Failla memorial lecture. Risk, research, and radiation protection

Radiation protection concerns the risk of stochastic late effects, especially cancer, and limits on radiation exposure both occupationally and for the public tend to be based on these risks. The risks are determined, mainly by expert committees, from the steadily growing information on exposed human populations, especially the survivors of the atomic bombs dropped in Japan in 1945. Risks of cancer estimated up to the early 1980s were in the range 1 to 5 X 10(-2)/Sv, but recent revisions in the dosimetry of the Japanese survivors and additional cycles of epidemiological information suggest values now probably at the high end of this range. These are likely to require an increase in the values used for radiation protection. A major problem with risk estimation is that data are available only for substantial doses and must be extrapolated down to the low-dose region of interest in radiation protection. Thus the shape of the dose-response curve is important, and here we must turn to laboratory research. Of importance are studies involving (1) dose rate, which affects the response to low-LET radiation and often to high-LET radiation as well; (2) radiation quality, since the shapes of the dose-response curves for high- and low-LET radiation differ and thus the RBE, the ratio between them, varies, reaching a maximum value RBEM at low doses; and (3) modifiers of the carcinogenic response, which either enhance or reduce the effect of a given dose. Radiation protection depends both on risk information, and especially also on comparisons with other occupational and public risks, and on research, not only for extrapolations of risk to low doses but also in areas where human information is lacking such as in the effects of radiation quality and in modifications of response.     

Hyperthermic radiosensitization of thermotolerant Chinese hamster ovary cells

Synchronous G1 cells were given a priming dose of heat (45.5 degrees C for 15 min) and then heated and irradiated 6-120 h later. Compared to heat radiosensitization for cells irradiated 10 min after the priming heat dose (thermal enhancement ratio, TER of 2.6 for a 10-fold reduction in survival), heat radiosensitization 18-24 h after the priming heat dose was less (i.e., TER of 1.6 for radiation at 24 h compared with heat-radiation at 24 h). A thermotolerance ratio (TTR) at 24 h was calculated to be 2.6/1.6 = 1.6. TERs at 100-fold or 1000-fold reduction in survival and ratios of slopes of radiation survival curves also showed that the cells developed a similar amount of thermotolerance for heat radiosensitization at 18-24 h. Furthermore, since the TER for heat radiosensitization increased with heat killing either from the priming heat dose or the second heat dose in a similar manner for single or fractionated doses, the TER for nonthermotolerant and thermotolerant cells was the same when related to the heat damage (i.e., amount of killing from heat alone). When the radiation response of cells heated and irradiated 6-120 h after the priming heat dose was compared with the response of cells receiving radiation only, changes in TER as a function of time after the initial priming heat dose were shown to involve: recovery of heat damage interacting with the subsequent radiation dose, thermotolerance for heat radiosensitization, and redistribution of cells surviving the first heat dose into radioresistant phases of the cell cycle. In fact, redistribution resulted in a minimal TER at 72 h for heat-radiation compared with radiation alone, instead of at 24 h where maximal thermotolerance for heat killing was observed [P. K. Holahan and W. C. Dewey, Radiat. Res. 106, 111 (1986)]. These observations are discussed relative to clinical considerations and similar results reported from in vivo experiments.     

Nonlinear Blending: A Useful General Concept for the Assessment of Combination Drug Synergy

Human diseases may involve cellular signaling networks that contain redundant pathways, so that blocking a single pathway in the system cannot achieve the desired effect. As such, the use of drugs in combination are particularly effective interventions in networked systems. However, common synergy measures are often inadequate to quantify the effect of two different drugs in complex cellular systems. This article proposes a general approach to quantifying the synergy of two drugs in combination. This approach is called strong nonlinear blending. Drugs with different relative potencies, different effect maxima, or situations of potentiation or coalism pose no problem for strong nonlinear blending as a way to assess the increased response benefit to be gained by combining two drugs. This is important as testing drug combinations in complex biological systems are likely to produce a wide variety of possible response surfaces. It is also shown that for monotone increasing (or decreasing) dose response surfaces that strong nonlinear blending is equivalent to improved potency along a ray of constant dose ratio. This is important because fixed dose ratios form the basis for many preclinical and clinical combination drug experiments. Two examples are given involving HIV and cancer chemotherapy combination drug experiments.     

