling to obtain initial yields of double-strand breaks (DSBs) in DNA fibers for proton irradiations of 3 and 50 MeV and compared the effect of including chemical reactions on the number and complexity of DSB induction. Over half of the DSBs were found to include chemical reactions with approximately 5% of DSBs caused only by chemical reactions. In conclusion, the TOPAS-nBio extension to the TOPAS MC application offers access to accurate and detailed multiscale simulations, from a macroscopic description of the radiation field to microscopic description of biological outcome for selected cells. TOPAS-nBio offers detailed physics and chemistry simulations of radiobiological experiments on cells simulating the initially induced damage and links to models of DNA repair kinetics.

Our understanding of radiation-induced cellular damage has greatly improved over the past few decades. Despite this progress, there are still many obstacles to fully understand how radiation interacts with biologically relevant cellular components, such as DNA, to cause observable end points such as cell killing. Damage in DNA is identified as a major route of cell killing. One hurdle when modeling biological effects is the difficulty in directly comparing results generated by members of different research groups. Multiple Monte Carlo codes have been developed to simulate damage induction at the DNA scale, while at the same time various groups have developed models that describe DNA repair processes with varying levels of detail. These repair models are intrinsically linked to the damage model employed in their development, making it difficult to disentangle systematic effects in either part of the modeling chain. These modeling chains typically consist of track-structure Monte Carlo simulations of the physical interactions creating direct damages to DNA, followed by simulations of the production and initial reactions of chemical species causing so-called "indirect" damages. After the induction of DNA damage, DNA repair models combine the simulated damage patterns with biological models to determine the biological consequences of the damage. To date, the effect of the environment, such as molecular oxygen (normoxic vs. hypoxic), has been poorly considered. We propose a new standard DNA damage (SDD) data format to unify the interface between the simulation of damage induction in DNA and the biological modeling of DNA repair processes, and introduce the effect of the environment (molecular oxygen or other compounds) as a flexible parameter. Such a standard greatly facilitates inter-model comparisons, providing an ideal environment to tease out model assumptions and identify persistent, underlying mechanisms. Through inter-model comparisons, this unified standard has the potential to greatly advance our understanding of the underlying mechanisms of radiation-induced DNA damage and the resulting observable biological effects when radiation parameters and/or environmental conditions change.

