Clinical and Physical Investigations of the Problem of Dosimetry in Roentgen Therapy

Abstract

The ideal for which we are striving in dosimetry in roentgen therapy is the establishment and acceptance of a well defined unit which will make any administered dose of roentgen rays reproducible. According to our present knowledge, this unit must be a physical unit because there should be no doubt left in any one's mind that the so-called erythema dose or skin unit dose can not answer the requirements. The logical unit appears to be the roentgen unit as defined by Behnken (1), based on the electrostatic unit. Unfortunately, its absolute measurements done in Berlin and in this country are at variance (Meyer and Classer, 2, 3), the cause of the discrepancy still being unknown. In practice, however, we can easily work with either one and use the chosen unit as basis for the therapeutic application of roentgen rays. It is, of course, necessary to correlate this physical unit in some way with the biological effect. In principle there is not much difference which biological dose unit represents the connecting link: erythema dose, tolerance dose, or safe maximum dose. Investigations in this field have been done by a number of clinicians and physicists; the analysis of their work reveals three groups. One finds a definite dependence between the average wave length of the radiation and the number of roentgen units required for the so-called erythema dose. There may be an increase of the primary energy with decreasing wave length (Meyer and Classer, loc. cit., and others) or just the opposite (Schreus, 4), while one author (Holthusen, 5) recommends the use of the same number of roentgen units through the entire range of radiation, beginning at 64 K.V., filtered through 6 mm. cardboard, up to 186 K.V., filtered through 1 mm. copper plus 1 mm. aluminum. Our own standpoint regarding that particular question will be set forth in detail in the second chapter of this article. When approaching the subject as a whole, we were confronted by this fact of discrepancy and assumed that there was a relation between penetration of roentgen rays and the number of roentgen units required for the erythema. In this case, and also if the measuring instrument is dependent upon the wave length, it will be necessary to give a quality factor in addition to the amount of energy used in order to make the dose reproducible. Numerous methods have been suggested for expressing the quality or penetration of roentgen rays; they have been briefly reviewed by one of us (Pohle, 6). Suffice it to say that we confined our study to the measurements of the half value layer in copper and aluminum, both by the ionometric and photographic method as compared with the effective wave length according to Duane.