Nonlinear blending: a useful general concept for the assessment of combination drug synergy

Human diseases may involve cellular signaling networks that contain redundant pathways, so that blocking a single pathway in the system cannot achieve the desired effect. As such, the use of drugs in combination are particularly effective interventions in networked systems. However, common synergy measures are often inadequate to quantify the effect of two different drugs in complex cellular systems. This article proposes a general approach to quantifying the synergy of two drugs in combination. This approach is called strong nonlinear blending. Drugs with different relative potencies, different effect maxima, or situations of potentiation or coalism pose no problem for strong nonlinear blending as a way to assess the increased response benefit to be gained by combining two drugs. This is important as testing drug combinations in complex biological systems are likely to produce a wide variety of possible response surfaces. It is also shown that for monotone increasing (or decreasing) dose response surfaces that strong nonlinear blending is equivalent to improved potency along a ray of constant dose ratio. This is important because fixed dose ratios form the basis for many preclinical and clinical combination drug experiments. Two examples are given involving HIV and cancer chemotherapy combination drug experiments.     

Risk of second primary thyroid cancer after radiotherapy for a childhood cancer in a large cohort study: an update from the childhood cancer survivor study

Previous studies have indicated that thyroid cancer risk after a first childhood malignancy is curvilinear with radiation dose, increasing at low to moderate doses and decreasing at high doses. Understanding factors that modify the radiation dose response over the entire therapeutic dose range is challenging and requires large numbers of subjects. We quantified the long-term risk of thyroid cancer associated with radiation treatment among 12,547 5-year survivors of a childhood cancer (leukemia, Hodgkin lymphoma and non-Hodgkin lymphoma, central nervous system cancer, soft tissue sarcoma, kidney cancer, bone cancer, neuroblastoma) diagnosed between 1970 and 1986 in the Childhood Cancer Survivor Study using the most current cohort follow-up to 2005. There were 119 subsequent pathologically confirmed thyroid cancer cases, and individual radiation doses to the thyroid gland were estimated for the entire cohort. This cohort study builds on the previous case-control study in this population (69 thyroid cancer cases with follow-up to 2000) by allowing the evaluation of both relative and absolute risks. Poisson regression analyses were used to calculate standardized incidence ratios (SIR), excess relative risks (ERR) and excess absolute risks (EAR) of thyroid cancer associated with radiation dose. Other factors such as sex, type of first cancer, attained age, age at exposure to radiation, time since exposure to radiation, and chemotherapy (yes/no) were assessed for their effect on the linear and exponential quadratic terms describing the dose-response relationship. Similar to the previous analysis, thyroid cancer risk increased linearly with radiation dose up to approximately 20 Gy, where the relative risk peaked at 14.6-fold (95% CI, 6.8-31.5). At thyroid radiation doses >20 Gy, a downturn in the dose-response relationship was observed. The ERR model that best fit the data was linear-exponential quadratic. We found that age at exposure modified the ERR linear dose term (higher radiation risk with younger age) (P < 0.001) and that sex (higher radiation risk among females) (P = 0.008) and time since exposure (higher radiation risk with longer time) (P < 0.001) modified the EAR linear dose term. None of these factors modified the exponential quadratic (high dose) term. Sex, age at exposure and time since exposure were found to be significant modifiers of the radiation-related risk of thyroid cancer and as such are important factors to account for in clinical follow-up and thyroid cancer risk estimation among childhood cancer survivors.     

Optimal Dimensions of Gold Nanorod for Plasmonic Nanosensors

Localized surface plasmon resonance (LSPR) for longitudinal mode of gold nanorod is simulated by using Gans theory. The parameters like surface scattering, radiation damping, and dynamic depolarization of radiation across the surface of nanorod affecting response of free electrons towards optical excitation are considered. Simulation results show that refractive index sensitivity linearly rises with size and aspect ratio, whereas this leads to the broadening of resonant line width also. Therefore, to optimize the size of nanorod, figure of merit (FOM) is calculated and observed that optimized width is 15 nm for an aspect ratio of 2, whereas it is 12 nm for aspect ratios 3 and 4. Further, optimization by using newly modified figure of merit (MFOM) shows that optimized width is 39 nm for aspect ratio of 2 and 24 nm for 3 and 4 aspect ratios. It is also found that at aspect ratio 2, both FOM and MFOM are higher than the aspect ratios 3 and 4. The quality factor calculation for LSPR response of nanorod explains its dependence with aspect ratio and optimized dimensions.     

