The low-dose problem in radiobiology

Three aspects of the problem of "low doses" in radiobiology are discussed, they are: (1) universality of the stimulatory effect of penetrating radiation on biological objects, (2) the doses used, and (3) the damages induced by irradiation with these doses.     

Toward a consensus on radiobiology teaching to radiation oncology residents

There are approximately 82 radiation oncology residency programs in the United States, which provide training opportunities for about 400 residents. All accredited radiation oncology residency programs must have at least one basic scientist on the faculty, and it is these individuals who often assume, wholly or in part, the responsibility of teaching radiation and cancer biology to radiation oncology residents in preparation for the American College of Radiology (ACR) In-Training Examination in Radiation Oncology and the American Board of Radiology (ABR) written examinations. In response to a perceived lack of uniformity in radiation and cancer biology curricula currently being taught to residents and a perceived lack of guidance for instructors in formulating course content for this population, a special session was presented at the Forty-eighth Annual Radiation Research Society meeting on April 23, 2001. The session, entitled "Toward a Consensus on Radiobiology Teaching to Radiation Oncology Residents", was focused on issues related to teaching radiobiology to radiation oncology residents and targeted for individuals who actively teach radiation and cancer biology as well as coordinators of residency training programs. The speakers addressed current challenges and future problems facing instructors and programs. Among these were lack of feedback on resident performance on ABR and ACR written examinations and on course content, uncertainty about what topics residents must know to pass the ABR examination, and, in the near future, a reduction (due to retirement) of instructors qualified to teach radiobiology. This article provides a synopsis of the information that was presented during that session, offers a glimpse into how the ABR and ACR examinations are prepared and details of the content of past and future examinations, and summarizes the activities of the Joint Working Group on Radiobiology Teaching which was formed to educate instructors, to establish a consensus for course curricula, and to improve the overall quality of resident teaching.     

Biological low-dose radiation effects.

It is theorized that biological responses to ionizing radiation in the low dose range are determined according to a doubly dichotomous pattern. Energy depositions fall into 2 categories: events at thermal energy levels where they may be experienced by cells as rates even at background exposure conditions, and events at energy levels of the order of 10-100 eV where damage to DNA may be caused. Variations in background exposure intensity may or may not lead preemptively to changes in the cell's capacity for response to radiation damage. High-level energy depositions lead post hoc to an initial stabilizing reaction largely leading to the fixation of the initial DNA damage, and to a subsequent restorative or palliative repair process. This model entails reinterpretation of some experimental results. The model has implications for the relationship between scientific analysis of low-dose effects and the regulatory needs for simplicity and homogeneity in risk evaluation. This represents a new challenge for the acceptability of radiation protection norms.     

Fullerenes in radiobiology

Molecule of fullerene, having a spherical or ellipsoidal shape, is made of rings consisting of five or six carbon atoms, combined with conjugated pi bonds. Delocalization of pi electrons in the molecule of fullerene makes it easy to scavenge free radicals. But, despite being the effective antioxidants and radical scavengers fullerenes may be prooxidants by reactive oxygen species generation. Mammalian cells consist mainly of water (about 70%). Thus, the radical and non-radical products of water radiolysis are the basic sources of radiation damage to biomolecules. Reactive oxygen species, such as hydroxyl (HO*) and superoxide (O2-*) radicals and hydrogen peroxide (H2O2), are responsible for radiation-induced damage in aerated systems. Free radical mechanism of radiation damage suggests that scavengers of free radicals should protect cellular structures against damage. Electron donor compounds should also exhibit protective properties towards oxidized functional groups by reducing them. However, the electron transfer from fullerene to oxygen may generate superoxide radical. The shape of fullerenes allows them to act as carriers of radioactive atoms of isotopes used in the therapy and medical diagnostics. Fullerenes and their derivatives due to its properties are new promising chemicals for application in radiobiology. Fullerenes may be radioprotectors, radiosensitizer or auxiliary compounds in diagnostic imaging. What they are depends on the experimental system used.     

Thyroid neoplasia following low-dose radiation in childhood

The thyroid gland is highly sensitive to the carcinogenic effects of ionizing radiation. Previously, we reported a significant increase of thyroid cancer and adenomas among 10,834 persons in Israel who received radiotherapy to the scalp for ringworm. These findings have now been extended with further follow-up and revised dosimetry. Overall, 98 thyroid tumors were identified among the exposed and 57 among 10,834 nonexposed matched population and 5392 sibling comparison subjects. An estimated thyroid dose of 9 cGy was linked to a fourfold (95% Cl = 2.3-7.9) increase of malignant tumors and a twofold (95% Cl = 1.3-3.0) increase of benign tumors. The dose-response relationship was consistent with linearity. Age was an important modifier of risk with those exposed under 5 years being significantly more prone to develop thyroid tumors than older children. The pattern of radiation risk over time could be described on the basis of a constant multiplication of the background rate, and an absolute risk model was not compatible with the observed data. Overall, the excess relative risk per cGy for thyroid cancer development after childhood exposure is estimated as 0.3, and the absolute excess risk as 13 per 10(6) PY-cGy. For benign tumors the estimated excess relative risk was 0.1 per cGy and the absolute risk was 15 per 10(6) PY-cGy.     

