Doses Of Low LET Radiation Research Articles

Cataractogenic Effects щf Low-Dose Radiation цith Low Let: More not Than There. Report 2. Epidemiological Studies

Radiation damage to the lens is considered to be the third most important effect of radiation, after mortality from cancer and diseases of the circulatory system (ICRP-118). In terms of the effects of low dose radiation with low LET (up to 100 mGy), interest in the problem of cataractogenic disorders is growing, although there is no clarification of the issue. In the present study, two reports attempt to fill this gap. Report 1 reviewed the work on cataractogenic effects of the lowest doses of radiation with low LET in experiments in vitro and in vivo and concluded that there was no significant confirmation of them in animal experiments; Report 2 presents the results of epidemiological studies relevant to the problem. Data are presented on the uncertainties associated with such epidemiological studies: the ambiguity of the relationship between disorders in the lens and the formation of cataracts, their dependence on age, as well as the dependence of the estimation on the accepted system for classifying opacities. These uncertainties have had the consequence that since 1977 the ICRP has proposed five successively decreasing threshold doses (limits) for lens disorders. The dose patterns for radiogenic damage to the lens in the nine exposed groups mentioned in the reviews are considered: victims of atomic bombings (LSS), liquidators of the Chernobyl accident, medical radiologists (rentgenologists, technologists), patients after computed tomography and radiotherapy, industrial radiographers, nuclear industry workers, residents living with an increased radiation background (natural and man-made), for cosmonauts/astronauts and pilots. For some groups, there were statements about the effects of low doses of radiation, however, the presence of a number of epidemiological uncertainties (reverse causality in diagnostic exposure, the contribution of radiation with high LET, UV and solar radiation in cosmonauts/astronauts and pilots, doses above 100 mGy for the upper limit of the studied range in residents, etc.) do not allow us to consider these statements as proven. Therefore, for most exposed groups, a threshold of 300 mGy should be adhered to, regardless of acute or chronic exposure, according to ICRP-118, although due to the precautionary principle, based on the data discussed in Reports 1 and 2, the limit can be reduce to 200 mGy. Exceptions are medical radiologists (rentgenologists, technologists) and industrial radiographers, for whom the cataractogenic effects of low doses (several tens of milligray; working minimum – 20 mGy) can be real. This is probably due to the direct involvement of the organ of vision in professional manipulations with radiation. It is concluded that it is expedient to limit ourselves to these two groups in the future when studying radiogenic disorders in the lens after exposure to low doses of low-LET radiation, while the other groups are unpromising in this regard.

Prophylactic administration of podophyllotoxin and rutin combination assists the revival of radiation-induced hematopoietic suppression in lethally irradiated mice

Hematopoietic syndrome contributes to mortality after exposure to high doses of low LET radiation. In this context, we have earlier demonstrated the potential of G-003 M (a combination of podophyllotoxin and rutin) in alleviating radiation-induced bone marrow suppression. Similarly, we here demonstrate that G-003 M protected mice from death (>83% protection) and increased the populations of CD 34 (Cluster of differentiation 34) as well as CD 117 (Cluster of differentiation 117) positive cell population and their colony forming capacity. This was accompanied with increase in the serum titre of granulocyte colony stimulating factor (G-CSF), granulocyte-macrophage colony stimulating factor (GM-CSF). Interestingly, G-003 M lowered down the titre of fms-like tyrosine kinase (Flt-3) ligands. Our results furthermore demonstrates that G-003 M facilitated the nuclear translocation of β-catenin and upregulated the expression of Wnt 10b. Conditioning of animal with G-003 M activated the expression of survivin, inhibited the activation of Caspase-3 in CD 34/117+ progenitor stem cells and protected the bone marrow vascularity and splenic colonies in lethally irradiated animals, which collectively promoted hemopoietic recovery in lethally irradiated mice.

