Major types of radiation injury
Any living organism can be killed by radiation if exposed to a large enough dose, but the lethal dose varies greatly from species to species. Mammals can be killed by less than 10 Gy, but fruit flies may survive 1,000 Gy. Many bacteria and viruses may survive even higher doses. In general, humans are among the most radiosensitive of all living organisms, but the effects of a given dose in a person depend on the organ irradiated, the dose, and the conditions of exposure.
The biologic effects of radiation in humans and other mammals are generally subdivided into (1) those that affect the body of the exposed individual—somatic effects—and (2) those that affect the offspring of the exposed individual—genetic, or heritable, effects. Among the somatic effects, there are those that occur within a short period of time (e.g., inhibition of cell division) and those that may not occur until years or decades after irradiation (e.g., radiation-induced cancer). In addition, there are those, called non-stochastic effects, that occur only in response to a considerable dose of radiation (e.g., ulceration of the skin) and those, termed stochastic, for which no threshold dose is known to exist (e.g., radiation-induced cancer).
Every type of biologic effect of radiation, irrespective of its precise nature, results from injury to the cell, the microscopic building block of which all living organisms are composed. It therefore seems useful to open a review of such effects with a discussion of the action of radiation on the cell.
Effects on the cell
The effects of radiation on the cell include interference with cell division, damage to chromosomes, damage to genes (mutations), neoplastic transformation (a change analogous to the induction of cancer), and cell death. The mechanisms through which these changes are produced are not yet fully understood, but each change is thought to be the end result of chemical alterations that are initiated by radiation as it randomly traverses the cell.
Any type of molecule in the cell can be altered by irradiation, but the DNA of the genetic material is thought to be the cell’s most critical target, since damage to a single gene may be sufficient to kill or profoundly alter the cell. A dose that can kill the average dividing cell (say, 1–2 Sv) produces dozens of lesions in the cell’s DNA molecules. Although most such lesions are normally reparable through the action of intracellular DNA repair processes, those that remain unrepaired or are misrepaired may give rise to permanent changes in the affected genes (i.e., mutations) or in the chromosomes on which the genes are carried, as discussed below.
In general, dividing cells (such as cancer cells) are more radiosensitive than nondividing cells. As noted above, a dose of 1–2 Sv is sufficient to kill the average dividing cell, whereas nondividing cells can usually withstand many times as much radiation without overt signs of injury. It is when cells attempt to divide for the first time after irradiation that they are most apt to die as a result of radiation injury to their genes or chromosomes.
The percentage of human cells retaining the ability to multiply generally decreases exponentially with increasing radiation dose, depending on the type of cell exposed and the conditions of irradiation. With X rays and gamma rays, traversal by two or more radiation tracks in swift succession are usually required to kill the cell. Hence, the survival curve is typically shallower at low doses and low dose rates than at high doses and high dose rates. The reduced killing effectiveness of a given dose when it is delivered in two or more widely spaced fractions is attributed to the repair of sublethal damage between successive exposures. With densely ionizing particulate radiations, on the other hand, the survival curve is characteristically steeper than with X rays or gamma rays, and its slope is relatively unaffected by the dose or the dose rate, implying that the death of the cell usually results from a single densely ionizing particle track and that the injury produced by such a track is of a relatively irreparable type.