The Biological Action of X Rays—A Theoretical Review

Experiments, voluntary and involuntary, on the action of X rays on biological material, have been in progress from the discovery of the radiation by Röntgen in 1895. The literature of the subject is immense, and is scattered through an unusual variety of scientific and medical journals. It is no part of my purpose in this lecture to attempt to summarise it; even if such a summary were humanly possible in the time at my disposal. My intention is rather to discuss what light, if any, may be gleaned from this mass of experiment on the problem as to how X rays, including their energetic brethren the gamma rays, produce their biological effects. In other words, by what stages is the energy of the photon transformed into biological destruction. The problem, like many others in the hinterland between physics and biology, has its intrinsic interest for the scientific explorer. It is at least possible that its solution may do something to facilitate the work of the radiotherapist. It is a fairly good rule in research, that fundamental information is more readily attained by the study of simple, rather than complex, systems, and I propose to consider more particularly the action of X rays on single cells and unicellular organisms. Much of the early work was defective either on the biological or on the physical side, and some startling observations were recorded which later work has failed to substantiate.