Modeling of Molecular Mechanisms of Radiation Adaptive Response Formation

The phenomenon of adaptive response is expressed in the increase of resistance of a biological object to high doses of mutagens under the conditions of previous exposure to these (or other) mutagens in low doses. Low doses of mutagen activate a number of protective mechanisms in a living object, which are called hormetic. Thus, the adaptive response and hormesis are links in the same chain. Radiation hormesis refers to the generally positive effect of low doses of low LET radiation on biological objects. The phenomenology of radiation-induced adaptive response and radiation hormesis for biological objects of different levels of organization is considered; the review of existing theories describing the dose-effect relationship has been reviewed. The hypothesis proposing one of the mechanisms of formation of radiation adaptive response of cells taking into account the conformational structure of chromatin has been submitted. The analysis of modern concepts of the phenomenon of hormesis on the basis of modeling of molecular mechanisms of formation of hormetic reactions to low-dose low LET radiation has been carried out. The parameters that can be used for quantitative and graphical evaluation of the phenomenon of hormesis was considered, and a formula for calculating the coefficient of radiation-induced adaptive response has been proposed. A review of mathematical models describing the radiation relative risk of gene mutations and neoplastic transformations at low-dose irradiation of cohorts has been performed. The following conclusions have been made: radiation hormesis and adaptive response are generally recognized as real and reproducible biological phenomena, which should be considered as very important phenomena of evolutionarily formed biological protection of living organisms from ionizing radiation. The hormesis model of dose-response relationship makes much more accurate predictions of a living object's response to radiation (or other stressors) in the low-dose range than the linear threshold (LNT) model does. The LNT model can adequately describe reactions only in the region of high doses of radiation, and, therefore, extrapolation modeling of biological object’s reactions from the zone of high doses to low doses is not correct.

Genetic background of enhanced radioresistance in an anhydrobiotic insect: transcriptional response to ionizing radiations and desiccation.

It is assumed that resistance to ionizing radiation, as well as cross-resistance to other abiotic stresses, is a side effect of the evolutionary-based adaptation of anhydrobiotic animals to dehydration stress. Larvae of Polypedilum vanderplanki can withstand prolonged desiccation as well as high doses of ionizing radiation exposure. For a further understanding of the mechanisms of cross-tolerance to both types of stress exposure, we profiled genome-wide mRNA expression patterns using microarray techniques on the chironomid larvae collected at different stages of desiccation and after exposure to two types of ionizing radiation-70Gy of high-linear energy transfer (LET) ions (4He) and the same dose of low-LET radiation (gamma rays). In expression profiles, a wide transcriptional response to desiccation stress that much exceeded the amount of up-regulated transcripts to irradiation exposure was observed. An extensive group of coincidently up-regulated overlapped transcripts in response to desiccation and ionizing radiation was found. Among this, overlapped set of transcripts was indicated anhydrobiosis-related genes: antioxidants, late embryogenesis abundant (LEA) proteins, and heat-shock proteins. The most overexpressed group was that of protein-L-isoaspartate/D-aspartate O-methyltransferase (PIMT), while probes, corresponding to LEA proteins, were the most represented. Performed functional analysis showed strongly enriched gene ontology terms associated with protein methylation. In addition, active processes of DNA repair were detected. We assume that the cross-tolerance of the sleeping chironomid to both desiccation and irradiation exposure comes from a complex mechanism of adaptation to anhydrobiosis.

Abstract 3713: Targeting alternative DNA repair pathways for cancer cell killing with heavy ion particles

Abstract Heavy ion therapy is a promising approach for cancer treatment, particularly for locally advanced solid tumors. High linear energy transfer (LET) radiation induces greater lesions density in close proximity (clusters) compared that in low-LET radiation. Such clustered DNA damage is thought to be more difficult to repair than isolated DNA damage produced by equivalent doses of low-LET radiation and persist longer in irradiated cells. It has been known that homologous combination contributes to the repair of clustered DNA damage other than classic non-homologous end-joining (NHEJ). DNA double-strand breaks are also repaired by an alternative NHEJ pathway with PARP1, DNA ligase III and Mre11. This alternative pathway is error prone and contributes to genomic instability, such as chromosomal translocation and telomere fusion. Genetic analysis of tumors in TCGA database revealed that these genes are amplified in human tumors and their overexpression is correlated with poor overall survival of cancer patients. However, influence of alternative NHEJ pathways on the relative biological effectiveness (RBE) of heavy ions remains unclear. Here we investigated the effect of PARP-1 and DNA ligase III on RBE of heavy ion particles on cell survival and chromosomal aberration in breast cancer cells and pancreatic cancer cells. Cells were irradiated with X-ray, Carbon (290 MeV/n, 13 KeV/μM) and Iron (1 GeV/n, 150 KeV/μM). We found that overexpression of PARP-1 and DNA ligase III increased RBE of carbon and iron. PARP inhibitor reduced chromosomal aberration induced by heavy ions. This study may provide predictor of radiation responsive of tumor cells from cancer patients that would lead to patient stratification for heavy ion therapy selection Citation Format: Huichen Wang, Premkuma Saganti. Targeting alternative DNA repair pathways for cancer cell killing with heavy ion particles. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3713.

