The fact that ionising radiations could cause biological damage became apparent very soon after Roentgen’s discovery of X-rays in 1895 when a researcher using early X-ray tubes developed radiation burns to the skin of his hand. The first case of radiation-induced cancer was reported seven years later. Evidence of the harmful effects of large exposures to ionising ra­diations grew as the use of radiation and radioactive materials spread throughout industry and medicine. Early examples were damage to the hands and cancer among X-ray workers, lung cancer among uranium miners and bone cancer among workers in the radium industry, particularly luminous dial painters. How­ever, the long-term biological significance of smaller, repeated doses of radiation was not widely appreciated until relatively recently and most of our knowledge of these effects has been accumulated since World War 2.

All radiation is absorbed by matter with a transfer of energy from the radiation to the absorber. Radiant heat and light, when absorbed, produce a temperature rise in the absorber. Nuclear radiation differs only in that each particle or photon possesses sufficient energy to cause ionisations in the absorber; thus it is known as ionising radiation. Body cells are about 80<Го water, so radiation absorbed in body tissue will usually cause ionisation of the water in the cells. Ionisation in the cell leads to the formation of chemical species that may damage the cell. If sufficient cells are damaged then the body itself may be affected. The process of radiation damage to cells after interaction with cell water can be characterised in four stages:

(a) Ionisation, typically

H2O radiation H20 + + e“

This process has a reaction time of the order of 10“16 seconds.

(b) Formation of free radicals H20 + H+ + OH*

or H20 + + e" H20′ H* + OH-

H+ and OH’ are ions which are always present in water and so have no significance in this case, but H* and OH* are free radicals and are chemically highly reactive.

(c) The chemical stage The free radicals are able to react with organic molecules in the functional parts of the cell and alter them. This process takes a few seconds.

(d) The biological stage Over a period varying from a few minutes to a few years the biological con­sequences of the chemical changes in the cell be­come apparent.

Alternatively there is the possibility, although with lower probability, that radiation may interact directly with functional organic molecules in the cell, such as DNA, and produce direct damage to them by ion­isation. Biological damage will then become apparent as in (d). The effect of this damage varies with the nature of the damage. It may be so slight as to be insignificant or so subtle that it goes unnoticed. If it is severe enough to affect the function of the cell a reaction may be triggered. Cells have the ability to recover from limited radiation damage by repair of the damaged structure. The repair may be suc­cessful or may result in a functioning cell which is different from the original.

From this we have two effects of radiation damage, the first, where the cell is altered but continues to re­plicate, is called cell transformation. The second, where the cell is unable to divide, is cell death, which re­quires in general a higher radiation dose than the first.

Knowing the ability of cells to repair damage we can make observations about the incidence of cell death. Firstly, cells which are dividing rapidly (e. g., skin cells) will be more susceptible than those which are not (e. g., nerve cells) because there is less time between cell division for repair. Secondly, dose rate as well as total dose will be important because, put simply, at low dose rates repair is better able to keep pace with damage.