The Biological Effects of Ionising

SINCE the high incidence of iatrogenic hypothyroidism (40%/. eight years after treatment) in thyrotoxic patients treated with radioiodine was appreciated, the radiobiology of the thyroid gland has 'become a subject of great clinical importance. The method of treatment in its orthodox form and dosage is clearly unsatisfactory and efforts are being made to improve it in various centres. Unfortunately, these efforts are having to take place against a background of inadequate iniformation ajbout fundamental radiobiological mechanisms. It is my intention to review some of the literature relevant to the radiobiology of the thyroid but before doing so I would like to mention some common radiobiological concepts. The events following bombardment of a tissue by ionising radiation are discussed by Bacq and Alexander (1961). When X-rays or y-rays enter a tissue they eject electrons from the atoms by which they are absorbed. These ejected electrons in turn eject electrons from other atoms and are responsible for most of the ionisations which occur. In the case of a-rays, P-rays, protons and neutrons electrons are again ejected with similar consequences.