Commentaries on Viewpoint: A call for research to assess and promote functional resilience in astronaut crews

Commentaries on Viewpoint: A call for research to assess and promote functional resilience in astronaut crews

A role for reactive oxygen species in the resolution of persistent genomic instability after exposure to radiation

Reactive oxygen species (ROS) are involved in amplifying the radiation-induced damage to macromolecules as well as in downstream early responses leading to DNA repair or cell death and in delayed responses leading to tissue damage and/or tumorigenesis. We found in immortalized normal human bronchial epithelial cells (HBEC-3KT) following exposure to 1 Gy low (X-rays) or high (Fe ions) LET radiation that ROS levels persist elevated for up to eight population doublings (2 weeks) in the progeny of cells that repaired the radiation-induced damage and survived. To evaluate the function of persistently elevated ROS levels, we interrogated cells over the same period for genomic instability, proliferation and senescence biomarkers and evaluated whether ROS are functionally related to any of these phenotypes. We found that increased ROS production overlapped temporally with the persistence of reporters for genomic instability, increased micronucleus frequency and the presence of γH2AX-53BP1 positive ionizing radiation-induced foci. Although low LET radiation at high doses and high LET radiation induced a senescence-like phenotype dependent on ATM and p38 MAPK activity, this pathway was not a causative factor in ROS generation and did not affect cell proliferation rates at day 7 post-irradiation. On the contrary, when this pathway was inhibited, ROS levels increased further and micronucleus formation frequency was reduced. Because these results suggest a positive effect for increased ROS levels in the resolution of persistent micronucleus formation, their role was further confirmed by exposure to exogenous hydrogen peroxide, which resulted in a reduced micronucleus frequency. These results implicate ROS as an effector in the resolution of genomic instability and suggest that the senescence phenotype induced by high LET radiation or high low LET radiation dose promotes genomic instability by interfering with the cellular mechanism regulating ROS levels or the DNA repair machinery.

Proteomic analysis of proton beam irradiated human melanoma cells.

Proton beam irradiation is a form of advanced radiotherapy providing superior distributions of a low LET radiation dose relative to that of photon therapy for the treatment of cancer. Even though this clinical treatment has been developing for several decades, the proton radiobiology critical to the optimization of proton radiotherapy is far from being understood. Proteomic changes were analyzed in human melanoma cells treated with a sublethal dose (3 Gy) of proton beam irradiation. The results were compared with untreated cells. Two-dimensional electrophoresis was performed with mass spectrometry to identify the proteins. At the dose of 3 Gy a minimal slowdown in proliferation rate was seen, as well as some DNA damage. After allowing time for damage repair, the proteomic analysis was performed. In total 17 protein levels were found to significantly (more than 1.5 times) change: 4 downregulated and 13 upregulated. Functionally, they represent four categories: (i) DNA repair and RNA regulation (VCP, MVP, STRAP, FAB-2, Lamine A/C, GAPDH), (ii) cell survival and stress response (STRAP, MCM7, Annexin 7, MVP, Caprin-1, PDCD6, VCP, HSP70), (iii) cell metabolism (TIM, GAPDH, VCP), and (iv) cytoskeleton and motility (Moesin, Actinin 4, FAB-2, Vimentin, Annexin 7, Lamine A/C, Lamine B). A substantial decrease (2.3 x) was seen in the level of vimentin, a marker of epithelial to mesenchymal transition and the metastatic properties of melanoma.