Incidence of female breast cancer among atomic bomb survivors, Hiroshima and Nagasaki, 1950-1990

An incidence survey of the Life Span Study (LSS) population found 1093 breast cancers among 1059 breast cancer cases diagnosed during 1950-1990. As in earlier breast cancer surveys of this population, a linear and statistically highly significant radiation dose response was found. In the analysis, particular attention was paid to modification of radiation dose response by age at exposure (e) and attained age (a). Dose-specific excess relative risk (ERR(1Sv)) decreased with increasing values of e and a. A linear dose-response model analysis, with e and a as exponential age modifiers, did not conclusively discriminate between the two variables as modifiers of dose response. A modified isotonic regression approach, requiring only that ERR(1Sv) be monotonic in age, provides a fresh perspective indicating that both e and a are important modifiers of dose response. Exposure before age 20 was associated with higher ERR(1Sv) compared to exposure at older ages, with no evidence of consistent variation by exposure age for ages under 20. ERR(1Sv) was observed to decline with increasing attained age, with by far the largest drop around age 35. Possible explanations for these observations are discussed, along with research approaches that might provide more information.     

Determination of Sample Sizes for Demonstrating Efficacy of Radiation Countermeasures

Summary .  In response to the ever increasing threat of radiological and nuclear terrorism, active development of nontoxic new drugs and other countermeasures to protect against and/or mitigate adverse health effects of radiation is ongoing. Although the classical LD₅₀ study used for many decades as a first step in preclinical toxicity testing of new drugs has been largely replaced by experiments that use fewer animals, the need to evaluate the radioprotective efficacy of new drugs necessitates the conduct of traditional LD₅₀ comparative studies ( FDA, 2002 ,  Federal Register   67, 37988–37998). There is, however, no readily available method to determine the number of animals needed for establishing efficacy in these comparative potency studies. This article presents a sample-size formula based on Student's  t  for comparative potency testing. It is motivated by the U.S. Food and Drug Administration's (FDA's) requirements for robust efficacy data in the testing of response modifiers in total body irradiation experiments where human studies are not ethical or feasible. Monte Carlo simulation demonstrated the formula's performance for Student's  t , Wald, and likelihood ratio tests in both logistic and probit models. Importantly, the results showed clear potential for justifying the use of substantially fewer animals than are customarily used in these studies. The present article may thus initiate a dialogue among researchers who use animals for radioprotection survival studies, institutional animal care and use committees, and drug regulatory bodies to reach a consensus on the number of animals needed to achieve statistically robust results for demonstrating efficacy of radioprotective drugs.     

Vascular response to radiation injury in the rat lung.

Changes in relative left-to-right lung blood flow ratios were followed as an index of vascular radiation injury in left-hemithorax-irradiated Sprague-Dawley rats. Single doses of 11 to 21 Gy gamma radiation resulted in a dose-dependent decrease in relative blood flow to the irradiated lung from 3 to 5 weeks after exposure during the development of pneumonitis. Blood flow returned to near normal by 5 weeks after lower doses (11-13.5 Gy). After a single dose of 15 Gy the left-to-right blood flow ratio recovered to 75% of normal at 12 weeks and leveled off. Following 18 Gy irradiation a second period of reduced flow began 16 weeks after exposure. After 21 Gy irradiation flow to the irradiated side remained low for 1 year after exposure. Rats that received a single dose of 18 Gy to the left hemithorax were also treated with one or two of the following drugs: captopril, cyproheptadine, dexamethasone, diethylcarbamazine, penicillamine, or theophylline. Dexamethasone was most effective at preventing the decrease in blood flow to the irradiated lung when treatment was continued through the pneumonitis period and dose was not tapered until 8 weeks after radiation exposure. All other drugs and drug combinations were, for the most part, virtually ineffective after the pneumonitis period. There was a relatively poor correlation with earlier vascular permeability surface area product studies. This suggests that endothelial damage, as well as damage to other cell types, contributes to the development of post-irradiation fibrosis in the lung.     

Low-dose hyper-radiosensitivity: a consequence of ineffective cell cycle arrest of radiation-damaged G2-phase cells

This review highlights the phenomenon of low-dose hyper- radiosensitivity (HRS), an effect in which cells die from excessive sensitivity to small single doses of ionizing radiation but become more resistant (per unit dose) to larger single doses. Established and new data pertaining to HRS are discussed with respect to its possible underlying molecular mechanisms. To explain HRS, a three-component model is proposed that consists of damage recognition, signal transduction and damage repair. The foundation of the model is a rapidly occurring dose-dependent pre-mitotic cell cycle checkpoint that is specific to cells irradiated in the G2phase. This checkpoint exhibits a dose expression profile that is identical to the cell survival pattern that characterizes HRS and is probably the key control element of low-dose radiosensitivity. This premise is strengthened by the recent observation coupling low- dose radiosensitivity of G2-phase cells directly to HRS. The putative role of known damage response factors such as ATM, PARP, H2AX, 53BP1 and HDAC4 is also included within the framework of the HRS model.     

