Protective Methods in Radiology 269 



and Radium Protection take a value of 0.1 r per day as the 

 tolerance dose. 



At the fifth international congress of radiology, held at Chicago 

 in 1937, the definition of the rontgen was modified in such a 

 way that it became a unit of gamma rays as well as of X-rays. 

 As regards the tolerance dose of radium gamma rays, the early 

 evidence indicated that it was likely to be of the same order 

 of magnitude as that for X-rays. Accordingly, we find that the 

 current recommendations of the International and British Com- 

 mittees state that "the evidence at present available suggests 

 that a person in normal health can tolerate with impunity ex- 

 posure to X-rays and radium gamma rays to an extent of about 

 0.2 international rontgen (r) per day or 1 r per w^ek." In 

 this respect, the American Advisory Committees have again 

 chosen the lower tolerance dose of 0.1 r per day. 



Integral Dose and Tolerance 



It will be seen that the present tolerance doses are expressed 

 in terms of the radiation falling upon the surface of the body. 

 It has been emphasized by Mayneord ^'' and others that the total 

 quantity of energy absorbed throughout the body of an irradiated 

 person, or "integral dose" as it is called, is of considerable im- 

 portance, both physically and clinically. For a given dosage 

 rate of radiation (expressed in rontgens per unit time) incident 

 upon the surface of the body, the dosage rates at various depths 

 in the body will be greater the more penetrating the radiation. 

 It follows, therefore, that the integral dose per unit surface dose 

 will depend on the quality of the radiation. 



A suggested unit of integral dose is the gram-rontgen, which 

 is the quantity of energy absorbed when 1 rontgen of radiation 

 is delivered to 1 gram of air. Mayneord and Clarkson ^^ have 

 drawn attention to the possible importance of integral dose in 

 protection problems. For X-rays excited at 40 kilovolts ( Siemens' 

 "Doglas" therapy tube with no added filter; HVL of 0.037 

 millimeter Cu), they find that the integral dose is of the order 

 of 13,000 gram-rontgens per rontgen measured on the patient's 