Estimating the Lowest Detectable Dose of Ionizing Radiation by the Cytokinesis-Block Micronucleus Assay

The frequency of binucleated cells containing one or more micronuclei (MNBN cells) in cytokinesis-blocked peripheral blood lymphocytes can be used to determine whether a person has received an overexposure to ionizing radiation. However, the absence of a pre-exposure sample can preclude precise dosimetry. Here we use a database of MNBN cell frequencies in peripheral blood lymphocytes from 3,104 apparently healthy, unexposed, control subjects aged birth to 88 years, contributed by laboratories participating in the HUMN project. To determine whether a putatively exposed person has actually received a measurable dose, that person's peripheral blood lymphocyte MNBN frequency is compared to values from age and gender-matched controls in the database. If the subject's frequency is significantly higher than the controls, then a cobalt-60 dose-response curve obtained with the cytokinesis-block micronucleus (CBMN) assay in human peripheral blood lymphocytes is used to estimate the minimum dose of low-LET radiation that could have caused the increase. The response curve was generated with 11 acutely administered doses ranging from 0-4 Gy; the majority of doses were in the low end of this range to provide an accurate estimate of the linear portion of the response. The minimum detectable acute whole-body dose at the 95% prediction level and their corresponding 95% confidence intervals are 0.18 Gy (0.15-0.22) and 0.20 (0.17-0.24) Gy for 20-year-old males and females, respectively. Corresponding values for 50 year olds are 0.23 Gy (0.19-0.26) and 0.25 (0.21-0.29) Gy, and for 70 year olds are 0.24 (0.21-0.28) Gy and 0.26 (0.22-0.31) Gy. The minimum detectable chronic doses are approximately fivefold higher for both genders. These types of analyses, including knowledge of assay variability, will improve our understanding of the requirements and limitations for biodosimetry when a pre-exposure micronucleus value is unavailable and reliance on historical baseline micronucleus values is required.

Effect of low doses of low-let radiation on the innate anti-tumor reactions in radioresistant and radiosensitive mice.

BALB/c and C57BL/6 mice differ in their Th1/Th2 lymphocyte and M1/M2 macrophage phenotypes, radiosensitivity, and post-irradiation tumor incidence. In this study we evaluated the effects of repeated low-level exposures to X-rays on the development of artificial tumor colonies in the lungs of animals from the two strains and cytotoxic activities of natural killer (NK) cells and macrophages obtained from these mice. After ten daily irradiations of BALB/c or C57BL/6 mice with 0.01, 0.02, and 0.1 Gy X-rays NK cell-enriched splenocytes collected from the animals demonstrated significant and comparable up-regulation of their anti-tumor cytotoxic function. Likewise, peritoneal macrophages collected from the two irradiated strains of mice exhibited the similarly stimulated cytotoxicities against susceptible tumor cells and produced significantly more nitric oxide. These results were accompanied by the significantly reduced numbers of the neoplastic colonies induced in the lungs by intravenous injection of syngeneic tumor cells. The obtained results indicate that ten low-level irradiations with X-rays stimulate the generally similar anti-tumor reactions in BALB/c and C57BL/6 mice.

Interaction between the biological effects of high- and low-LET radiation dose components in a mixed field exposure