Verification of electron beam parameters in an intraoperative linear accelerator using dosimetric and radiobiological response methods

BackgroundThe availability of linear accelerators (linac) for research purposes is often limited and therefore alternative radiation sources are needed to conduct radiobiological research. The National Centre for Radiation Research in Poland recently developed an intraoperative mobile linac that enables electron irradiation at energies ranging from 4 to 12 MeV and dose rates of 5 or 10 Gy/min. The present study was conducted to evaluate the electron beam parameters of this intraoperative linac and to verify the set-up to evaluate out-of-field doses in a water phantom, which were determined through dosimetric and biological response measurements.Materials and methodsThe distribution of radiation doses along and across the radiation beam were measured in a water phantom using a semiconductor detector and absolute doses using an ionisation chamber. Two luminal breast cancer cell lines (T-47D and HER2 positive SK-BR-3) were placed in the phantom to study radiation response at doses ranging from 2 to 10 Gy. Cell response was measured by clonogenic assays.Results and ConclusionThe electron beam properties, including depth doses and profiles, were within expected range for the stated energies. These results confirm the viability of this device and set-up as a source of megavoltage electrons to evaluate the radiobiological response of tumour cells.     

Imaging dose from cone beam computed tomography in radiation therapy

Imaging dose in radiation therapy has traditionally been ignored due to its low magnitude and frequency in comparison to therapeutic dose used to treat patients. The advent of modern, volumetric, imaging modalities, often as an integral part of linear accelerators, has facilitated the implementation of image-guided radiation therapy (IGRT), which is often accomplished by daily imaging of patients. Daily imaging results in additional dose delivered to patient that warrants new attention be given to imaging dose. This review summarizes the imaging dose delivered to patients as the result of cone beam computed tomography (CBCT) imaging performed in radiation therapy using current methods and equipment. This review also summarizes methods to calculate the imaging dose, including the use of Monte Carlo (MC) and treatment planning systems (TPS). Peripheral dose from CBCT imaging, dose reduction methods, the use of effective dose in describing imaging dose, and the measurement of CT dose index (CTDI) in CBCT systems are also reviewed.     

Low-dose-rate radiation attenuates the response of the tumor suppressor TP53

Acute low-dose irradiation (0.1-1 Gy, 1.33 Gy/min) of cells of a human glioblastoma cell line, A-172, induced a dose-dependent monophasic accumulation of TP53 (formerly known as p53) and CDKN1A (formerly known as WAF1). In contrast, chronic gamma irradiation (0.001 Gy/min) produced a clear biphasic response of accumulation TP53 with the first peak at 1.5 h (0.09 Gy) and the second peak at 10 h (0.54 Gy). Significantly, when the cells were preirradiated with a chronic dose of gamma irradiation for 24 h (1.44 Gy) or 50 h (3 Gy), they no longer responded to an acute challenging dose to produce a dose-dependent response of the TP53 pathway. These findings suggest that chronic irradiation at low dose rate alters the TP53-dependent signal transduction pathway. Wearing away of the TP53 pathway by chronic exposure to radiation may have important implications for radiation protection.     

Different sensitivity of carp (Cyprinus carpio) and rainbow trout (Oncorhynchus mykiss) to the immunomodulatory effects of UVB irradiation

In order to study the sensitivity of two fish species, carp (Cyprinus carpio) and rainbow trout (Oncorhynchus mykiss), to the immunomodulatory effects of ultraviolet B (UVB) radiation, the fish were exposed to a single UVB dose of 50, 250, 500 or 1,000 mJ cm(-2). These species represent different phylogenetic groups of fish, and they differ also in their behaviour inhabitating often dark and turbid (carp) or clear and transparent waters (salmonids). Immune responses were studied on day 1 post-irradiation. Unexposed fish, and fish exposed to radiation depleted of UV wavelengths served as controls. UVB irradiation markedly enhanced the blood respiratory burst and cytotoxic activity in carp, but in the head kidney these parameters were significantly suppressed. Rainbow trout respiratory burst was affected only after exposure with the highest dose of UVB. Lymphopenia and granulophilia were noted in both fish blood after exposure. This study indicates that UVB irradiation modulates immune functions in both fish species studied, and that rainbow trout is more tolerant than carp against UVB. Fish are clearly adapted to the environmental UVB levels prevailing in their usual living habitats, but are also a target of undesired effects of UVB on immune functions whenever exposed to increased radiation levels.     

