242 Applied Biophysics 



shorter wave lengths, due to the sharper dehmitation of the 

 heam edges. 



A third factor arises which might he called the patient's vitality, 

 over which the therapist has only some small control ; namely, 

 in making certain that the total radiation energy absorbed by the 

 patient is a minimum commensurate with the necessary lesion 

 dose.* Provided that all stray radiations have been excluded, 

 the energy absorption during the treatment then becomes a 

 question of the most effective geometric distribution of the re- 

 quired X-ray beams, both physically and clinically, and of the 

 physical properties of the radiation used. 



Treatment at wave lengths shorter than those obtained with 

 a 200 kilovolt equipment had l)een carried out in the use of 

 radium on surface lesions, interstitially, in body cavities, or in 

 mass in the radium-bomb units. The nearest approach to the 

 methods employed in X-ray therapy are those of the radium 

 bomb. The main difference is that owing to the low gamma-ray 

 output from radium bombs, treatments can be carried out only 

 at short distances from the patient, limiting the use of the bomb 

 to lesions at short distances from the skin surface. In order to 

 obtain the same radiation intensity as that emanating from a 200 

 kilovolt tube operating at 10 milliamperes 40 centimeters FSD 

 [Focus-Skin Distance], 1.0 millimeter copper HVL, 1,000 grams 

 of radium would be required. 



However, it had been estaljlished from theory and experiment 

 that the shorter the wave length of the X-rays, i.e., the higher 

 the voltage applied to the X-ray tube, the more penetrating the 

 rays would be and the less the absorption would vary with the 

 density of the medium. One of the problems in 200 kilovolt 

 therapy was, and is, the distortion, due to intervening bone, of 

 the theoretical dosage distribution by an unknown factor. With 

 the shorter wave lengths this unknown factor .should become less 

 disturbing. 



* The lesion dose is the average dose throughout the lesion specified in rontgens. 

 It is estimated from a mathematical analysis of the dose distrihution in the patient, 

 arrived at hy the summation of dose-distribution charts for each X-ray beam. These 

 charts are obtained by ionization-chamber measurements in a water-phantom. 