Purpose: The relative biological effectiveness of two epithermal neutron sources, a reactor based source at Studsvik, Sweden, and a proton accelerator-based source in Birmingham, UK, was studied in relation to the proportional absorbed dose distribution as a function of neutron energy. Evidence for any interactions between the effects of biological damage induced by high- and low-linear energy transfer (LET) dose components, in this ‘mixed field’ irradiation, was also examinedMaterials and methods: Clonogenic survival in Chinese Hamster-derived V79 cells was used to assess biological effectiveness in this study. Cells were irradiated in suspension at 4°C at depths of 20, 35, 50 and 65 mm in a water phantom. This prevented the repair of sublethal damage, predominantly that produced by both incident and induced γ-rays in the field, over the variable periods of exposure required to irradiate cells with the same total absorbed dose. Cell survival, as a function of the absorbed radiation dose and depth in the phantom, was compared with Monte Carlo N-Particle (MCNP) calculations of the proportional absorbed dose distribution as a function of neutron energy for the two sources.Results: In terms of the dose-related reduction in clonogenic cell survival, the epithermal neutron source at Studsvik was more biologically effective than the Birmingham source at all depths considered in the phantom. Although the contribution from the high-LET dose component was greater for the Studsvik source at 20 mm depth in the phantom, at greater depths the dose contribution from the high-LET dose component at Studsvik overlap with those for the Birmingham source. However, the most striking difference is in the fast neutron component to the dose of the two sources, neutron energies > 1 MeV were only associated with the Studsvik source. The relative biological effectiveness (RBE) of both sources declined slightly with depth in the phantom, as the total high-LET dose component declined. The maximum source RBE for Studsvik was 2.70 ± 0.50 at 20 mm; reduced to 2.10 ± 0.35 at depths of 50 and 65 mm. The corresponding values for Birmingham were 1.68 ± 0.25 and 1.31 ± 0.19, all values relate only to the surviving fraction of V79 cells at 37%, since RBE values are only applicable to the selected endpoint. Based on a dose reduction factor (DRF) of 1.0 for the total low-LET component to the absorbed dose, the RBE values for the high-LET dose component (fast neutrons and induced protons from the nitrogen capture reaction) was 14.5 and 7.05 for the Studsvik and Birmingham neutron sources, respectively. This is well outside the range of RBE historically reported values for V79 cells for the same level of cell survival for fast neutrons. The calculation of RBE values, based on the proportional absorbed dose distribution as a function of neutron energy, from historical data, and using a RBE of 1.8 for the dose from the nitrogen capture reaction, suggests RBE values for the total high-LET dose component of 3.1–2.8 and 2.5–2.0 for Studsvik and Birmingham, respectively, values again declining with depth in the phantom.Conclusions: The overall biological effectiveness of the mixed field irradiation from an epithermal neutron sources depends on the composition and quality of the different dose components. The experimentally derived RBE values for the total high-LET dose components in these ‘mixed field’ irradiations are well in excess of historical data for fast neutrons. The difference between the historically expected and the observed RBE values is attributed to the interactions between the damage produced by high- and low-LET radiation.

Association of Inter- and Intrachromosomal Exchanges with the Distribution of Low- and High-LET Radiation-Induced Breaks in Chromosomes

To study the effects of low- and high-linear energy transfer (LET) radiation on break locations within a chromosome, we exposed human epithelial cells in vitro to (137)Cs γ rays at both low and high dose rates, secondary neutrons at a low dose rate, and 600 MeV/u iron ions at a high dose rate. Breakpoints were identified using multicolor banding in situ hybridization (mBAND), which paints chromosome 3 in 23 different colored bands. For all four radiation scenarios, breakpoint distributions were found to be different from the predicted distribution based on band width. Detailed analysis of chromosome fragment ends involved in inter- or intrachromosomal exchanges revealed that the distributions of fragment ends participating in interchromosomal exchanges were similar between the two low-LET radiation dose rates and between the two high-LET radiation types, but the distributions were less similar between low- and high-LET radiations. For fragment ends participating in intrachromosomal exchanges, the distributions for all four radiation scenarios were similar, with clusters of breaks found in three regions. Analysis of the locations of the two fragment ends in chromosome 3 that joined to form an intrachromosomal exchange demonstrated that two breaks with a greater genomic separation can be more likely to rejoin than two closer breaks, indicating that chromatin folding can play an important role in the rejoining of chromosome breaks. Comparison of the breakpoint distributions to the distributions of genes indicated that the gene-rich regions do not necessarily contain more breaks. In general, breakpoint distributions depend on whether a chromosome fragment joins with another fragment in the same chromosome or with a fragment from a different chromosome.

