III. A Comparison of Fast Neutrons and X Rays in Relation to the “Oxygen Effect” in Experimental Tumours in Rats

Dr. Fowler has already pointed out in this symposium (Fowler, Morgan and Wood, 1963) that possible advantages of using neutrons in radiotherapy lie in their relation to the “oxygen effect”. He has also indicated the magnitude of the effect in a hypothetical tumour. The relevance of the oxygen effect to radiotherapy depends upon the supposition that there is a differential between the radiosensitivity of normal tissue and the tumour. This is because of the possible presence within the tumour of some cells which are protected from radiation injury by low concentrations of oxygen and which are able to continue to multiply in the environment produced by the doses of radiation in general use. Most of the evidence that there are such cells in human tumours is indirect. However, the clinical observations of Dr. Churchill-Davidson of the effect of using oxygen at high pressures in radiotherapy, when about 20 per cent of tumours respond to a greater extent than would be expected from conventional radiotherapy, lend support to the supposition (Churchill-Davidson, 1962). This is further confirmed by the results of Dr. Van den Brenk and his colleagues in Melbourne (Van den Brenk, 1962). There is also direct evidence (Cater and Silver, 1960; Naylor and Evans, 1962) from measurements of oxygen tension that there are areas in both human and animal tumours where the oxygen tension is very low and where it rises, sometimes after considerable delay, with the breathing of oxygen at higher partial pressures.