In the same way, the eventual state of tissues that have been irradiated depends on how badly they were damaged. This, in turn, depends on the type and dose of radiation and just how it was given. Let us again use skin as an example. After a small dose of radiation, skin can look normal and function normally. After a larger dose it may be thinner than normal, look tightly stretched, move less freely, be darker in colour, have less hairs than before and remain dry when hot. In general, more specialised structures (like hair follicles and sweat glands) are less likely to be restored to normal than less specialised structures after an injury of any sort. Many of the specialised cells are replaced by scar tissue, rather than the original type of cell. Thus the processes of repair do not produce skin which looks or feels completely normal. Neither is irradiated skin completely normal in its ability to respond to further injury, as we have seen. However, unless it has received very high doses, irradiated skin does serve its main purpose of providing a protective covering for the body. On the other hand, after very high doses of radiation, the damage can be so severe that no healing is possible and the skin is lost. An ulcer then forms— one which will never heal without grafting. Naturally, your radiotherapist will be Very careful to try to prevent such a serious reaction as this.

The same principles apply for other tissues. For each one, it is known how much radiation can be given on average, without causing damage that cannot be satisfactorily repaired. What is ‘satisfactory’ is different for different tissues, partly because the loss of specialised structures is more serious for some tissues than others. For example, it is much less serious to have an area of skin which is hairless and can’t sweat than to have a kidney which doesn’t work.

To summarise this section, then, there are two facts which tend to make cancer by its very nature more vulnerable to radiation than normal tissues. One is that the part of the cell that is most easily damaged by radiation is the part to do with reproduction. The other is that cancer cells cannot repair damage while normal tissues can.

*259/40/1*

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