Any means of increasing the sensitiveness of human tumours to X-rays or gamma rays would certainly change the outlook on the treatment of cancer in many sites, more especially for the types of tumours which are sometimes termed radio-resistant. The experiments recorded here were mainly carried out with a fairly rapidly growing sarcoma of the rat (F. 16), originally given us by the Imperial Cancer Research Fund in 1921. We (1933) have already referred to the modification which it has since undergone, particularly to the extraordinary increase in its rate of growth and cellularity of structure. These changes are probably due to the technique used in its transplantation, and to the use of a strain of rats particularly sensitive to tumour implantation. With Jensen’s rat sarcoma, 100% of progressive tumours is generally obtained from grafts and 90% is looked upon as a rather poor result. Another obvious advantage for experiments of the type described here is that spontaneous absorption of an established tumour of either F. 16 or J. R. S. is extremely rare. In the first experiments established tumours were exposed for varying lengths of time to carefully measured doses of X-rays generated at about 170 kV and filtered by 10 mm of aluminium. The minimum amount of radiation necessary to cause the disappearance of
all
tumours of each type is referred to here as the “lethal” dose. It is expressed by time—for sarcoma F. 16 it is 200 minutes, approximately equivalent to 1000 r. A dose of X-rays amounting to 30-40% of the lethal dose generally has some deterrent effect on the growth of the tumour; 40% of the lethal dose very rarely causes the disappearance of a well-established tumour. The effect becomes more definite and lasts longer as the dose is increased and the majority of tumours disappear after a dose which is 75% of the lethal dose. Great care has always been taken to expose only the tumour, as it has been observed that the dose of X-rays required to cause complete disappearance is considerably less when even a small area of surrounding tissue is irradiated. The actual size of the tumour at the time of irradiation also influences the final result; quite small, young tumours may disappear with a comparatively small dose of X-rays, while very large ones are sometimes little affected by a much larger dose, in spite of strict attention to the principle of dosage which is measured by the ionization at the lowest level of the tumour. Most of the tumours used for these experiments were between 16 mm and 22 mm in diameter at the time of exposure to X-rays.
THE ACTION OF X-RAYS ON EUPLOTES TAYLORI AND ASSOCIATED BACTERIA