High Frequency of Simple and Complex Chromosome Aberrations Detected in the Tokai-mura Survivor four and Five Years after the 1999 Criticality Accident

In September 1999 a criticality accident occurred in a uranium processing plant in Tokai-mura, Japan. During the accident, three workers (A, B and C) were exposed to high acute doses of neutrons and γ-rays: workers A and B fatally and worker C to an estimated whole body absorbed dose of 0.81 Gy neutrons and 1.3 Gy γ-rays. We obtained fixed peripheral blood lymphocytes (PBL) preparations from worker C approximately four and five years after the accident and assayed by 24 colour karyotyping (M-FISH) to determine the frequency and complexity of chromosome aberrations present. We observed a high frequency of simple reciprocal translocations, which we used to provide a rough estimation of dose and, in addition, for the assessment of the emergence of any clinically-relevant clonal exchanges. We did not observe any evidence of clonality but did find some evidence suggesting chromosome 1 as being preferentially involved in exchanges in stable cells. We also detected a relatively high frequency of damaged cells containing complex chromosome aberrations, of both the stable and unstable types. Qualitatively these complex aberrations were consistent with those observed to be induced after exposure to low doses of high-LET radiation or moderate doses of low-LET radiation, supporting the suggestion that heavily damaged cells can be quite long-lived in vivo.

Radiation-Induced S-Nitrosylation of ER-α Predisposes MCF-7 Breast Cancer Cells to Aggressive Invasion and Migration.

Abstract The objective of this study is to decipher the potentials of breast cancer cells that survive radiation exposure at clinical doses to acquire invasive and metastatic determinants. In this study we exposed estrogen receptor positive (MCF-7) and negative (MDAMB-231) breast cancer cells to clinical doses of low LET radiation including doses used as single fraction (2Gy), cumulative dose (10Gy) and scattered dose (10cGy). The cells were exposed to 137Cs source at a dose rate of 1.27 Gy/min. The mock irradiated cells were used as controls. The mRNA transcript level, protein expression and protein-protein interaction were analyzed by QPCR/RT-PCR, immunoblotting, immunoprecipitation and mammalian two-hybrid system, respectively. Intracellular nitric oxide (NO) levels were determined by immunofluorescence and electron spin resonance spectroscopy (ESR) using DAF-FM and Fe-MGD as NO traps, respectively. NO-dependent ER-α s-nitrosylation was determined through immunoprecipitating s-nitrosylated proteins from the cell lysates with an s-nitrosylated-cysteine IgG antibody followed by ER-α western blot analysis. Transactivation of ER-α was measured by EMSA and luciferase reporter assay. Cell invasion and migration were examined by co-culture system using thin inserts.Radiation induces eNOS expression and activation through the phosphorylation of eNOS at Ser1177 site resulting in increased bioavailability of NO in a dose- and time-dependent manner. The three-fold bioavailability of NO s-nitrosylates ER-α leading to a two-fold binding of ER-α to ERE. Paradoxically, this binding did not translate into the transcriptional activation of ER-α dependent gene regulation. Altered structure due to s-nitrosylation of ER-α contributes to enhanced cell invasion and cell migration. In consistent with these results, there was an associated 2.3-fold increase in the expression of MMP-2 and MMP-9 and corresponding TIMP-1 decreased expression leading to a 3.5-fold increase in MMP activity. Radiation, while controlling tumor growth, could simultaneously play a significant role in breast cancer relapse and metastasis through the activation of eNOS and the generation of NO. The bioavailability of NO s-nitrosylates ER-α leading to re-defining the genomic functions of ER-α and thereby impart metastatic transformation potentials to ER-α positive breast cancer cells. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 6162.

The Dose Window for Radiation-Induced Protective Adaptive Responses

Adaptive responses to low doses of low LET radiation occur in all organisms thus far examined, from single cell lower eukaryotes to mammals. These responses reduce the deleterious consequences of DNA damaging events, including radiation-induced or spontaneous cancer and non-cancer diseases in mice. The adaptive response in mammalian cells and mammals operates within a certain window that can be defined by upper and lower dose thresholds, typically between about 1 and 100 mGy for a single low dose rate exposure. However, these thresholds for protection are not a fixed function of total dose, but also vary with dose rate, additional radiation or non-radiation stressors, tissue type and p53 functional status. Exposures above the upper threshold are generally detrimental, while exposures below the lower threshold may or may not increase either cancer or non-cancer disease risk.

