Models for mixed irradiation with a 'reciprocal-time' pattern of the repair function. (9/96)

Suzuki presented models for mixed irradiation with two and multiple types of radiation by extending the Zaider and Rossi model, which is based on the theory of dual radiation action. In these models, the repair function was simply assumed to be semi-logarithmically linear (i.e., monoexponential), or a first-order process, which has been experimentally contradicted. Fowler, however, suggested that the repair of radiation damage might be largely a second-order process rather than a first-order one, and presented data in support of this hypothesis. In addition, a second-order repair function is preferred to an n-exponential repair function for the reason that only one parameter is used in the former instead of 2n-1 parameters for the latter, although both repair functions show a good fit to the experimental data. However, according to a second-order repair function, the repair rate depends on the dose, which is incompatible with the experimental data. We, therefore, revised the models for mixed irradiation by Zaider and Rossi and by Suzuki, by substituting a 'reciprocal-time' pattern of the repair function, which is derived from the assumption that the repair rate is independent of the dose in a second-order repair function, for a first-order one in reduction and interaction factors of the models, although the underlying mechanism for this assumption cannot be well-explained. The reduction factor, which reduces the contribution of the square of a dose to cell killing in the linear-quadratic model and its derivatives, and the interaction factor, which also reduces the contribution of the interaction of two or more doses of different types of radiation, were formulated by using a 'reciprocal-time' pattern of the repair function. Cell survivals calculated from the older and the newly modified models were compared in terms of the dose-rate by assuming various types of single and mixed irradiation. The result implies that the newly modified models for mixed irradiation can express or predict cell survival more accurately than the older ones, especially when irradiation is prolonged at low dose rates.  (+info)

Studies about space radiation promote new fields in radiation biology. (10/96)

Astronauts are constantly exposed to space radiation of various types of energy with a low dose-rate during long-term stays in space. Therefore, it is important to determine correctly the biological effects of space radiation on human health. Studies about biological the effects at a low dose and a low dose-rate include various aspects of microbeams, bystander effects, radioadaptive responses and hormesis which are important fields in radiation biology. In addition, space radiations contain high linear energy transfer (LET) particles. In particular, neutrons may cause reverse effectiveness at a low dose-rate in comparison to ionizing radiation. We are also interested in p53-centered signal transduction pathways involved in the cell cycle, DNA repair and apoptosis induced by space radiations. We must also study whether the relative biological effectiveness (RBE) of space radiation is affected by microgravity which is another typical component in space. To confirm this, we must prepare centrifuge systems in an International Space Station (ISS). In addition, we must prepare many types of equipment for space experiments in an ISS, because we cannot use conventional equipment from our laboratories. Furthermore, the research for space radiation might give us valuable information about the birth and evolution of life on the Earth. We can also realize the importance of preventing the ozone layer from depletion by the use of exposure equipment to sunlight in an ISS. For these reasons, we desire to educate space researchers of the next generation based on the consideration of the preservation of the Earth from research about space radiation.  (+info)

Development of an ion microbeam system for irradiating single plant cell[s]. (11/96)

An ion microbeam system for irradiating single plant cells was developed to analyze exact biological effects of ion beams. Tobacco BY-2 protoplasts were used as a model of single plant cells. Protoplasts were cultured in thin agarose medium on a specially designed irradiation-vessel, which has a CR-39 nuclear track detector (a 100-micrometer thick sheet). The colony formation rate of unirradiated protoplasts was 22.7 +/- 6.7% (mean +/- SE of 3 different experiments) after a month of culture. Protoplasts were irradiated with programmed numbers of 18.3 MeV/u carbon ions that had been collimated by a 20-micrometer phi micro-aperture. After the irradiation, the positions within the protoplasts that were hit with ions were accurately determined by etching the CR-39 sheet in 13.4M KOH solution at 27 degrees centigrade for 9 h. The hit rate of the carbon ion microbeam, i.e., the percent of the ion particles that hit the protoplast that they were aimed at, was 56.9 +/- 2.4% (mean +/- SE of 7 different replications).  (+info)

A multi-port low-fluence alpha-particle irradiator: fabrication, testing and benchmark radiobiological studies. (12/96)