The response of murine stem spermatogonia to radiation combined with 3-aminobenzamide

The influence of 3-aminobenzamide (3-AB) on the radiation response of the stem spermatogonia of the CBA mouse has been investigated. Doses of 3-AB from 66 to 450 mg/kg, administered 1 h before irradiation, significantly enhanced stem-cell killing. Enhancement was observed when 3-AB (450 mg/kg) was given up to 5 h before, but not if administered after, irradiation. When radiation was delivered at a lower dose rate (5 cGy/min compared to 180 cGy/min) significant dose sparing was achieved for radiation alone. Pretreatment with 3-AB resulted in slightly less enhancement at the low dose rate than at the high. Split-dose studies (9 Gy total dose) with radiation alone resulted in a recovery ratio of 1.4-1.5. Administration of 3-AB before the first dose resulted in a similar recovery ratio, but if given immediately after the first dose the ratio was smaller. Pretreatment of mice with the radiosensitizer RSU-1069 indicated that at least some of the stem cells were radiobiologically hypoxic. We suggest therefore that the enhancement of spermatogonial stem-cell killing by 3-AB is not entirely due to inhibition of repair processes but may also involve modification of the oxygen status of the testis.     

Rapid acquisition of preference in concurrent schedules: Effects of d-amphetamine on sensitivity to reinforcement amount

In the present study, effects of d-amphetamine on sensitivity to reinforcement amount under concurrent schedules were examined using a rapid-acquisition procedure. Four pigeons key pecked under single concurrent variable-interval 30-s schedules of grain presentation. Two different reinforcer-amount ratios (7:1 and 1:7) changed across sessions according to a 31-step pseudo-random binary sequence (PRBS). After at least four times through the PRBS, response ratios generally tracked the session-to-session changes in amount ratios; estimates of sensitivity ranged from 0.26 to 0.31 across the four pigeons. Effects of a range of doses of d-amphetamine (0.3-5.6 mg/kg) then were determined. For 3 of 4 pigeons, at least one dose, which did not dramatically alter overall response output or bias, decreased sensitivity to reinforcement amount. These results suggest that reducing sensitivity of responding to reinforcement amount may be one behavioral mechanism of stimulants, which may have implications for interpreting drug effects on self-control.     

Multi-endpoint biological monitoring of phosphine workers

5-Aminosalicylic acid (5ASA), a prescribed drug for ulcerative colitis, is a potent scavenger of oxygen-derived free radicals. The present study was undertaken to ascertain its ability to protect against radiation-induced damage. The drug dose-dependent effect, optimum time of drug administration and radiation dose-dependent effect (0-4 Gy) on in vivo radiation protection against micronuclei induction in polychromatic erythrocytes (PCE) and normochromatic erythrocytes (NCE) were studied in the bone marrow of mice. Intraperitoneal injection of 10-125 mg/kg of the drug 30 min before whole body irradiation with 3 Gy produced a significant reduction in the frequency of micronucleated erythrocytes at 24 h after exposure. The optimum dose for protection without drug toxicity was 25 mg/kg body weight. Injection of 25 mg/kg of the drug 60 or 30 min before or within 15 min after 3 Gy whole body gamma-irradiation resulted in a significant decrease in the radiation-induced PCE and NCE with micronuclei (MPCE and MNCE) and an increase in the ratio of PCE to NCE (P/N), at 24 h post-irradiation. Maximum effect was seen when the drug was administered 30 min before irradiation. Therefore, to study the radiation dose-response, mice were pre-treated with 25 mg/kg of 5ASA 30 min before 1-4 Gy of gamma-irradiation. Radiation increased the MN frequency linearly (r(2)=0.99) with dose. Pre-treatment with 5ASA significantly reduced the MN counts to 40-50% of the radiation (RT) alone values, giving a dose modification factor (DMF) of 2.02 (MPCE) and 2.53 (MNCE). Irradiation resulted in a dose-dependent decline in the P/N ratio at all the doses of radiation studied. 5ASA produced a significant increase in the P/N ratio from that of irradiated controls, at all doses of radiations tested. These results show that 5ASA protect mice against radiation-induced MN formation and mitotic arrest.     

The Trp53 pathway is induced in vivo by low doses of gamma radiation

The induction of the Trp53 response after very low doses (0.01-1 Gy) of ionizing radiation was studied in the adult mouse using immunochemical and immunohistochemical methods. We found a detectable response at 0.01 Gy and an increased induction of Trp53 with increasing dose in both radiation-resistant and radiation-sensitive tissues. These results suggest that there is no lower threshold for induction. This response was heterogeneous, since cells that received the same dose had different staining intensities, suggesting that the induction of Trp53 is not based simply on dose-dependent responses to DNA damage. These data also demonstrate the exquisite sensitivity of the Trp53 pathway and show that this response is controlled by cell- and tissue-specific factors that have yet to be defined.