During the course of experimental work on the effects of X-radiations on the growth of Jensen's rat sarcoma, it became clear that the mere disappearance or continued growth of the tumours after irradiation afforded a very inadequate criterion of the effect of the radiations. The rate of growth of the tumour was, therefore, investigated, daily observations of the linear dimensions of grafted tumours being made in a series of rats. Usually the length and breadth of the tumour were measured, but since the ratio of these measurements remained approximately constant throughout growth, we may confine our attention to one set of results, say for length only. The results of such measurements carried out for quite a different purpose by my colleague Mr. Harold Burrows, F.R.C.S., on his experimental animals, are given in Figs. 1 and 2. These represent measurements on “control” sarcomata, i.e. untreated rats. Similar results have been obtained with many such tumours. Examples of the actual tumour silhouettes are given in Fig. 4, the numbers above the tumours representing the number of days since implantation.

The rather surprising fact emerges that the increase in long diameter of the implanted tumour follows a linear law; that is, if 1 equals the length of the tumour in centimeters after t days, then over a fair region of time 1 = k[t - t0] where t0 varies somewhat but is usually of the order of six to eight days, while k is a constant whose value is different for different tumours, but usually of the order of 0.3 cm./day; t0 represents (presumably) the time taken for the implant to obtain a blood supply and commence growth.

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