258 



Applied Biophysics 



L.A.L. 18x8 



30° 



18x3 

 90" 



18x8 

 65" 



FIG. 5. IsoDosES ON A Transverse Section of a Carcinoma of hie Rectum 



a. 10—18 X 8 40 cm FSD fields 

 with 200 kv DC. X-rays (HVL 

 2.0 mm Cu) 



b. 10—18 X 8 100 cm FSD fields 

 with 1,000 kv DC. X-rays 

 (HVL 9.0 mm Cu) 



dose declines rapidly at 1,000 kilovolts, whereas at 200 kilovolts, 

 even up to the skin, the dose is still 80% of the lesion dose, 

 unnecessarily causing damage to normal tissue and disturbances 

 to the patients. The maximum skin dose is the same in both 

 cases. If, now, 6,000 r is to be given to the lesion in 5 weeks, 

 the following results are obtained : 



Data 1,000 kv 200 kv 



Lesion dose (5 weeks) 6,000 r 6,000 r 



Dose per field 1,620 r 2,400 r 



Dose per day 650 r 960 r 



Maximum skin dose 3,400 r 5,030 r 



Integral dose* 40 Mgr 65 Mgr 



It is doubtful if it would be possible to attain 6,000 r at the 

 lesion, at 200 kilovolts, since the skin dose is probably above 

 the tolerance, also the dose per day is high and would impair 

 the patient's vitality. The integral dose is a measure of the 

 dose absorbed by the patient, being the sum of the products 

 of volumes of tissue and their respective doses. 40 Mgr is nearly 



* Megagram-rontgens. 



