THE rate of growth of human tumours is in clinical practice generally determined by a visual comparison of tumour sizes as displayed by a series of radiographs. Collins, Loeffler and Tivey (1956) found, at the time of their study of human tumours, that there were no quantitative terms for tumour-growth rate in clinical use. Their work was mainly concerned with the justification of using the concept of doubling time as a consequence of exponential growth, suggested by them as a hypothesis. They also considered the use of this concept in predicting the course of a disease. Since then there have been a number of growth-rate studies on human tumours and metastases (Schwartz, 1961 ; Garland, Coulson and Wl'ollin, 1963;Spratt, Spjut and Roper, 1963;Breur, 1965). So far, however, very little can be said regarding the laws of tumour growth.In this paper we present some data on growth rates measured from radiographs obtained in ordinary clinical diagnostics. METHODSAs a measure of the volume of a metastasis or a tumour we have used a " relative volume" defined as the volume of an ellipsoid with the semiaxes equal to three dimensions of the shadows measured from the radiographs. These dimensions were in most cases taken as the breadth 2a and 2b and the height 2c of the shadows obtained from the antero-posterior and the lateral pictures. In the case of definitely oblate or prolate shadows, the semiaxes of the shadows were applied. Th-e volume of the ellipsoid is 47r abc/3. This relative volume and the real volume of the measured body are equal in the special case when the body is oriented in the three directions corresponding to the three dimensions a, b and c.If only one radiograph could be used, two dimensions were obtained by taking as the third dimension the mean of the measured semiaxes. It is obvious that the determination of the volume of very eccentric ellipsoids will be erroneous. The method used here is, however, justified by the fact that it gives a correct size if the body is spherical, and gives a fairly good approximation for slightly ellipsoid shapes. If the shape remains unchanged during growth the calculated volume will be proportional to the real volume, i.e. the ratio between the calculated and the real volume would be constant during growth.

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The doses to which an examining radiologist is subjected have been measured by fluoro-glasses in 100 contrast-media studies of the gastrointestinal tract, 100 lower extremity venographies, and 50 urethrocystographies on male patients. The maximum doses were measured on the forehead, the forearms and the shoulders. The average dose of radiation accumulated in the forehead in a single contrast-media study of the gastrointestinal tract was about 0.1 mrems, 10 rems in venography, and 30 mrems in urethrocystography.