A Dosage System for Interstitial Radium Therapy. Part II—Physical Aspects

Part II is concerned with the physical basis of the dosage rules given in Part I. The guiding principles for the manipulation of single- and multiple-plane implants are derived from previous results relating to surface applicators. The foundation of the more general volume dosage lies in a study of the radiation fields created by distributions of a hypothetical radioactive fluid throughout the volume considered. The treatment has been restricted to cases in which two regions of different fluid density, the so-called rind and core, are permitted. The radiation fields have been evaluated for the sphere, circular and elliptic cylinders, ellipsoids and cuboids (rectangular parallelepipeds). The mean dosage-rate within solids of the same volume and radium content is independent of the shape, except for a small correction factor determined by the elongation. Dosage is given by the formula milligramme-hours per 1,000 r = 21·57 × (Volume)⅔ × e0·07 (Elongation — 1) The modifications introduced into dosage and distribution by the substitution of actual radium needles for the fluid are described.