Relation between Radiation Quality Factors and Depth

Abstract

THE effects of radiant energy are quantitatively proportional to the radiation absorbed, and, therefore, whatever clinical result is to be produced in tissues will be proportional to the radiation absorbed in the tissues and at the location of the lesion to be treated. This is one of the axioms of modern physics and now a definitely accepted working principle of clinical radiation therapy known as Grotthus-Draper's law. As a further condition, because of the limitations imposed by skin erythema, the actual dose will be restricted to the accepted maximum skin tolerance dose and hence the tumor itself will receive but a fraction of this skin dose. Therefore, the efficiency of a given radiation for the therapeutic treatment of a deep-seated lesion is measured and expressed by the ratio of the dose tolerated by the skin to the dose absorbed by the tumor. I t is customary to represent this ratio for various depth locations by curves which are generally known as absorption curves, and the various methods used today for defining the quality of radiations are expedients to define such absorption curves (1). A curve of this kind, however, is an absorption curve in the true sense only when the actual intensity of the beam is measured by total absorption; if but a small fraction of the beam is measured, as we do with the commonly used ionization instruments and the radiation is monochromatic, then the curve is a fractional absorption curve (2). However, if the radiation beam is polychromatic and also measured with the small ionization chamber, then the measurements are not intensities but doses stopped in air and the true intensity of the radiation either at the surface or at the various depths is entirely unknown. The reason for this is that in the latter case as the radiation penetrates into the absorbing substance it is further filtered, with the result that the intensity as well as the average wave length and the absorption coefficient become smaller. A curve of this kind, therefore, represents the intensity of the beam multiplied by a fraction which indicates what portion of the beam is absorbed in the ionization chamber or, in other words, the product of the intensity times the absorption coefficient in air. It now becomes quite evident that if the absorption coefficient is small, then even though a substantial radiation intensity may reach the selected depth, since there is but little absorption, there also can be but little biologic effect. A curve of this kind does not represent the doses absorbed in a given volume or mass of tissues but, rather, it represents the intensity times the absorbability in air of the radiation at various depths. The latter factor is indicated by the slope of the curve, or the tangent or first differential coefficient, at the particular depth and this is equal to the absorption coefficient in air. Therefore, to distinguish such a curve from one which is a true absorption curve, we shall designate it as an absorbability curve.