Low-dose radiation risk extrapolation fallacy associated with the linear-no-threshold model

Managing radiation risks typically involves establishing regulations that limit radiation exposure. The linear-no-threshold (LNT) dose-response model has been the traditional regulatory default assumption. According to the LNT model, for low a linear-energy-transfer (LET) radiation-induced stochastic effects (e.g., neoplastic transformation and cancer), the risk increases linearly without a threshold. Any radiation exposure is predicted to increase the number of cancer cases among a large population of people. Cancer risk extrapolation from high to low doses based on this model is widespread. Here, indirect evidence is provided that the excess cancer risk calculated at very low doses of low-LET radiation (e.g., around 1 mGy), based on extrapolating from high dose data for an irradiated human population using the LNT model, is likely a phantom excess risk. Indirect evidence is provided, suggesting that for brief exposures to low-LET radiation doses on the order of 1 mGy, that a decrease below the spontaneous level is many orders of magnitude more probable than for any increase in risk as would be predicted by extrapolating from high to low doses using the LNT model. Such a decrease is, however, not expected after exposure to high-LET alpha radiation. The risk reduction has been largely attributed to the induction of a protective apoptosis-mediated (PAM) process that selectively eliminates cells that contain genomic instability (e.g., mutant and neoplastically transformed cells). The PAM process appears to require a dose-rate-dependent stochastic threshold for activation whose minimum is estimated to possibly be as low as 0.01 mGy for X-rays and gamma rays. However, if the dose is too high (e.g., above 250 mGy for brief exposure at a high rate to X-rays or gamma rays), the PAM process is not expected to be activated. For protracted exposure to X-rays or gamma rays, doses as high as 400 mGy (and possibly higher) may activate the PAM process.

Should positive phase III clinical trial data be required before proton beam therapy is more widely adopted? No

Purpose Evaluate the rationale for the proposals that prior to a wider use of proton radiation therapy there must be supporting data from phase III clinical trials. That is, would less dose to normal tissues be an advantage to the patient? Methods Assess the basis for the assertion that proton dose distributions are superior to those of photons for most situations. Consider the requirements for determining the risks of normal tissue injury, acute and remote, in the examination of the data from a trial. Analyze the probable cost differential between high technology photon and proton therapy. Evaluate the rationale for phase III clinical trials of proton vs photon radiation therapy when the only difference in dose delivered is a difference in distribution of low LET radiation. Results The distributions of biological effective dose by protons are superior to those by X-rays for most clinical situations, viz. for a defined dose and dose distribution to the target by protons there is a lower dose to non-target tissues. This superiority is due to these physical properties of protons: (1) protons have a finite range and that range is exclusively dependent on the initial energy and the density distribution along the beam path; (2) the Bragg peak; (3) the proton energy distribution may be designed to provide a spread out Bragg peak that yields a uniform dose across the target volume and virtually zero dose deep to the target. Importantly, proton and photon treatment plans can employ beams in the same number and directions (coplanar, non-co-planar), utilize intensity modulation and employ 4D image guided techniques. Thus, the only difference between protons and photons is the distribution of biologically effective dose and this difference can be readily evaluated and quantified. Additionally, this dose distribution advantage should increase the tolerance of certain chemotherapeutic agents and thus permit higher drug doses. The cost of service (not developmental) proton therapy performed in 3–5 gantry centers operating 14–16 h/day and 6 days/week is likely to be equal to or less than twice that of high technology X-ray therapy. Conclusions Proton therapy provides superior distributions of low LET radiation dose relative to that by photon therapy for treatment of a large proportion of tumor/normal tissue situations. Our assessment is that there is no medical rationale for clinical trials of protons as they deliver lower biologically effective doses to non-target tissue than do photons for a specified dose and dose distribution to the target. Based on present knowledge, there will be some gain for patients treated by proton beam techniques. This is so even though quantitation of the clinical gain is less secure than the quantitation of reduction in physical dose. Were proton therapy less expensive than X-ray therapy, there would be no interest in conducting phase III trails. The talent, effort and funds required to conduct phase III clinical trials of protons vs photons would surely be more productive in the advancement of radiation oncology if employed to investigate real problems, e.g. the most effective total dose, dose fractionation, definition of CTV and GTV, means for reduction of PTV and the gains and risks of combined modality therapy.

