Synonyms

Radiation interactions with living matter

Definition

Radiation biology is an interdisciplinary subject that describes the biological effects of ionizing radiations. It is based on studies in physics, chemistry, biology, and medicine.

History

The history of radiation biology started shortly after the discovery of X-rays in 1895 by Wilhelm Conrad Röntgen, who was awarded the first Nobel Prize in Physics in 1901. Within short time, X-rays were being used not only to take pictures of the internal organs of living people but also to treat a variety of diseases. The discovery of natural radioactivity by Antoine Henri Becquerel in 1896 was the prerequisite for the detection of terrestrial and cosmic radiation. In 1903, Becquerel shared the Nobel Prize in Physics with Pierre and Marie Curie “in recognition of the extraordinary services he has rendered by the discovery of spontaneous radioactivity.” Interaction of radiation energy with living matter was observed, and the radiation sensitivity law was formulated by Jean Alban Bergonie and Louis Tribondeau as early as 1906. The genetic effects of radiation, including the effects on cancer risk, were recognized much later. In 1946, Hermann Joseph Muller was awarded the Nobel Prize in Physiology or Medicine, for the discovery of the production of mutations by means of X-ray irradiation.

Overview

Life on Earth has been shaped by interactions of the organisms with their environment, including radiation of terrestrial and cosmic origin, by numerous adaptive responses. Radiation biology describes the interactions of radiation with living matter through primarily ionization and excitation of electrons in atoms and molecules. Starting with the energy absorption of irradiated water molecules, a series of follow-up reactions is initiated (water radiolysis) in cells and tissue which give rise to the indirect effects of radiation in addition to the direct energy absorption effects in biological key substances, such as proteins, RNA, and DNA. Direct consequences of molecular radiation damage on cellular level are inactivation or mutation induction. Dependent on the organizational level of life, cellular damage may result in tissue damage or even in the death of the exposed organism. Cell killing by ionizing radiation depends on different factors, such as DNA content (target size: number and size of chromosomes) and the quality of radiation (sparsely/densely ionizing radiation). The effect of radiation on inactivation (killing) is described in terms of survival curves. In radiobiological terms, cell survival is understood as the ability for indefinite cell reproduction. In dose-effect curves, the surviving fraction of irradiated cells relative to that of nonirradiated cells is plotted in a half-logarithmic manner, showing the number of survivors to decrease with increasing dose. The parameter D0 describing radiation sensitivity is derived from the slope of the terminal part of the dose-effect curve as −1/slope (Fig. 1). While human cells will be killed by doses of about 1.5 Gy (D0), yeast cells and bacteria tolerate doses up to 50–200 Gy (D0). The most radiation-resistant bacterium is Deinococcus radiodurans (D0 >5,000 Gy). In response to the harmful effects of environmental radiation, life has developed a variety of defense mechanisms, including the expression of stress proteins, the activation of the immune defense, and a variety of efficient repair systems for radiation-induced DNA injury. The radiation response of each life form is influenced by the physical properties of the radiation in question. The biological effectiveness of radiation largely depends on the local energy distribution, the linear energy transfer (LET, measured in keV/μm). The relative biological effectiveness (RBE) is defined as the ratio of physical doses of the densely ionizing radiation (heavy ions, neutrons) compared to the doses of sparsely ionizing standard radiation (usually 250 kV X-rays) leading to the same effect. The RBE value is different for different biological systems and end points, depending on their stage in the growth cycle and a series of environmental factors, for example, oxygen content. For interplanetary space radiation, which is composed of different radiation types and qualities, RBE values are still to be investigated.

Radiation Biology, Fig. 1
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Survival curves of Bacillus subtilis spores in response to X-rays (open squares) and accelerated heavy ions: helium (open triangles up), carbon (open triangles down), silicon (open diamonds), and iron (open circles) (From Moeller et al. 2010)

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