Factors in Dosage Determination in Interstitial Radiation

Abstract

FROM a technical standpoint, radiation therapy may be divided into two general methods, namely, external and interstitial. The purpose of both methods is to produce within living tissue the identical biological effect. In certain instances either one may be indicated to the exclusion of the other, but, in general, they are complementary and offer two technical means to the same end. Ordinarily they can best be utilized when employed together. The present discussion deals exclusively with the problems of dosage in interstitial radiation and, therefore, the advantages of external radiation as an important adjunct have been purposely omitted. The determining factor in successful radiation therapy is the “tissue dose” or the quantity of radiant energy reaching all parts of a volume of neoplastic tissue. The term “tissue dose” is not to be confused with the millicurie, millicurie-hour, or milliampereminute doses. The latter deal only with the physical qualities of the radiating source. Due to the physical nature of the radiations themselves, the tissue dose can not be made uniform or diffuse, and consequently some portions of a mass will always receive many times the intensity of others. This difficulty is very significant in interstitial radiation, with which we are particularly concerned in this discussion. The tissue dose of any mass should, therefore, be considered as of that portion which receives the smallest quantity of energy. Accurate dosage implies the production, in a neoplastic mass, of the smallest tissue dose sufficient to cause death of all neoplastic cells within it. In order to attain such accuracy, it is essential, first, that we know beforehand the minimum lethal tissue dose of the tumor in question, and, secondly, that we be able to produce at least that intensity in all parts of a given tumor mass. The difficulties in attaining this ideal are several. Although the comparative radiosensitivity of a neoplasm can be predicted by its general classification and by its individual morphology, there is at present no means of predetermining its minimum lethal dose. The question of dosage in a given lesion is not answered by stating that it is radiosensitive or radioresistant. We are also confronted by the lack of a satisfactory unit for measuring dosage below the surface of skin. At present, the only biological unit of radiation is the skin erythema dose and since the skin reaction varies in the individual and is subject to various interpretations, depending on the observer, it can never be sufficiently definite. In using physical standards, we find the very short (though significant) distances difficult to measure and practically impossible to duplicate in practice. It may as well be admitted that accurate dosage is not possible in practice, and that at present our best results are obtained by intentional overdosage.