Total Energy Absorption in Biological Objects

Abstract

THE total energy absorbed in a biological system when irradiated by a beam of X- or gamma-rays is a quantity of both theoretical and practical interest. The letter by Dr. Happey¹ indicates one method of attempting to solve such a problem, but it seems that the matter may be carried much further. When a beam of X- or gamma-rays is sent into a scattering medium such as a block of tissues, the distribution of radiation absorbed within the tissues may be conveniently represented by constructing ‘isodose surfaces’. During the last two years we have carefully studied these surfaces, and they seem to afford the best method of attacking the total energy absorption problem. If the isodose surfaces have an axis of rotation, the volume of tissue between any two surfaces representing given dosage levels may readily be determined, so that by multiplication of volume (or more accurately mass of substance) and average dose we obtain the ‘integral dose’. In this way we have studied the integral dose over a wide range of clinical conditions. An indication of the kind of results obtained is given in the accompanying table. The most striking feature is the very large variation in integral dose (always taken to the 10 per cent isodose surface) which exists between the different techniques. The results may be correlated with the very varying constitutional effects produced in the patients by the treatments.