Thymocyte apoptosis in response to low-dose radiation

Thymocyte apoptosis was assessed by counting apoptotic bodies with flow cytometry (FCM) and measuring DNA fragmentation with fluorescence spectrophotometry (FSP). J-shaped dose-response curves were obtained after both whole-body irradiation (WBI) of mice and in vitro irradiation of EL4 cells with doses ranging from 0.025 to 4 Gy X-rays. There was a significant reduction of apoptosis rate to below control level with doses within 0.2 Gy, and a dose-dependent increase in apoptosis with doses above 0.5 Gy. When thymocytes were cultured 24 h after WBI with 75 mGy X-rays in complete RPMI 1640 medium, a reduction in apoptosis was observed in the course of incubation for 72 h, and the presence of Con A in the medium accentuated this reduction in a dose- and time-dependent manner. The implications of these observations and the possible molecular mechanisms for future studies are proposed.     

Attenuation of diabetes-induced cardiac inflammation and pathological remodeling by low-dose radiation

In the present study, novel preventive effects of repeated low-dose radiation exposure on diabetes-induced cardiac inflammation and cardiac damage were investigated. C57BL/6J mice were given multiple low doses of streptozotocin (STZ, 60 mg/kg × 6) to generate type 1 diabetes. A week after the last STZ injection, hyperglycemic mice were diagnosed and treated with and without whole-body low-dose radiation exposure (25 mGy X rays) once every 2 days for 2, 4, 8, 12 and 16 weeks. Diabetes caused significant increases in cardiac inflammation, shown by time-dependent increases in mRNA and protein expressions of interleukin 18 (IL-18), tumor necrosis factor-alpha (TNF-α), intercellular adhesion molecule 1 (ICAM-1), plasminogen activator inhibitor 1 (PAI-1), and monocyte chemoattractant protein 1 (MCP-1). Repeated exposure of control mice to low-dose radiation caused mild increases in these inflammatory factors, except for ICAM-1. Repeated exposure of diabetic mice to low-dose radiation significantly reduced diabetes-increased cardiac expression of IL-18, TNF-α, MCP-1 and PAI-1 at both the mRNA and protein levels. Furthermore, cardiac histopathological abnormalities, oxidative damage and fibrosis were significant in diabetic mice but to a lesser extent in diabetic mice with repeated low-dose radiation exposure. These results suggest that although low-dose radiation contributes to mild cardiac inflammation in control mice, it can significantly reduce diabetes-induced cardiac inflammation and associated pathological changes. Therefore, low-dose radiation may potentially become a novel approach to the prevention of diabetic cardiovascular complications.     

Apoptosis in HeLa Hep2 cells is induced by low-dose,low-dose-rate radiation

Radioimmunotherapy with radiolabeled antibodies may cause inhibition of the growth of epithelial tumors, despite low total radiation doses and comparatively low radiosensitivity of epithelial tumor cells. The induction of apoptosis by low-dose radiation, such as delivered in radioimmunotherapy, was investigated in vitro in human HeLa Hep2 carcinoma cells. The cultured cells were exposed to defined radiation doses from a (60)Co radiation therapy source. The radiation source delivered 0.80 +/- 0.032 (mean +/- SD) Gy/min and the cells were given total doses of 1, 2, 5, 10 and 15 Gy. Using fluorescein-labeled Annexin V, followed by flow cytometry and DNA ladder analysis, apoptotic cells were detected and quantified. Radiation doses below 2 Gy did not cause any significant increase in apoptosis. Compared to control cells, apoptosis was pronounced after 5-10 Gy irradiation and was correlated to the radiation dose, with up to 42 +/- 3.5% of the cells examined displaying apoptosis. Clonogenic assays confirmed significantly decreased viability of the cells in the interval 2 to 10 Gy with low-dose-rate radiation, 60 +/- 2% compared to 2 +/- 2%. Lethal effects on the tumor cells were also evaluated by an assay of the cytotoxic effects of the release of (51)Cr. Significant cytotoxicity, with up to 64 +/- 6% dead cells, was observed at 5 Gy. Similar results were obtained when the dose rate was reduced to 0.072 +/- 0.003 Gy/min (mean +/- SD). In the case of the (137)Cs source, the dose rate could be reduced to 0.045 Gy/h, a level comparable to radioimmunotherapy, which induced significant apoptosis, and was most pronounced at 72-168 h postirradiation. It can be concluded that in vitro low-dose and low-dose-rate radiation induces apoptosis in epithelial HeLa Hep2 cells and thus may explain a mechanism by which pronounced inhibition of growth of HeLa Hep2 tumors at doses used in radioimmunotherapy has been obtained previously.     

