Radiation Units

Cellular reproducibility and conservation of genetic stability are the two cellular functions most important for the development and maintenance of life. However, both tasks can be disturbed by ionizing radiation with the result of cell death or induction of mutations. Cell survival, in radiobiological terms, is understood as the ability for indefinite reproduction. In dose-effect curves, the surviving fraction of irradiated cells relative to that of non-irradiated cells is plotted on the ordinate logarithmically versus dose on a linear abscissa scale (Figure 7-08).

With increasing dose the number of survivors decreases. The corresponding dose-effect curve declines continuously and can be simply characterized by two parameters. One is the D0, which is defined as the dose necessary to reduce survival to e"1 (=0.37), it can be calculated from the slope of the terminal straight part of the curve (-1/slope). The other parameter is the extrapolation number n, which is calculated from the backward extrapolation of the straight proportion of the effect curve.

Radiation sensitivity of different organisms can be compared on the basis of these parameters. Their sensitivity is related to the amount of genetic material per cell and to their DNA repair capacity. The most resistant organisms are exclusively single-stranded viruses, followed by double-stranded viruses, bacteria, algae, and yeast. For simple eukaryotes, it could be shown that haploid cells are about twice as sensitive as diploid cells. The most radiation resistant bacterium known is Deinococcus radiodurans. It was originally isolated from samples of canned meat that were thought to be sterilized by high doses of y-radiation. Typically, it is found in locations where most other bacteria have died under extreme conditions, ranging from the shielding pond of a radioactive cesium source to the surfaces of Arctic rocks. D. radiodurans can tolerate doses up to 4 kGy without remarkable cell death (Figure 7-08).

Ionizing radiation is measured in the S.I. unit of absorbed dose per mass unit, the Gray (Gy), with 1 Gy equal to the net absorption of 1 J in 1 kg of water. Compared to the previously used unit rad: 1 Gy = 100 rad. However, the biological effectiveness of radiation largely depends on the local energy distribution, the Linear Energy Transfer (LET). Therefore, different qualities of radiation can have different biological effectiveness, even at the same physical dose. The Relative Biological Effectiveness (RBE) describes this dependence of the biological effectiveness on LET. RBE is the ratio of the physical doses of the test radiation and e.g., X-rays, leading to the same biological effect. The RBE value can be different for different biological systems, depending on their stage in the growth cycle and other environmental factors, such as the oxygen content.

In order to assess the effectiveness posed by radiation to humans and also the whole biosphere, estimates must be made of both the amount and type of radiation under consideration as well as the radiobiological effectiveness of the different components of the radiation. For this purpose, the Quality Factor (Q) has been introduced. Q is the biological weighting function of ionizing radiation and has been obtained by averaging over a variety of RBE values for the same LET value. Its relation to the LET of the radiation is shown in Figure 7-09.

It should be stressed that Q is an estimate of maximum RBE for the biological endpoint cancerogenesis only. For X-rays and y rays, Q is equal to unity. For a given dose of high-LET radiation, the dose equivalent H is the product of the quality factor Q and the absorbed dose D (Equation 7):

H=QD

The S.I. unit for the dose equivalent is Sievert (Sv)5. For a mixed radiation field composed of ionizing radiations of different radiation qualities i (as encountered in space), the dose equivalent H is given by (Equation 8).

with Dj = absorbed dose, deposited in biological matter by the radiation i (Gy), Qj = radiation quality which is described as a function of LET, and Ni = a special factor which accounts for specific exposure conditions (e.g., dose rate, fractionated exposure, microgravity) or special physiological properties. For terrestrial radiation protection applications, N is set equal to unity.

Figure 7-09. The quality factor Q is the biological weighting function of ionizing radiation and is dependent on the linear energy transfer (LET) of the radiation under consideration.

Figure 7-09. The quality factor Q is the biological weighting function of ionizing radiation and is dependent on the linear energy transfer (LET) of the radiation under consideration.

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