C. Relative Biological Effectiveness (RBE) 



RBE is a concept designed to express the comparative effectiveness of two dif- 

 ferent radiations. It is the ratio of the doses, measured in the physical sense as 

 ergs/gram, that produce the same biological effect. 



Certain ionizing particles, especially those in which the energy transfer per unit 

 path length (linear energy transfer, or LET) is great, produce relatively intense bio- 

 logical changes in higher organisms. The exact relationship between LET and biological 

 effects is not a proportional one and appears to have a certain maximum value. The 

 use of RBE in assessing radiation hazards presents certain difficulties in that it is 

 not readily applicable in all instances. Experimental work has thus far failed to yield 

 satisfactory information on the RBE relating alpha and beta rays with respect to acute 

 or chronic mammalian damage-*-". 



The term RBE has validity only when the mode, type, and method of radiation 

 are stated, the particular biological effects clearly circumscribed, and the acute or 

 chronic nature of the study properly evaluated. For excimple, the RBE of a group of 

 particles for interference with DNA synthesis may differ from the RBE for certain 

 cellular changes, such as chromosomal breakage or inhibition of mitosis, and it may 

 again differ from the RBE in a complex organism for a long-range effect such as the 

 production of neoplasms. Also, in a complex organism the RBE's of two particles 

 may differ widely for long-term and short-term effects. The RBE becomes even more 

 difficult to assess under special physiological conditions, such as changes in the state 

 of oxygenation. 



Because of the many difficulties, the concept of RBE can be applied only in a very 

 general way, particularly to internal emitters, and great care must be exercised when 

 utilizing it in establishing standards of radiation safety for various types of ionizing 

 radiation. It may be stated categorically that an RBE assigned to any particular ionizing 

 radiation cannot be applied uniformly to all biological effects- under all conditions. 



