32 BIOPHYSICALLY ACTIVE X-RAYS 



Half- Value Layer 



The quality of a heterogeneous x-ray beam can be described by the 

 thickness of a filter which will reduce the intensity of the beam to one 

 half its initial value. The absorption curves shown in Figs. I— 11 and 

 1-12 indicate that, the shorter the effective wavelength of the x-radia- 

 tion, the thicker the half-value layer of the filtering material must be. 

 Inasmuch as the composite absorption coefficient varies with thickness 

 of the added filter, there is no simple direct quantitative relation con- 

 necting effective wavelength with half-value layer. 



The intensity of a heterogeneous beam of x-radiation, if evaluated 

 by the half-value layer or effective wavelength method, shows that the 

 latter has the advantage, in that it presents a clearer physical picture 

 of the radiation quality. 



Action of X-Radiation on Living Tissues 



Almost immediately after the discovery of x-rays was announced it 

 was suggested that, by virtue of their penetrating power, they might be 

 used therapeutically to influence deep-seated pathological processes. 

 In order to evaluate their physiological effectiveness it is essential to 

 know the relative penetration, absorption, and resulting ionization of 

 the x-radiation, for the energy must be absorbed to be effective. 



The energy penetrating the tissue may be completely absorbed and 

 excite secondary phenomena. It is to these secondary phenomena that 

 the action of the primary radiation is attributed. 



The pioneer workers in radiography and roentgen therapy developed 

 cancer of the fingers after exposure to the soft x-rays emitted by the 

 then prevalent gas x-ray tubes. Repeated exposure of the hands to 

 soft radiation results first in keratosis (horny excrescences) that are 

 usually multiple. At first dry and scaly, the surface epithelium in time 

 becomes superficially eroded and the lesions become moist. This con- 

 dition is an important danger signal indicating activity of the process. 

 The next progressive stage is the development of carcinomata (rarely 

 sarcomata). It is therefore important to appreciate the danger of small, 

 repeated exposures to soft x-radiation. 



As in inorganic material, the relative amount of the x-radiation ab- 

 sorbed by tissue in its superficial layers is also determined by its wave- 

 length. Since the absorption is very great for long wavelengths, the 

 superficial layers of tissues irradiated by the entering energy are affected 

 to a greater extent than the deeper-lying tissue. As pointed out in 

 previous sections, the control of physical factors governing the quality 

 and quantity of x-radiation with which a patient is radiated is repro- 



