I. The Continuing Evolution of Linear Accelerators

In presenting a paper as part of a symposium, there is a danger of repeating information given in the other papers. This is especially true when the papers represent the contributions made by particular organisations to achieve the same objective and there has been a large measure of similarity in their development programmes. The present paper, therefore, does not deal in any detail with a particular machine although it should be appreciated that the organisation with which the author is associated produces a standard model of clinical accelerator which is in general use at a number of centres. Rather, the title of the symposium has been interpreted freely, and an attempt is made to set the perspective of accelerator development and to indicate some of the remaining problems. The story of clinical accelerators is not a very old one, and therefore the whole of the development may be called recent. It was in November, 1946 that electrons were first accelerated in a travelling-wave accelerator by a group led by D. W. Fry at Malvern (Fry, Shersby-Harvie, Mullet and Walkinshaw, 1947). Largely due to the efforts of the chairman of the present symposium, Dr. Constance Wood, and those of Dr. L. H. Gray, the possibility of clinical application was quickly appreciated.