At the present time, there is a considerable tendency to use rather weak sources of radiation applied for a long time ; in some cases, the exposures last as long as 12 or 14 days. Care has however to be taken that the source is not too weak, for there is ample evidence to show that cells can adapt themselves to very weak irradiation. Some idea of the doses commonly used in gamma ray therapy may be obtained from the statement, that a concentration of about 1 milligram of radium element for each cubic centimetre of a malignant growth is often aimed at, and an adequate time of exposure with this concentration is in many cases found to be from 5-10 days. It is perhaps unnecessary to add that this dose is often departed from when conditions make it advisable. In most cases, it is easier to manipulate the gamma rays from radium than X-rays, and for this reason partly there is a growing tendency to use them for the irradiation of deep-seated organs.
It is often very difficult, and in some cases impossible, to get the best physical conditions for arranging the necessary uniform irradiation of a malignant mass ; but there is no doubt that those who practise radiotherapy in any form nowadays, and have a sound knowledge of the physical characteristics of the radiation they use, can get much nearer the ideal conditions than was possible in the early days of radiotherapy. The encouraging signs of collaboration between clinical and laboratory investiga tions are likely in time to establish the subject on a sound quan titative basis. Although much remains to be done to explain the processes set up when the tissues are irradiated by ultra-violet, X-rays, or gamma rays, some of the main changes occurring in cells both normal and malignant, are now well recognized and have been made the subject of detailed study. It has been found too, that cells react differently according to their state of activity at the time of irradiation, and it is generally believed that cells in active division are more vulnerable to radiation than when in a resting stage.
Effects on Cell Life.—It is reasonable to expect that the effects upon cell life will depend on the amount of radiation to which they are exposed. The general principle enunciated by Grothus in 1818 that, "in photo-chemical actions only those rays which are absorbed can produce chemical change," is found to have a much more general application. The reactions of living cells appear to depend largely, though not wholly upon the amount of energy they absorb from the beam of radiation going through them. It has already been stated that, if the intensity of the radiation is too small, the cells can cope with it to a large extent, even though it be prolonged for a very long time. It has been found that the changes shown by cells depend to some extent upon the wave-length of the radiation producing the action, although the amount of energy actually absorbed may be the same, the result may be entirely different; these processes begun in living structures depend not only upon the energy let loose in the structures, but also upon the way in which it is set free.
Generalizations must always be difficult on a subject which is comparatively young ; experimental advances too are apt to modify views which can only be held somewhat tentatively, but it is thought that the following may be stated as expressing views which are fairly generally held at present. They are taken from Radium, X-rays and the Living Cell by Colwell and Russ.
I. The cells of some tissues are more affected by a given dose of radiation than are the cells of other tissues when exposed to the same dose. This is generally known as selective action.
2. In some cases, at least, the cells of a tissue are more affected by a given amount of energy of one range of wave-lengths than they are by the same amount of energy of another range of wave lengths. This is known as differential action.
3. Some cells, when in an active state of division are more affected by a measured dose of radiation than are similar cells in the resting stage.
4. Some cells respond to a dose of radiation in different ways, according to whether such radiation is administered so that a large intensity is coupled with a short period of exposure or a small intensity is coupled with a long period of exposure.
Instruction for Practitioners.—The widespread application of radiology in medicine has inevitably affected the courses of instruction laid down for qualifying medical degrees.
Not many years after the first therapeutic trials with X-rays and radium, injurious action on the skin was noticed, and later it was discovered that those who were giving frequent doses of the rays to patients were themselves being damaged ; in some cases there was obvious damage to the skin which led eventually to a cancerous condition; in others, a state of anaemia existed, and others were sterilized. Deaths from repeated but uninten tional doses of radiation had undoubtedly occurred. With such facts known, protective committees were formed in various countries to lay down measures for the protection of all workers.
In Great Britain, there has been very wide acceptance of the general guiding principles laid down by the X-ray and Radium Protection Committee whose publications are issued from the British institute of radiology. This Institute came into being in 1924 and marks a distinct era in the development of a branch of medicine, whose services in the diagnosis and treatment of diseases have certainly not yet reached the zenith of their attain ment. See the article RADIOTHERAPY; X-RAYS; X-RAY TREAT