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hard and hard X-rays are used where a layer or mass of tissue some little distance beneath the surface is to be treated.

For the better treatment of new growths removed some distance from the surface of the body, there is a tendency at the present time (1921) to increase the hardness of the rays to the utmost extent with the object of producing a radiation that approximates in some degree to the gamma rays of radium in penetrating power. Chief amongst the new growths that are the subject of this " deep therapy " with intensely penetrating X-rays are uterine fibroids. The production of soft to medium X-rays requires an apparatus capable of generating about 100,000 volts, for deep therapy a voltage of about 200,000 is necessary and the aim of radiologists is to get a still higher voltage. The method -employed is essentially use of a series of transformers.

It is unnecessary here to enter into a detailed examination of the methods whereby X-ray dosage is determined, but it is obvious that estimation of the dose is one of the most important points in connexion with radiotherapy. In the case of radium, the matter is relatively simple, for the output of rays from the radium is constant, but in the case of the X-ray bulb, quite apart from variations in the primary current, the conditions of the bulb vary within wide limits, and the output of the bulb in X-rays varies accordingly. An important advance has been made in recent years by the introduction of the Coolidge tube in which by means of a different working principle the output of X-rays can be kept fairly steady. Under all circumstances, the output of an X-ray bulb is heterogeneous, the bundle of rays emitted is partly penetrating, partly soft, and in order to produce a more or less homogeneous bundle of rays for purposes of treatment, it is necessary to eliminate the softer varieties by means of niters. These filters are of different kinds but the chief are aluminium, zinc, copper and lead. For absorption of the specific secondary radiations produced when gamma or X-rays impinge upon metals, such substances as rubber, gauze, cardboard are used.

When a more or less homogeneous beam has been produced, it is necessary to calculate the dose employed in any given case for comparison with other cases. Various means have been adopted to this end, of which the commonest is the Sabouraud's pastille which consists of barium platinum cyanide and changes colour from green to yellow under a certain dosage of the rays. It was thought at first that this colorimetric test would be generally applicable, but it was soon found that the change is nDt brought out by X-rays of all degrees of penetration and is fallacious as a guide to gamma radiation of radium. It is now largely employed as a test of dosage during the X-ray treat- ment of ringworm and other skin diseases, but it is recognized that it must only be employed with caution, with rays of medium hardness, and for superficial conditions. Another form of test is electrical (ionto quantimeter) in which the rate of dischargs of a charged gold leaf forms a measure of the output of X-rays. Another method depends upon the chemical reduction of iodine from iodoform in a chloroform solution and is probably the most scientifically accurate of all the methods which have been devised. Yet other methods depend upon the correlation be- tween the effects produced by X-rays on the one hand and the gamma rays of radium on the other. Here the production of identical degrees of fluorescence on the fluorescent screen or of silver deposit on the photographic plate or of biological effects on the animal cell has been aimed at in standardization.

So far as treatment is concerned, it is obvious that a biological test is the most satisfactory. The one commonly used is known as the " erythema dose," meaning thereby that dose of X-rays which leads to a reddening of the skin a few days after applica- tion and subsequently to slight bronzing, without blistering or other damage. On the other hand there is no doubt that the various cells of the body do not react to radiations in the same degree and partly on this account and partly because of the operation of the law of inverse squares, it is obvious that the skin over a tissue being irradiated may itself receive an injurious dose while the tissue in question is receiving the correct dose. Hence when a tissue some distance beneath the surface is under

treatment it becomes necessary to irradiate it through different portals, so that each area of skin shall receive less than the erythema dose, although the tissue in question gets the full amount that the radiologist wishes to give it; this method of cross fire is largely employed.

Radio-active Substances. Treatment by means of radio- active substances largely resolves itself into treatment by means of the beta and the gamma rays of radium or occasionally meso- thorium. Just as X-rays vary in degree of penetration, so do the rays of radium. The so-called alpha rays are little pene- trating, being stopped by about 35cm. of air. The beta rays, which are particulate negative electrons, are more penetrating but their penetrating power varies over a wide field, some of the softest being as easily absorbed as alpha rays, some of the hardest approximating to the soft gamma rays in penetrating power. Gamma rays are aether vibrations and they, too, vary in degree of penetrating power. Their wave length is the short- est of any form of vibration known, and the most penetrating gamma rays can be detected through several inches of lead.

The alpha rays are but little used, the only methods of em- ployment being in the way of radium emanation dissolved in saline solution, or of needles upon which " active deposit " from radium emanation has been collected. In either case the emana- tion water or the active deposit needles must be introduced into the system whether intravenously or into the solid tissues otherwise the alpha rays would have no power to act. In either case, too, they act along with the beta and gamma rays pro- duced by the active deposit.

Beta radiation is always used in conjunction with gamma radiation, but inasmuch as the ionizing power of the beta ray is about 50 times as great as that of the gamma ray, it follows that when beta radiation is being employed, the gamma radia- tion may probably be ignored. Beta radiation is used for merely superficial conditions and the radium salt which supplies it is spread over a flat or curved applicator and is covered with a thin layer of varnish, mica or aluminium or is placed in a thin glass tube; the beta rays which traverse thin solid filters act upon the tissue in the neighbourhood of which the radium is placed. Instead of a radium salt one of its products, viz. radium emanation is often employed clinically. No essential difference is introduced by the use of this emanation excepting that its intensity undergoes a progressive diminution with time since it falls to half value in 3-85 days. Early rodent cancer, certain conditions of the eyelids, some cutaneous non-malignant tumours and birth-marks, are treated successfully in this way.

Gamma radiations are used where deep penetration is required, but the law of inverse squares approximately holds good in their case also, a matter of fundamental importance in treatment. The substances used as filters when radio-active materials are employed in treatment are not quite the same as those used along with X-rays. Since one of the main objects in employ- ing radium is to utilize the highly penetrating gamma rays, the filters employed are generally of the higher atomic weights, silver, brass, gold, lead, platinum, and there is some reason for believing that the more highly penetrating the rays, (i.e. the denser the filter through which they have passed) the less is undesirable damage suffered by the tissues.

Mode of Action of Radiations. The method by which X-rays and radium rays produce their effects are not thoroughly under- stood, but it is certain that dosage must vary according to the type of cell which it is desired to influence. Thus the vulner- ability of skin is not the same in different individuals nor even in different regions in the same individual; the vulnerability of the squamous cell is not the same as that of the columnar cell; the vulnerability of renal cells differs in the convoluted tubules and in the conducting tubules. Even in the fur of animals it is possible to recognize a differentiation, for a certain amount of X-radiation will lead to a destruction of the pigment forming cells in the hair of a black rat, while a little more radia- tion will affect the cells themselves. In the former case there is no epilation but the hair comes white instead of black, in the latter the hair falls out and baldness results.