Radiation-Induced Neoplastic Transformation and Cancer Risk Assessment at Low Doses of Low-LET Radiation

Epidemiologic studies of cancer incidence are insensitive at low doses (<10 cGy). For this reason it is necessary to turn to relevant experimental systems in order to shed light on what may be happening regarding cancer risk at these low doses. In terms of in vitro studies, a cancer-relevant endpoint is neoplastic transformation. Over the past 10 years we have investigated the effects of a range of doses for a variety of sources of low-LET radiation with the goal of determining the shape of the dose-response curve, and underlying biologic responses, at low doses. The first conclusion we have arrived at is the dose-response for high dose-rate radiation is not linear but J-shaped. The transformation frequency only begins to rise above background at doses above 10 to 20 cGy. Secondly, we have evidence that the low dose suppression is most likely a consequence of multiple mechanisms (upregulation of DNA repair and antioxidant status as well as hyper-radio-sensitivity of cells destined to become neoplastically transformed). Thirdly, we have evidence that this suppression of transformation also occurs for radiation delivered at low (mGy/min) to very low (mGy/day) dose-rates. In addition, we have shown that exposure to very low dose-rate radiation (mGy/day) over 2 to 3 months confers a resistance to neoplastic transformation by a subsequent challenge dose, i.e. an adaptive response as classically defined. It would therefore appear that low doses of low-LET radiation mount responses that protect cell populations against neoplastic transformation in vitro. Clearly, in vitro systems have limitations compared the in vivo situation, nonetheless, they have historically given comparable conclusions at high radiation doses to those seen in vivo in terms of the effect of dose-fractionation, dose-rate, LET and chemical modifiers. A priori, there is therefore no reason to doubt that the same will be true at low doses. Indeed, there are several in vivo studies that would support this contention. Additionally, the relative risk for neoplastic transformation determined from our studies agrees well with those for cancer incidence from radiation epidemiology in the case of breast cancer and leukemia. Based on the above, and other extensive evidence in the literature, it is unlikely that cancer risk from low-LET radiation has a linear-no-threshold dose-response and that threshold, or even hormetic, responses are more probable. Acknowledgement. Supported by the U.S. Department of Energy Low Dose Radiation Research Program.

Detrimental and protective bystander effects: a model approach.

This work integrates two important cellular responses to low doses, detrimental bystander effects and apoptosis-mediated protective bystander effects, into a multistage model for chromosome aberrations and in vitro neoplastic transformation: the State-Vector Model. The new models were tested on representative data sets that show supralinear or U-shaped dose responses. The original model without the new low-dose features was also tested for consistency with LNT-shaped dose responses. Reductions of in vitro neoplastic transformation frequencies below the spontaneous level have been reported after exposure of cells to low doses of low-LET radiation. In the current study, this protective effect is explained with bystander-induced apoptosis. An important data set that shows a low-dose detrimental bystander effect for chromosome aberrations was successfully fitted by additional terms within the cell initiation stage. It was found that this approach is equivalent to bystander-induced clonal expansion of initiated cells. This study is an important step toward a comprehensive model that contains all essential biological mechanisms that can influence dose-response curves at low doses.

Protective Bystander Effects Simulated with the State-Vector Model

Apoptosis induced in non-hit bystander cells is an important biological mechanism which operates after exposure to low doses of low-LET radiation. This process was implemented into a deterministic multistage model for in vitro neoplastic transformation: the State-Vector Model (SVM). The new model is tested on two data sets that show a reduction of the transformation frequency below the spontaneous level after exposure of the human hybrid cell line CGL1 to low doses of gamma-radiation. Stronger protective effects are visible in the data for delayed plating while the data for immediate plating show more of an LNT-like dose-response curve. It is shown that the model can describe both data sets. The calculation of the time-dependent numerical solution of the model also allows to obtain information about the time-dependence of the protective apoptosis-mediated process after low dose exposures. These findings are compared with experimental observations after high dose exposures.