Million-Volt Therapy 257 



process, and one which should be avoided unless the most 

 elaborate equipment and calculating devices are available, 



4. Where originally at 200 kilovolts it was quite impossible 

 to attain a uniform and sufficient dose owing to the patient's 

 size, e.g., carcinoma of the breast of a large woman, even with 

 the small increase in depth dose in large beams at 1,000 kilovolts, 

 few cases have been encountered where it is impossibe to ad- 

 minister a greater uniform dose to the lesion than to the skin. 



5. Where, at 200 kilovolts, lesions have had to be approached 

 by beams through organs, the damaging of which incapacitates 

 the patient, e.g., glancing beams in carcinoma of the esophagus 

 damaging lung tissue, at 1,000 kilovolts, most of the lesion dose 

 can, because of the increase in depth dose and the reduction in 

 skin response, be contributed by the anterior and posterior fields, 

 leaving only a small portion to be administered by the glances 

 through the lung. 



Comparison of the Physical Data Obtained for 

 Treatment of Carcinoma of the Rectum 



Figure 5 gives the cross-section outline at the level of the 

 pubic crest in the case of carcinoma of the rectum. This type 

 of case has been chosen because it shows very well many of the 

 advantages of million-volt therapy, when compared with 200 

 kilovolt therapy. The case is treated with ten 18 X 8 centimeter 

 beams at the angles indicated, each field being given 100 units 

 of X-rays on the skin. On the left half of the section is shown 

 the isodose if the case is treated at 200 kilovolts with the usual 

 40 centimeter FSD and Thoraeus filter. On the right-hand side 

 is the isodose if the patient is- treated at 1,000 kilovolts 100 

 centimeters FSD, 2 millimeter lead filter (HVL 9.3 millimeter 

 Cu ) . 



The dififerences are obvious. The lesion, which is a small one, 

 is surrounded by the 370*^^ contour at 1,000 kilovolts and by 

 the 250% contour, approximately, at 200 kilovolts, indicating a 

 50% improvement with 1,000 kilovolt rays in the lesion dose, 

 for the same input dose on each field. Outside the lesion, the 