Methods: Patients with 10 most common solid cancers were identified from the cohort (n=73,673). Patients already diagnosed with cancers in 2002 (n=8,047) or patients with short term follow up less than 2 years (n=8,449) were excluded. Finally 52,459 adult cancer patients (25–85 years) were analyzed for estimat-ing the prevalence, mortality and healthcare cost of pre-existing and new-onset CVDs. Results: About sixty five percent of patients already had at least one pre-existing CVD when they were diagnosed with cancer and these patients showed signif-icantly higher overall mortality, 5-year adjusted mortality and healthcare cost. 49.8% of cancer patients without pre-existing CVD were also newly diagnosed with CVD during the follow up period, mostly within 5 years. Adult cancer patients with new-onset CVD also showed significantly higher overall mortality, 5-year adjusted mortality and healthcare cost than patients without new-onset CVD. Except for cancer related death, CVD was the most common cause of death in adult cancer patients irrespective of pre-existing or new-onset CVD. Multivariate analysis revealed that new-onset CVD, male gender, old age, prior history of diabetes and chronic kidney disease, suburb residents and low income status were significant risk factor for all-cause mortality. Conclusions: Two thirds of patients already had pre-existing CVD at the time of cancer diagnosis and half of cancer patients without pre-existing CVD suffer from newly diagnosed with CVD mostly within 5 years. Because both pre-existing and new-onset CVD predicted an increased risk of higher mortality and healthcare cost, strict management of pre-existing CVD and pre-emptive prevention of new- onset CVD are necessary to reduce its burden in cancer patients. Background: Only a few studies have investigated the relation between depres- sion and cardiovascular disease (CVD) over time, and uncertainties remain on the underlying mechanisms linking depression and subclinical atherosclerosis. It is unclear whether inflammation is a mechanism linking depression to CVD. Purpose: The purpose of this study was to examine prospectively whether in- flammation explains the relationship between depression and CVD. Methods: Depression status (Beck Depression Inventory, BDI), selected CHD risk factors, inflammation markers, measures of heart rate variability (HRV), and indices of endothelial function (flow-mediated dilation, FMD) were evaluated in 415 subjects free of CHD, diabetes mellitus, and other life-threatening conditions, with at least two CHD risk factors among the following: older age, male gender, current smoking, hypertension, and dislipidaemia. The main outcome was incidence of CVD events (hospital stays for nonfatal myocardial infarction, stroke, congestive heart failure, and CVD-related mortality)during 10 year of follow-up. Results: Overall, 51.7% of the participants were males, aged 57.6+8.8 years on average (minimum 30, maximum 70). Almost half were ypertensive,43.9% were dyslipidemic, 30.4% current smokers, and 23.1% showed a depressive symp- tomatology (BDI > 10).Compared with subjects without depression, subject with depression had a 70% higher CRP (p=0.0008) and more likely to have altered HRV and their FMD was severely impaired (adjusted odds ratio of 1% increase 14 0.72; 95% CI: 0.61–0.86).). Depression was a significant predictor of CVD (haz- ard ratio 2.58, p=0.0009). Adjustment for other patient factors did not substan-tially affect the results. Addition of CRP decreased the estimate for depression by 13% and addition of HRV decreased it by 4%. Both depression and inflammatory biomarkers remained independent predictors of outcome. Conclusion: Despite their robust association with depression, HRV, systemic inflammatory, explain only a small portion of the association between depression and CVD incidence We compared demographics, blood pressure, echocardiography and vascular function in 1 year after normal versus abnormal pregnancy complicated by PrE or DM at a large tertiary university hospital. Results: See table: In PrE, but not in DM, multiple CV abnormalities suggest evolving cardiac hypertrophy and vascular stiffening. and functional CV ab- normalities in women at 1 year after pregnancy complicated by PrE. These find-ings suggest increased clinical vigilance for evolving CV disease in these women. Background: While there is a well-established link between ionizing radiation and cancers, the link between radiation and cardiovascular (CV) diseases at low doses is uncertain. Numerous cohorts exist of individuals with medical or occupa- tional radiation exposure, estimable radiation dose and cause of death in the form of International Classification of Diseases (ICD) codes. However the relationship between clinically meaningful CV endpoints and these thousands of ICD codes is poorly defined. Purpose: To develop a classification system based on ICD codes to enable study of potential radiation related CV death, and to apply this to large cohorts

Purpose:The purpose of this study is to investigate and quantify the biological effects of ion radiation using several human cell lines. We aim to answer the question of whether carbon ion the most ideal ion species for heavy ion radiotherapy.Methods:The cells were irradiated at different positions along the pristine Bragg peak of several ions with different atomic number. The biological effectiveness was evaluated using the clonogenic cell survival assay. Irradiation of three human lung cancer cell lines and a fibroblast cell line were undertaken using the charged particle beam at the NASA Space Radiation Laboratory at Brookhaven National Lab. Four mono‐energetic ion beams (carbon, oxygen, helium and lithium) were used to irradiate the cells. Water or media‐filled T25 flasks were lined up along the beam line so that the cell‐containing surfaces of the flasks were placed at a specific depth along the pristine Bragg curve. Four depths along the curve, representing entrance point, rising peak, peak and distal fall off, were selected to determine biological effectiveness. Gaf‐chromic films were placed between the flasks to monitor the irradiation as soon as it was finished.Results:For all ion radiations, the maximum cell killing effect occurs at either peak or distal fall off, depending on the cell lines. For instance, for the fibroblast cell line AGO1522, RBEs of 1.4, 1.2, 1.4 and 1.9 were observed at the Bragg peak for Helium, Lithium, Carbon and Oxygen ions. Comparing positions, RBEs of 0.9, 1.2, 1.4 and 1.8 were observed for carbon irradiation of AGO‐1522 cells positions corresponding to entrance, rising peak, peak and distal fall off.Conclusion:RBE values differ with position in the Bragg peak, ion species and cell line. Ions other than carbon may prove more effective in certain irradiation conditions and may contribute to optimized heavy ion therapy.

