Biological Effects of Ionising Radiation

Abstract

The effects of radiation at atomic and molecular levels have been discussed in the previous chapter: events that occur over a time frame of nanoseconds to milliseconds. The present chapter relates to the effects of radiation on visible structures, in other words cells and tissues: events that occur within a time range of seconds to months or even years. To begin with, the theories relating to the infliction of radiation damage at the cellular level must be mentioned. It is considered that this may occur in one of two main ways. There is the Direct Action theory, whereby the ionisation of and lesion to the target, most probably DNA, is the primary event. The Indirect Action theory refers to the formation of DNA lesions produced by free radicals, which have in this instance been the primary target for ionisation by the radiation. Experimental work shows that the effect of oxygen on radiosensitivity is mediated by indirect action, since the tissue pO₂ affects the formation of the free radicals, which damage the DNA. Today, both modes of action are considered to be important in producing cell death. It may be mentioned that while there is, as stated above, broad agreement that the most important site of damage is the DNA of the cell nucleus, it may not be the only one. Some radiation effects, for example radiation oedema, indicate that cell membranes may also be important targets.