The factor (T.D.F.) defined by ORTON and ELLIS for regularly distributed discontinuous irrdiation may be dissociated from Ellis' equation. The essential fact is that, in principle, the T.D.F. is given as 100 for all techniques which are associated with full tolerance or, if desired, exact saturation of the normal supporting tissue. 2. From this standpoint, all techniques in which T.D.F. = 100 have the same effectiveness as the N.S.D., i.e. 1,800 Rads given at a flow of approximately 10 Rads per minute. A TD.F. greater or less than 100 should be considered as evaluating the effective dose as a percentage of the N.S.D. The total dose administered effectively as a percentage of N.S.D., Dt/18 represents by contrast the actual dose. 3. With discontinuous treatment of 50 or 100 Rads/minute, the ratio effective dose over actual dose = T.D.F. 18/Dt represents the coefficient of reduction in effectiveness due to fractioning. Crf (the T.D.F. of which is given in the tables of Orton and Ellis). In the case of continuous uninterrupted treatment, the coefficient of reduction of effectiveness by protraction Crp is calculated using Orton's equation. With treatment given on an interrupted basis (split) the reduction in effectiveness Crs is calculated on the basis of the appendix ratio of Orton and Ellis. 4. In low flow rate, low fractioned treatment, in interrupted sections, the overall coefficient of reduction of effectiveness equals the product of the three mentioned above: Crg = Crf-Crp-Crs. 5. Dissociation of the T.D.F. factors from Ellis' equation makes it possible to generalise its principle and to extend it to all normal tissue (kidney, lung, intestine, marrow), the T.D.F. of which represents that state in which radiolesions are maximally compatible with reversibility, in the immediate or in the shortest possible time. It would be theoretically possible, using a suitable experimental technique, to develop, for each of these particularly senstive organs, tables analogous to those provided by ORTON and ELLIS for supporting tissue in conditions which seem acceptable to many.