Purpose:The purpose of this study is to investigate and quantify several factors affecting biological effects of carbon ions such as cell type, dose, energy and position where the cells are irradiated along the pristine Bragg curve.Methods:Experiments to quantify clonogenic cell survival in three human lung cancer cell lines and a fibroblast cell line were performed at the NASA Space Radiation Laboratory, BNL, Upton, USA. A system of water or media‐filled T25 flasks lined up along the beam axis was designed so that the cell‐containing surfaces of the flasks were placed at specific depths along the Bragg curve. Gaf‐chromic films were placed between the flasks to monitor the dose distribution in the sample as soon as the irradiation was finished. Additional studies were conducted at four selected depths along the Bragg curve to obtain full cell survival dose response curves for the four cell lines.Results:The percent depth dose of the beams was determined using an ionization chamber and showed that the physical Bragg peak is at 22.5 cm water depth. However, the clonogenic cell survival data indicated that the maximum cell killing occurred at 21.5 cm. Gaf‐chromic films revealed some inhomogeneity in the dose distribution on the flasks near the peak, presumably due to lack of scattering from the sides of the flasks, which might account for the differences. Depending on the cell line and radiation dose, the maximum cell killing (i.e., the greatest RBE) is at the 21.5 (the peak) or 24 cm (distal fall off) depth.Conclusion:There is a difference in biological effect along the Bragg curve of a carbon ion beam, indicating an elevated RBE at or beyond the end of the range. Gaf‐chromic films are proven to be effective in monitoring the 2D irradiation pattern to the flasks.Research supported by NIH/NCI through grant no. R21 CA182259.

Chondrosarcoma is well known as a radioresistant tumor, but the mechanisms underlying that resistance are still unclear. The bystander effect, whereby cells that are not traversed by ionizing radiation exhibit various responses when in proximity to irradiated cells, is well documented in the field of radiation biology. In the present work, we investigated the bystander response induced by ionizing radiation in chondrosarcoma cells. Human chondrosarcoma cells (HTB94; ATCC, Manassas, VA) were irradiated with 0.1, 0.5, 1, and 2 Gy X-ray irradiation using a transwell insert co-culture system that precludes physical contact between targeted and bystander cells. We investigated the formation of micronuclei and p53 binding protein 1 (53BP1) staining in bystander and irradiated cells when bystander cells were co-cultured with irradiated cells immediately after irradiation. Micronucleus formation was assayed 72 hours after irradiation, and 53BP1 foci formation was measured 5 hours after irradiation. In addition, we investigated the bystander signaling between mixed cultures of chondrosarcoma cells and normal human skin fibroblasts (AGO1522). The percentages of cells with micronucleus formation in control, 0.1 Gy, 0.5 Gy, 1 Gy and 2 Gy irradiated chondrosarcoma cells were 2.8%, 5.1%, 18.8%, 37.6% and 63.8%, respectively. The percentages of irradiated cells showing 53BP1 foci formation were0.3%, 4.4%, 28.8%, 72.1% and 93.8%, respectively. These showed dose-dependent increases. However, the rates of micronucleus formation in all bystander chondrosarcoma cells were 2.4 ± 0.65% and those of 53BP1 foci formation were 0.3 ± 0.33%. These did not show changes from the rates of control cells. In the bystander signaling between mixed cultures of chondrosarcoma cells and fibroblasts, the rates of micronucleus formation in all bystander chondrosarcoma cells with irradiated fibroblasts were 2.3 ± 0.45%, the same as the rates for control cells. However, all bystander fibroblasts co-cultured with irradiated chondrosarcoma cells showed 10.1 ± 0.86% micronucleus formation, a significant increases from the rates of control cells (6.2 ± 0.53%, p = 0.002). Chondrosarcoma cells did not show bystander responses in the transwell insert co-culture system. In mixed cultures, chondrosarcoma cells co-cultured with irradiated fibroblasts did not show bystander responses. However, fibroblasts co-cultured with irradiated chondrosarcoma cells showed the induction of bystander response. These results suggested that bystander stimulations could be released by chondrosarcoma cells but those cells could not develop bystander responses.

