Radiation Effects

Interaction of Radiation With Matter

When radiation penetrates any medium (matter), rays interact with the molecules of the medium through a collision process. This results in a transfer, a giving up, of energy by the radiation to the medium. This transferred energy is also referred to as the absorbed energy.

In air, the molecules are simple, lightweight, and widely separated, while in denser materials such as bone, concrete, and lead, the molecules are closely packed, heavy, and complex in structure. These differences strongly affect the way the interactions take place and the amount of energy absorbed, as does also the energy of the radiation involved.

Radiation Exposure/Dose

Radiation is similar to sunlight in certain respects. Exposure to sunlight, or a sun lamp, will produce a certain effect on our skin. For x-rays and gamma radiation, exposure is the measurement in air expressed in roentgens. The effect of the radiation in tissue is expressed in rads and referred to as the dose. This dose is, of course, the energy absorbed from the beam of radiation.

Therapeutic doses are usually in hundreds or thousands of rads. Diagnostic doses are as low as possible, but seldom exceed a few hundred millirads. Since the rad is difficult to measure directly, radiation exposure (the roentgen) is usually measured. The roentgen is a unit related to the amount of ionization that is produced in air by gammas or x-rays. A third unit often used is the dose equivalent, the Roentgen Equivalent Man (rem). This unit takes into account the biological effect of the particular type of radiation and is equal to rads times a quality factor (QF). Since the QF for x-rays, gammas, and betas is equal to l, rads are equal to rems for these types of radiation.

For practical purposes, the roentgen, rad, and rem are equal. Radiation exposures can be measured and expressed as roentgens, or milliroentgens (mR), one-thousandth of a roentgen. While occupational doses should be maintained ALARA, the maximum permissible dose equivalent to the whole body of a radiation worker is 5,000 mrem per year. As a comparison, a person receives approximately 20 mrem from an ordinary chest x-ray.

Biological Effects

Different types of body tissues respond differently to radiation. In general, tissues with cells that divide rapidly (e.g., blood cells) are more susceptible to radiation damage than those that divide slowly (e.g., nerve cells). This fact accounts, in part, for the usefulness of radiation when treating certain types of fast-growing tumors, and also for the deleterious effects of radiation to the fetus and certain normal tissues.

In this guide, radiation effects are discussed in a general way. It should be noted that the more serious radiation effects discussed are not likely to occur for the low radiation exposures in the hospital setting.

Everyone is exposed to some radiation. In our modern society, people are exposed to radiation from natural radiation (e.g., cosmic rays and naturally occurring radioisotopes) and man-made sources (e.g., medical radiation from x-rays, fluoroscopy, and radioisotopes, and from other sources such as luminescent watch dials, clock faces, and smoke detectors).

Somatic

Table 1 lists some effects of radiation exposures for various tissues, organs, and/or systems. It should be noted that 1) the maximum permissible dose equivalent for occupational workers is 5000 mrem (5 rem) per year, far less than the doses listed in the table, 2) that listed values are many times greater than those likely for hospital personnel, and 3) most of the effects noted in Table 1 result from acute radiation exposures (a large dose received in a short time) rather than chronic exposures (small amounts spread over a long time, like those in the hospital).

Genetic

Exposure of the reproductive organs to radiation can produce changes in the genes of the reproductive cells. These changes, or mutations, may be passed on to the children. Apparently, mutations can occur at any level of radiation exposure, however, the greater the exposure of the reproductive organs to radiation, the greater the likelihood of mutations.

There are many mechanisms other than radiation exposure that can cause genetic damage. Some genetic changes occur spontaneously and some damage can apparently be repaired.

Since the effects resulting from radiation exposure are small, the only way to measure them is to study large populations over several generations. While this has been done in laboratory animals, the information is not necessarily applicable to humans. Information from these experiments would indicate that the usual level of radiation exposure to the population as a whole is less than that which would be expected to produce a significant genetic change.

This effect is obviously not important for people past the reproductive age.

TABLE 1: RADIATION EFFECTS AT VARIOUS DOSES

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