Energy Absorption and Integral Dose in X-Ray and Radium Therapy: A Review

Abstract

In latter years conceptions of x-ray and gamma-ray dosage have been considerably extended in order to take account of the total energy absorbed throughout irradiated tissues (11, 12, 14, 15, 19, 20, 21). The roentgen is essentially a measure of energy absorption in a given mass of air, and previous investigations had been concerned almost entirely with an attempt to measure at each point in the tissues the energy absorbed near that point, the points considered being most often within the tumour itself. In most methods of radiation therapy, however, particularly in those methods which utilise multiple radiation beams or large radium surface applicators, much radiation is absorbed by the patient which contributes not to the destruction of the tumour itself, but possibly to that of normal tissues of the body. In this respect, it is a familiar fact that the severe constitutional effects of an intensive course of radiation often form an effective bar to higher tumour dosage, and the increase in the degree of these effects with the volume of irradiated tissue is also well known2 (4, 26). In considering these effects from the physical point of view, one is presumably concerned with the total energy absorbed by the body throughout the whole irradiated volume (the integral or volume dose). The physical problems of protection of staff and patients also seem to be intimately related to the integral dose, since it seems reasonable to suppose that the blood changes and general effects would depend on the total energy absorbed in the body as a whole and not on the dosage at a particular point (20). With the recognition of the possible importance of integral dose and the development of a suitable unit for its measurement, considerable work has been done in two main directions which converge toward a common goal. On the one hand, much attention has been given to methods suitable for assessing integral dose in the variety of circumstances that arise in practice and to the chief factors which control integral dose. On the other hand, attempts have been made to correlate integral dose with such clinical and biological observations of radiation effects as would be expected to bear some relation to it. In what follows we shall attempt to give a brief account of this work and to draw some conclusions regarding the present position and value of integral dose and its related conceptions. Roentgen and Energy Absorption in Air and Other Media (14, 20) As already stated, the roentgen is essentially a measure of energy absorption in air, the factor of proportionality being dependent upon the energy required to produce one ion pair. There is good evidence (8) that this quantity is approximately 33 electron-volts, whence it may be shown (20) that a dose of one roentgen corresponds approximately to the absorption of 85 ergs per gram of air. This quantity, which will be considered a great deal in what follows, may be referred to as a gramroentgen (20).