At the low particle fluences of radiation to which astronauts are exposed in space, "non-targeted" effects such as the bystander response may have increased significance. The radiation-induced bystander effect is the occurrence of biological responses in unirradiated cells near to or sharing medium with cells traversed by radiation. The objectives of this study were to establish the responses of AG01522 diploid human fibroblasts after exposure to several heavy ions and energetic protons, as compared to X-rays, and to obtain initial information on the bystander effect in terms of cell clonogenic survival after Fe ion irradiation. Using a clonogenic survival assay, relative biological effectiveness (RBE) values at 10% survival were 2.5, 2.3, 1.0 and 1.2 for 1 GeV/amu Fe, 1 GeV/amu Ti, 290 MeV/amu C and 1 GeV/amu protons, respectively, compared to 250 kVp X-rays. For induction of micronuclei (MN), compared to the low LET protons, Fe and Ti are very effective inducers of damage, although C ions are similar to protons. Using a transwell insert system in which irradiated and unirradiated bystander cells share medium but are not touching each other, it was found that clonogenic survival in unirradiated bystander cells was decreased when irradiated cells were exposed to Fe ions or X-rays. The magnitude of the decrease in bystander survival was similar with both radiation types, reaching a plateau of about 80% survival at doses of about 0.5 Gy or larger.

Individuals suffering from the rare chromosomal instability disorder Fanconi Anemia (FA) typically exhibit pronounced hypersensitivity to cancer therapies that employ crosslinking agents or ionizing radiation (IR). However, for reasons largely unknown, fibroblast lines derived from FA patients generally show very little or no radiation hypersensitivity in tissue culture. It was recently reported that human fibroblasts from the FA groups C and G are hypersensitive to IR under anoxic conditions, compared to non-isogenic wild-type cells. In this study, we sought to expand on these findings and elucidate the role of the central FA gene product D2 (FANCD2) in determining cellular resistance to IR. We utilized a well-characterized isogenic pair of fibroblasts derived from patients with FA group D2 (PD20) and their retrovirally complemented counterparts expressing wild-type FANCD2. We first determined clonogenic survival under room air conditions (20% oxygen) and found that FANCD2 deficiency did not confer radiation hypersensitivity. However, under anoxic conditions (0% oxygen), the FANCD2 mutant cells were approximately two-fold more radiation sensitive compared to the wild-type complemented cells. Importantly, there was no difference in sensitivity to anoxia/reoxygenation alone between the two cell lines. The OER for wild-type cells was 3.2, and the OER for mutant cells was slightly decreased at 2.3. Interestingly, when we determined survival under 3% oxygen, which more closely resembles the physiologic environment in human tissues, the difference between FANCD2 mutant and wild-type cells was maintained, although it was less pronounced. Radiation under hypoxic conditions was previously reported to cause more DNA interstrand crosslinks (ICLs) than radiation at room air, and ICLs are a substrate for the repair function of the FA pathway. However, both of our cell lines demonstrated proficiency in forming Rad51 subnuclear foci in response to radiation under 0% oxygen, i.e., from 5–6% of untreated cells to approximately 21% in irradiated cells. In contrast and consistent with our previous data, FANCD2-mutant PD20 cells were unable to form Rad51 foci in response to treatment with the crosslinking agent mitomycin C (MMC). Interestingly, PD20 cells subjected to anoxic IR showed a frequency of induced apoptosis of 15% after 24–48 hours, compared to only 7% in wild-type complemented cells. In comparison, after MMC treatment only 5–7% of cells demonstrated induced apoptosis regardless of genotype. Thus, the mode of cell death appears to be different for anoxic radiation damage versus ICLs, at least in our cell lines. These findings resemble other data from our lab showing that FANCD2 deficiency results in apoptosis and decreased clonogenic survival in response to oxidative DNA damage. In conclusion, we hypothesize that the observed hypersensitivity of FANCD2-mutant cells to anoxic irradiation and oxidative stress have a common underlying mechanism, which may be related to a recently postulated role of FANCD2 in the regulation of pro-survival pathways that respond to reactive oxygen species. In part supported by Susan G. Komen for the Cure (to LAK and HW) and NIH P01 CA095227 (to KDH).

