In the five years (1895-190o) following the discovery of X rays, they were much used for therapeutic purposes, mainly for superficial conditions. In 1897 Henri Becquerel discovered that certain substances, of which radium is the prototype, are radio active, in the sense that they give out rays spontaneously; these rays are of three kinds a, I and 7, the last being akin to X-rays. Similar attempts were soon made to test the therapeutic value of these rays (see RADIUM-THERAPY), and at the present time, there is a widely growing use of gamma rays and X-rays in the treatment of disease.
Tissue Reactions.—The tissues of the body respond to the five different groups of rays mentioned at the beginning, in very different ways. In what follows, references will mainly be made to the rays other than those constituting heat and light. Perhaps the most striking physical characteristic shared by ultra-violet, X-rays and gamma rays is that of liberating electrons from matter. No doubt this occurs when the tissues of the body are exposed to these rays.
Suppose that the skin of the abdomen is exposed to a beam. of X-rays comprised within certain wave-lengths. With an ade quate dose of radiation, a series of reactions occurs which lasts for several weeks; the skin becomes red, any hair there is will come out, a blister may develop and finally if the dose has been a very heavy one, a persistent sort of ulcer may form. Most of the X-rays falling upon the skin will penetrate to the deeper tissues and produce effects upon them which depend essentially upon the kind of organ or tissue irradiated. By suit able choice of the wave-lengths in a beam of X-rays, it is possible to give a sterilizing dose to the cells of the ovary, while causing no more damage to the skin than a transient erythema. Since the rays must be considerably weaker by the time they reach the ovary than they are on the skin, it is usual to say that X-rays have a selective action, meaning that they have apparently more effect upon some cells of the body than others and there is no doubt that this is so ; and that it is equally true of gamma rays. Upon this main principle, the foundations of X-ray and Radium therapy have been laid. The difference in the sensitiveness of the normal tissues of the body to these rays covers a wide range; cer tain cells of the ovary and testicle, glandular structures generally and lymphocytes are profoundly affected by doses of X-rays which do not have very much effect upon muscle, cartilage or nerve cells. How far this is due to the structures of the cell being
altered or to the function of the cell being interfered with can not at present be said.
Treatment of Malignant Tumours.—One of the most im portant therapeutic uses of X-rays and gamma rays is for the treatment of malignant tumours (see TUMOUR). One of the essential principles underlying the treatment of malignant growths was established by Dominici in 1909, who showed that it was possible to irradiate certain malignant tumours causing more damage to them than to the adjoining normal cells. Dominici traced the changes which occur in the tumour under these con ditions and found that degenerative changes were set up in the tumour cells which eventually led to their destruction. Such a process takes time for its full development and is influenced by the reaction of the normal tissues ; should these normal tissues receive an overdose of radiation, it may prejudice the whole sequence of events. Observations of this kind have established the view that the action of X-rays and radium, when properly used, is unlike that of a cautery but depends essentially on giving pathological tissues a dose, which though harmful enough to ensure their degeneration, will not severely damage any of the normal tissues of the host.
It has been found that the reactions of living cells depend not only upon the actual dose of radiation which they absorb, but also upon the way in which the dose is given. The two chief ways in which a definite physical dose can be varied are either by using a source of strong intensity and applying it for a short time or by using a relatively weak source and applying it for a correspondingly long time, keeping the product of these two quantities the same. Some researches of Lazarus-Barlow, in 1914, showed that the columnar and epithelial tissues of the rat react in quite different ways when the dose was varied in this way; the columnar type of cell being less damaged by keeping the intensity high and the time factor small, while the opposite was the case with cells of the squamous type. Observations of this kind have an obvious bearing upon treatment when decid ing the kind of dose required to ensure the degeneration of malignant growths in man.