Changing paradigms in radiobiology

The last 25 years have seen a major shift in emphasis in the field of radiobiology from a DNA-centric view of how radiation damage occurs to a much more biological view that appreciates the importance of macro-and micro-environments, hierarchical organization, underlying genetics, evolution, adaptation and signaling at all levels from atoms to ecosystems. The new view incorporates concepts of hormesis, nonlinear systems, bioenergy field theory, uncertainty and homeodynamics. While the mechanisms underlying these effects and responses are still far from clear, it is very apparent that their implications are much wider than the field of radiobiology. This reflection discusses the changing views and considers how they are influencing thought in environmental and medical science and systems biology.     

A microdosimetric understanding of low-dose radiation effects

This paper presents a microdosimetric approach to the problem of radiation response by which effects produced at low doses and dose rates can be understood as the consequences of radiation absorption events in the nucleus of a single relevant cell and in its DNA. Radiation absorption at the cellular level, i.e. in the cell nucleus as a whole, is believed to act through radicals. This kind of action is called 'non-specific' and leads to the definition of an 'elemental dose' and the 'integral response probability' of a cell population. Radiation absorption at the molecular level, i.e. in sensitive parts of the DNA, is thought to act through double-strand breaks. This kind of action is called 'specific' and leads to a 'relative local efficiency'. In general, both mechanisms occur for all types of radiation; however, it is the dose contribution of both specific and non-specific effects that determines the radiation quality of a given radiation. The implications of this approach for the specification of low-dose and low dose-rate regions are discussed.     

Quantitative risk in radiation protection standards

Summary Although the overall aim of radiobiology is to understand the biological effects of radiation, it also has the implied practical purpose of developing rational measures for the control of radiation exposure in man. The emphasis in this presentation is to show that the enormous effort expended over the years to develop quantitative dose-effect relations relationships in biochemical and cellular systems, animals and human beings, now seems to be paying off. The pieces appear to be falling into place, and a framework is evolving to utilize these data. Specifically, quantitative risk assessments will be discussed in terms of the cellular, animal and human data on which they are based; their use in the development of radiation protection standards; and their present and potential impact and meaning in relation to the quantity dose equivalent and its special unit, the rem. Recent neutron carcinogenesis data in man are evaluated.Invited paper, presented at the 14th Annual Meeting of European Society of Radiation Biology, Jülich, Germany, October 8–14, 1978     

Protection by pantothenol and beta-carotene against liver damage produced by low-dose gamma radiation.

Rats were exposed to a total dose of 0.75 Gy of gamma radiation from a 60Co source, receiving three doses of 0.25 Gy at weekly intervals. During two days before each irradiation, the animals received daily intragastric doses of 26 mg pantothenol or 15 mg beta-carotene per kg body mass. The animals were killed after the third irradiation session, and their blood and livers were analyzed. As found previously (Slyshenkov, V.S., Omelyanchik, S.N., Moiseenok, A.G., Trebukhina, R.V. & Wojtczak, L. (1998) Free Radical Biol. Med. 24, 894-899), in livers of animals not supplied with either pantothenol or beta-carotene and killed one hour after the irradiation, a large accumulation of lipid peroxidation products, as conjugated dienes, ketotrienes and thiobarbituric acid-reactive substances, could be observed. The contents of CoA, pantothenic acid, total phospholipids, total glutathione and GSH/GSSG ratio were considerably decreased, whereas the NAD/NADH ratio was increased. All these effects were alleviated in animals supplied with beta-carotene and were completely abolished in animals supplied with pantothenol. In the present paper, we extended our observations of irradiation effects over a period of up to 7 days after the last irradiation session. We found that most of these changes, with the exception of GSH/GSSG ratio, disappeared spontaneously, whereas supplementation with beta-carotene shortened the time required for the normalization of biochemical parameters. In addition, we found that the activities of glutathione reductase, glutathione peroxidase, catalase and NADP-dependent malate (decarboxylating) dehydrogenase ('malic enzyme') in liver were also significantly decreased one hour after irradiation but returned to the normal level within 7 days. Little or no decrease in these activities, already 1 h after the irradiation, could be seen in animals supplemented with either beta-carotene or pantothenol. It is concluded that pantothenol is an excellent radioprotective agent against low-dose gamma radiation.