230 Applied Biophysics 



isodose curves for beams incident at an angle to the skin, are 

 affected by omitting scatter material from the wedge-shaped 

 space between applicator and skin. Considerably greater depth 

 doses were obtained towards the margin of the beam remote from 

 the applicator edge in contact with the skin. 



Even though it is not possible for a radiotherapy center to 

 explore, in a water-phantom, all the fields used, a few check 

 measurements should be made, as wide deviations from pub- 

 lished values may occur. It cannot even be assumed that an 

 applicator end is filled with radiation ; sometimes strips as wide 

 as 1 centimeter at the sides are almost devoid of radiation. This 

 might be particularly detrimental when glancing-field techniques 

 are used. Studies of the distribution of dose rate in air across 

 various fields have been published by Thayssen,^^ Jacobsen,^^ 

 and Attlee and Trout.- Sometimes fields are badly asymmetric. 

 Ways in which these can be improved by specially designed filters 

 have been described by Spiegler,^' Meredith and Stephenson,-^ 

 and Flood and Smithers.^^ 



There still remains the possibility that dose distributions in 

 the human body may differ from water-phantom measurements. 

 The bones are more absorbent, particularly of the radiations of 

 longer wave length, and beams which are tangential to, say, the 

 ribs or skull, are likely to be considerably affected. The lungs 

 and air cavities, on the other hand, will give a greater trans- 

 mission than water. Ouimby, Copeland, and Woods -^ made an 

 extended series of measurements with 200 kilovolt radiation 

 filtered by 0.5 millimeter copper and 2.5 millimeters aluminum, 

 both in a cadaver and in the vaginas of patients wdio were 

 irradiated both from the anterior and the posterior surfaces of 

 the pelvis. Backscatter factors agreed well with water-phantom 

 values for all fields of irradiation. Depth doses in the pelvis were 

 also in agreement, but through the chest they became progres- 

 sively greater. Measurements in the thigh agreed with water 

 measurements until the bone was reached, beyond which they 

 were up to 30% less. Measurements of radiation transmitted 

 through the head of the humerus also gave definitely lower depth 

 dose values. 