The Gray Cancer Institute ultrasoft X-ray microprobe was used to quantify the bystander response of individual V79 cells exposed to a focused carbon K-shell (278 eV) X-ray beam. The ultrasoft X-ray microprobe is designed to precisely assess the biological response of individual cells irradiated in vitro with a very fine beam of low-energy photons. Characteristic C-K X rays are generated by a focused beam of 10 keV electrons striking a graphite target. Circular diffraction gratings (i.e. zone plates) are then employed to focus the X-ray beam into a spot with a radius of 0.25 mum at the sample position. Using this microbeam technology, the correlation between the irradiated cells and their nonirradiated neighbors can be examined critically. The survival response of V79 cells irradiated with a C-K X-ray beam was measured in the 0-2-Gy dose range. The response when all cells were irradiated was compared to that obtained when only a single cell was exposed. The cell survival data exhibit a linear-quadratic response when all cells were targeted (with evidence for hyper-sensitivity at low doses). When only a single cell was targeted within the population, 10% cell killing was measured. In contrast to the binary bystander behavior reported by many other investigations, the effect detected was initially dependent on dose (<200 mGy) and then reached a plateau (>200 mGy). In the low-dose region (<200 mGy), the response after irradiation of a single cell was not significantly different from that when all cells were exposed to radiation. Damaged cells were distributed uniformly over the area of the dish scanned (similar to25 mm(2)). However, critical analysis of the distance of the damaged, unirradiated cells from other damaged cells revealed the presence of clusters of damaged cells produced under bystander conditions. (C) 2003 by Radiation Research Society.

The radiobiological effectiveness of an epithermal neutron beam is described using cell survival as the end point. The M67 epithermal neutron beam at the Nuclear Reactor Laboratory, Massachusetts Institute of Technology, that was used for clinical trials of boron neutron capture therapy was used to irradiate Chinese hamster ovary cells at seven depths in a water-filled phantom that simulated healthy tissue. No boron was added to the samples. Therefore, this experiment evaluates the biological effectiveness of the neutron and photon components, which comprise 80-95% of the dose to healthy tissue. Cell survival was dependent upon the depth in the phantom, as a result of moderation and attenuation of the epithermal neutron beam components by the overlying water. The results were compared with 250 kVp X irradiations to determine relative biological effectiveness values. Cell survival as a function of the dose delivered was lowest at the most shallow depth of 0.5 cm, and increased at depths of 1.5, 3, 4, 5.6, 6.6 and 8.1 cm. The gradual increase in cell survival with increasing depth in the phantom is due to the exponential drop of the fast-neutron intensity of the beam. These results are applicable to clinical boron neutron capture therapy Phase I/II trials in which healthy tissue toxicity was an end point.