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tumour masses. A possible theoretical limit to what canbe achieved might be set by observing the selectiveaction of the irradiation on populations of tumour cells.Radio-iodine will tend to eliminate first those follicleswhich concentrate it best, and also those cells which aremost radiosensitive. This selection might change a

sensitive tumour which initially had a high uptake intoan insensitive one with a poor uptake. The length ofthe treatment would favour a selective mechanism ofthis sort. Fortunately there is not much evidence thatunfavourable selection actually takes place, for decreasinguptake by a tumour is usually associated with clinicalimprovement rather than deterioration. It is, in fact,difficult at present to define the factors which limit thesuccess of this treatment. The importance of the problemextends beyond the actual treatment of a relativelysmall number of patients with thyroid carcinoma. Forit is by no means improbable that a chemical substancewill eventually be found which is selectively concentratedby cancer cells in general. A radioactive isotope could beintroduced into such a substance and would be carriedwith it into the malignant cells. The experience gainedwith radio-iodine could then prove very valuable in

showing the most effective way of using a substance ofthis sort.

XERORADIOGRAPHY

SINCE our first reference 1 to this subject two notableBritish papers have appeared. Oliphant 2 has discussedthe design of the first British xeroradiography unitsuitable for hospital use, and Hills, Stanford, andMoore,3 working with this machine, have investigatedthe qualities of xeroradiographs.Xerography is a system of photography in which no

wet chemicals are needed to develop the image. It

depends on the properties of a group of substances knownas semi-conductors (selenium is one) which act as

insulators in the dark, but become conductors whenexposed to light. A thin layer of selenium on a metalplate is given a positive electrostatic charge in a light-tight chamber and is later exposed in the normal way.The charge is reduced by light or X rays falling on theplate, and the image is made visible by spraying thesurface with a fine powder, which adheres according tothe amount of the residual charge. The xeroradiographis rather like an etching, and is viewed by reflected light.Its most striking difference from a conventional film isthe presence of both bone and soft-tissue detail on thesame plate. The powder tends to concentrate alongthe edges of areas of residual charge, thus increasing thecontrast between two densities to a degree that is out ofproportion to the actual difference in radio-opacity.

Hills et al. have studied the variable factors in xero-radiography-namely, charging, exposure, and develop-ment times. They found that for any given plate thecharging time was the most critical-under-chargingreduced detail, over-charging caused distortion. Thelatitude in charging was small if distortion was to beavoided. There was a fairly rapid loss of charge from aplate immediately after charging ; this soon became aslow uniform loss and made it difficult to get comparableresults. Both exposure and development times were notcritical, and a 25% variation in development time wasnot unacceptable. Hills et al. found that the enhance-ment of contrast due to the aggregation of powder alongthe margins of varying densities, and the fairly lowover-all contrast between bone and soft tissues, wereoccasionally a help (e.g., in showing up small cracks inbone) ; but in general detail was better in a conventionalfilm. Though it was difficult to assess speed, the xero-graph needed two and a half times the exposure for non-screen film and up to ten times the exposure for screen

1. Lancet, 1955, i, 957.2. Oliphant, W. D. Brit. J. Radiol. 1955, 28, 543.3. Hills, T. H., Stanford, R. W., Moore, R. D. Ibid, p. 545.

films in the thicker areas of the body. Hills et al. con-cluded that the high tube-loadings and patient-doserestrict the use of xeroradiography, in its present form,to examination of the limbs ; but even at present it can,under certain circumstances, equal the qualities of theconventional radiograph and can be an advantage whennot all the films taken need be kept. It would beinteresting to know what xeroradiography can do withhigh-kilovoltage techniques.

DEFENCE OF RESEARCH

Humanity’s Rising Debt to Medical Research wasthe subject of Sir Henry Dale’s Stephen Paget lectureto the Research Defence Society on Wednesday, andhe was able to show how astonishingly the debt hasincreased even since he gave his previous StephenPaget lecture in 1931. Demonstrating repeatedly thatmost of the progress of medicine would have been

impossible without experiment on animals, Sir Henryrecalled how Ehrlich had to abandon his attemptedchemotherapy of malaria because " nobody could considerthe subjection of human patients, even if they had beenregularly available, to systematic trials of the curativeproperties of an indefinitely long series of new anduntested substances." The present prospect of riddingthe world of malaria dates from the decision, 35 yearslater, to test possible remedies on the malaria-likeinfections of small birds. Similarly the chemotherapyof bacterial infections, which in 1931 was generallyconsidered impracticable, dates from 1935, when Domagk" found the right kind of septicsemic infections in micefor the testing of potential remedies, and a syntheticsubstance which, when so tested, was found to havea clear therapeutic action." The application of

Fleming’s great discovery might have been indefinitelypostponed if Florey and his team had not made a newand thorough investigation of the properties of penicillinwith the help of an adequate series of experiments onliving animals ; and everybody should understand thatfurther exploration " in this still productive field of

life-saving and healing possibilities " could still be

delayed or halted by any successful attempt to hinderor prevent such experiments. Describing the improve-ment of anesthesia, Sir Henry complained, with reason,that " the essentially humane researches ... on which allthese beneficent advances are based, are still exposedto a cruel campaign of denunciation and obstruction,the best that can be said for which is that, hitherto,it has been so largely ineffective." One can feel a measureof respect, he said, for people whose imaginativesympathies lead them to reject medical service altogether,and to exclude all animal products from their food andclothing-though even such people must be preparedto answer a charge of cruelty by omission, when theyrefuse the chance of survival, or the relief of pain, forchildren and others for whose care they are responsible.But he had nothing but hard words for those who,without attempting such consistency, try to persuade or

, frighten the ignorant into refusing to be protected byimmunisation, or use misrepresentation to prevent

, experimental research, whether this is for the benefit

, of man ’or of animals. The cruelty of these activities is,in Sir Henry’s opinion, " none the less detestable on

. account of its origin in a self-indulgent sentimentalit " ;and he urged his hearers to counter it by joining the

, Research Defence Society, founded to fight the battlesL of the men and women who are doing medical research.l The society’s office is at 11, Chandos Street, Cavendish. Square, London, W.1.

Major-General W. A. D. DsuMMOND, C.B., F.R.C.S.,has been appointed director-general of Army MedicalServices, from April, 1956, in succession to Lieut.-

General Sir Frederick Harris.