II. The Transition from Röntgen to Rad

Early this year the British Committee on Radiological Units, after consultation with senior colleagues engaged in the practice of radiotherapy, resolved to recommend the adoption of the rad as the unit in terms of which absorbed radiation doses should be measured and recorded in radiotherapy departments throughout this country. In this respect British radiotherapy will, not for the first time, be giving a lead; but those who lead are the first to come to the rough ground. There may be alternative routes. My intention is to consider what is involved in making the transition from röntgen to rad so that when any choice is possible we may choose wisely. I should like to consider the implications of the proposed change for our thinking and teaching, for the measurement of dose, and for the prescription of dose. I have given precedence to a correct mental conception of the meaning of the röntgen and the rad because I believe that if we think aright the rest will follow. It is necessary to be quite clear at the outset that the rad is not just an improved version of the röntgen. The röntgen and the rad are two complementary units which attempt to define two different aspects of a beam of radiation. The radiation dose in röntgens within any volume element is determined by the number and energy of the photons passing through that element during the exposure, irrespective of the local distribution of matter.