The Cause of Cancer
By HARRY
ROBERTS
AT the end of the last century, it was generally sus- pected that cancer would turn out to be an infectious disease due to the invasion of the body by some specific organism. As a result of laboratory investigations and a better understanding of the details of the cancer process, the parasitic theory became discredited, and few pathologists in recent years have regarded it as worthy of consideration or discussion. Some six years ago, however, a con- siderable stir was caused by the publication of a report by Gye and Barnard of researches they had conducted and of the conclusion to which these researches had led them. Briefly their conclusion was that the cause of cancer is to be found in the conjunction within the malignant cell of a specific chemical substance and a non-specific living virus. This would, of course, place cancer in the category of infectious parasitic diseases. The new theory did not obtain wide acceptance in authori- tative quarters ; and it has by most pathologists long been relegated to the limbo of bygone speculations.
And now, after elaborate experiments carried out at the laboratories of the National Institute for Medical Re- search, an impressive volume on the cause of cancer, by Drs. Gye and Purdy, has just been published* ; in which these investigators again advance the explanation put for- ward by one of them six years ago. Nowhere will be found a fuller or more simply-expressed account of the significant features characteristic of cancer; and nowhere will be found so thorough, so complete, so effective, and so scientifically adequate, a. presentation of the case against the theory which, in spite of everything, the authors believe to be the true one. This part of advocatus diaboli, they have, indeed, played almost too well ; leaving themselves, as it were, hardly a leg to stand on. It is a very modest plea that they advance when they come to present their own case, and to state the reasons which have influenced them. They invite us, not so much to accept their conclusions as established, as to admit that they cannot be ruled out as incompatible with any of those facts on which all are agreed. What Drs. Gye and Purdy claim to have proved is that from certain fowl cancers—the so-called Rous sarcomas—an agent can be separated from the cells, which agent, even after repeated dilution, is able, when inoculated into healthy fowls, to set up malignant growth. Convincing evidence is given to justify the conclusion that this filterable agent is a living ultra-microscopic virus comparable with the specific viruses believed to be severally responsible for cerebro-spinal fever, smallpox and measles. The authors admit that they have not yet succeeded in extracting from any mammalian tumour a cell-free agent with pathologically causative potency ; but they suggest that the explanation of this failure may well be due to the inadequacy of present methods of laboratory extraction, and it is true that similar difficul- *The Cause, of Cancer. By W. Ewart Gye and W. J. Purdy. .308.) ties of separation have been met with in a number of diseases now recognized as being virus-caused. They claim that these filterable fowl cancers are new growths comparable in practically every respect with mammalian growths ; and that any abstract objections which may be raised to an " infective " explanation of human cancer should hold good equally of these avian tumours in which the presence of an infective agency has been demonstrated.
Cancer differs in many ways from almost any other disease, infective or non-infective. It apparently starts in a change in the functioning of some normal cell ; and it grows, not by the spread of this change to adjacent cells, but by the repeated and unrestrained multiplication of the original modified cell, the descendants of which inherit its hyper-progenitiveness. The process seems to be dissociate from that influence which normally limits growth and bodily reactions to the extent necessary to restore normality. Hence the popular comparison of cancer with anarchic rebellion. Moreover, the spread of the disease to other parts of the body is not, as with most other diseases, effected by the transmission through the blood or lymph stream of some provocant agent which deharmonizes local cellular metabolism, but by the migra- tion of actual cells descended from the nucleus cell of the original tumour—which daughter-cells themselves proceed in their new location, independent of their neighbours, to multiply and parasitically to establish themselves. The striking differences between this and the course of other diseases associated with infective organisms are obvious ; and it is clear that we must look upon the living and com- plete cell as the " fundamental indivisible unit of cancer."
The currently accepted theory—which is concerned with the nature rather than with the cause of the disease —is to the effect that malignancy starts with a permanent modification of certain normal cells, by the multiplication of which, the hereditary transmission of their abnormal qualities, and not by reason of any further provoking influence, the cancerous growth proceeds. The rival theory, now thrown once more into the arena, postulates the presence within the cancer cells of a continuously acting cause—living or non-living—to which, rather than to what is ordinarily understood by heredity, the pro- gressive development of the tumour is attributable. Whichever of the theories survives, it seems likely that the accessory factors—the factors that constitute indivi- dual susceptibility—will prove to be not only more im- portant, but also more humanly controllable, than any " specific agent." The facts that cancer is essentially a disease of middle and advanced life ; that its location is commonly, if not invariably, determined by long exposure to physical irritants ; and that the surgical extirpation of a malignant growth, before it has begun to colonize, often effects a complete and final cure ; are collectively difficult to harmonize within any theory, infective or metabolic, Which. has hitherto been propounded.
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