A new multi-port irradiator, designed to facilitate the study of the effects of low fluences of alpha particles on monolayer cultures, has been developed. The irradiator consists of four individual planar (241)Am alpha-particle sources that are housed inside a helium-filled Lucite chamber. Three of the radioactive sources consist of 20 MBq of (241)Am dioxide foil. The fourth source, used to produce higher dose rates, has an activity of 500 MBq. The four sources are mounted on rotating turntables parallel to their respective 1.5-microm-thick Mylar exit windows. A stainless steel honeycomb collimator is placed between the four sources and their exit windows by a cantilever attachment to the platform of an orbital shaker that moves its table in an orbit of 2 cm. Each exit window is equipped with a beam delimiter to optimize the uniformity of the beam and with a high-precision electronic shutter. Opening and closing of the shutters is controlled with a high-precision timer. Custom-designed stainless steel Mylar-bottomed culture dishes are placed on an adapter on the shutter. The alpha particles that strike the cells have a mean energy of 2.9 MeV. The corresponding LET distribution of the particles has a mean value of 132 keV/microm. Clonogenic cell survival experiments with AG1522 human fibroblasts indicate that the RBE of the alpha particles compared to (137)Cs gamma rays is about 7.6 for this biological end point.  (+info)

Sharpening the focus; the business of epithelial cell biology. An interview with Chris Potten by David Pearton. (13/96)

Dr Chris Potten is a singularly influential figure in the field of epithelial biology. His contributions have been seminal and include the introduction of the epidermal proliferative unit and of the concept of epidermal stem cells. With around 400 scientific papers and reviews to his credit as well as two books, he has certainly made his mark. His contributions have been recognised by the award of the Curie medal and recently the Weiss medal for radiation biology. Dr. Potten graciously agreed to be interviewed for this Special Issue of The International Journal of Developmental Biology. This interview was conducted via e-mail during June -August 2003.  (+info)

Medical ethics, clinical research, and special aspects in nuclear medicine. (14/96)

Medical ethics is the science of survival. It studies the working out of judgments on right or wrong referred to the human being as a biological entity interacting with the whole ecosystem. Medical ethics in clinical research raises numerous moral and technical issues. Methodological aspects are essential for carrying out the aim of clinical research. Medical ethics documents are inspired by the Nuremberg Code and culminate in the recently updated Helsinki Declaration of 1964. In Italy 2 ministerial decrees in 1997 and 1998 laid the basis for the work of a medical ethics committee. They acknowledge the European Good Clinical Practice Guidelines and set professional needs within ethical committees. In clinical research the use of ionising radiation merits special consideration. In the recent past, serious human rights abuses in radiation experiments of the 1950s and 1960s have been found. As regards research in this field we can refer to the publication of the International Commission on Radiological Protection (ICRP) and to the report of the World Health Organisation (WHO). Legislative decree no. 187 of May 26, 2000, which transposed the 97/43/ EURATOM Directive represents the most comprehensive and recent normative reference to clinical research using ionising radiation. However, law no. 39 of March 1, 2002 is important for the partial modifications of previous decrees (art. 108 of L.D. no. 230 of March 17, 1995 and, art. 4 and attachment III of L.D. no. 187 of May 26). In this paper medical ethics, research, methodological issues and aspects of ionizing radiation are discussed.  (+info)

Simulation of proton neutralization effect for neutron dosimetry. (15/96)

Neutron dose is transferred to biological materials through the recoil protons produced by elastic scattering. When a low-velocity proton collides with the atoms or molecules of a target, it changes to a hydrogen atom by electron capture; this hydrogen atom then changes to a proton by losing the electron. Because the hydrogen atom has a different ionization cross section from that of a proton, the charge exchange processes need to be considered to calculate stopping power for low energy protons. The proton neutralization effect has been simulated by using a proton track structure code developed by taking into account charge exchange processes. The microdosimetric spectrum for 1 MeV neutrons was calculated by assuming a continuous slowing down approximation (csda) and the results of the proton track code. It was found that hydrogen atoms after proton neutralized by electron capture contribute about 24% to neutron dose.  (+info)

Radiobiologic principles in radionuclide therapy. (16/96)

Although the general radiobiologic principles underlying external beam therapy and radionuclide therapy are the same, there are significant differences in the radiobiologic effects observed in mammalian cells. External beam and brachytherapy emissions are composed of photons, whereas radiations of interest in radionuclide therapy are particulate. The special features that characterize the biologic effects consequent to the traversal of charged particles through mammalian cells are explored with respect to DNA lesions and cellular responses. Information about the ways in which these radionuclides are used to treat cancers in experimental models are highlighted.  (+info)