The net result of any type of radiation moving through matter, including our cells and tissues, is that it deposits energy by ionizing atoms, according to the interac­tions described in the previous section. In order to make quantitative statements about the hazards of radiation, it is essential to define what is meant by a dose of radiation. A dose is a certain amount of energy deposited in a certain mass of material. However, there are different ways to specify the dose, depending on the type of radiation and the type of cells or tissues that are in its path.

The most basic definition of dose is called absorbed dose (D), which is given in units of gray (Gy) or rad. The official unit for absorbed dose is the gray, named after the British physicist Louis Gray who established a famous radiation labora­tory in Oxford, England, now known as the Gray Institute for Radiation Oncology and Biology. One gray is 1 joule of energy from ionizing radiation absorbed in 1 kilogram of matter. An older definition for absorbed dose is rad (radiation absorbed dose)—which is still frequently used in the United States—and is 100 ergs of energy absorbed in 1 gram of matter. One gray is equal to 100 rads. An absorbed dose of 1 Gy is independent of the type of radiation, so 1 Gy of у rays is equal to 1 Gy of protons is equal to 1 Gy of electrons is equal to 1 Gy of a particles, since it is always the same amount of energy absorbed in a given mass of matter.

The absorbed dose is not sufficient to understand the biological effects of radia­tion, however, since different types of radiation can produce different levels of damage in cells. The principal effect of radiation on cells is to cause damage to the DNA. Gamma rays and electrons are not nearly as efficient in doing this as a particles and neutrons, for example. In order to compare the biological effects of different types of radiation, radiation biologists irradiate cells and determine what dose of protons or neutrons or a particles cause the same amount of damage as a given dose of у or X rays. The results from these experiments determine the relative biological effectiveness (RBE) of different types of radiation compared to X-rays. These values of RBE for different radiations are assessed by national and international scientific agencies such as the National Council on Radiation Protection and Measurements (NCRP) in the United States and the International Commission on Radiological Protection (ICRP) and are formulated as radiation weighting factors (WR) for different kinds of radiation. A new measure of absorbed dose, known as the equivalent dose (H), is then defined as the dose (D) times the radiation weighting factor for a particular type of radiation. The equivalent dose, H, is still given as absorbed energy in a given mass but varies by the type of radia­tion. The official unit is sievert (Sv), which is the dose in Gy times the weighting factor, named after the Swedish physicist Rolf Sievert who invented the ionization chamber to measure doses of radiation (6). The older but still-used unit is rem (roentgen equivalent man), which is the dose in rads times the weighting factor (7). One Sv equals 100 rem. The various units for dose are given in Table 7.1 and the radiation weighting factors are given in Table 7.2.

Because of the radiation weighting factors, one Sv is equal to one Gy for у radia­tion or electrons, but for neutrons or a particles, one Sv is equal to about 20 Gy in terms of biological damage. For this reason, doses are normally specified in sievert or rem so that the biological effects are independent of the type of radia­tion. Then you can predict that the biological effects of 1 Sv of у radiation are the same as 1 Sv of a particles or 1 Sv of electrons or 1 Sv of neutrons.

There is one other factor to consider when a person is exposed to radiation, and that is the specific parts of the body that are exposed. For example, radia­tion workers may have a higher exposure to their hands or feet than the rest of

Table 7.2 Radiation Weighting Factors (ICRP)

Type of Radiation

Photons (X and y)

Electrons (P)

Protons

a-particles, fission fragments, heavy nuclei Neutrons

their body. People who breathe in radon are exposing the lungs. As it turns out, there are differences in the radiation sensitivity of different tissues, which leads to another factor, known as the tissue weighting factor (WT), to compare the various tissues (Table 7.3). Tissues with larger values of WT are more sensitive than those with smaller values of WT There is a big difference in sensitivity between the skin, for example, and the lungs or breast. Total WT is the summation of individual tissue weighting factors for each of the tissues. For a total human body, the total weighting factor is one, which just means that the sensitivity of the entire body is the sum of the sensitivities of the individual tissues. The effective dose is the equivalent dose times the tissue weighting factor.