THE cells of stratified squamous epithelium, whether normal or neoplastic, generally remain in contact with one another. The vascular stroma on which their nutrition depends lies in contact with the epithelium but the capillaries do not penetrate between the individual cells. Tumours composed of these cellsoften grow in solid rods which, whenseen in histological sections cut in a plane at right angles to their axes, appear as circular areas surrounded by stroma. In tumours of this kind the centres of the larger areas are necrotic and are surrounded by intact tumour cells which appear as rings.

It appeared that this particular structure might have an important bearing on the radiotherapy of such tumours, since there must exist a falling gradient in oxygen tension between the periphery and the centre of each tumour cord, and it is well known that cells which are anoxic at the time of irradition are generally much less damaged by a given dose of X-or y-radiation than those which are well oxygenated (Gray et al., 1953).