162 



R. H. MOLE 



pattern of cancer induction in Japanese exposed at Hiroshima wliere the 

 induced cancer rate appears to increase with the sixth power of the age just 

 as the natural cancer rate does (Harada and Ishida, 1960; Fig. 2). 



The basic hypothesis is that cancer follows the occurrence of two inde- 

 pendent cellular events which each have a constant probability of occurrence 

 with time if environmental conditions are constant. The first event is followed 

 by clonal growth of cells all carrying this first change, the second event occurs 

 in any of the cells carrying the first change and is inevitably followed by the 

 development of overt cancer. The kinds of event are not further specified. 



10,000 IT 



5,000 



2,000 



o 

 o 

 o 

 o 

 o 





0) 



1,000 



500 



200 



Non-exposed 

 (beyond 10,000m) 



L 



20 30 40 50 60 70 



Age (years) 



Fig. 2. Age-specific incidence rate of all malignant neoplasms in Japanese at Hiroshima 

 (from Harada and Ishida, 1960). The upper continuous hne gives the incidence rate of all 

 malignant neoplasms, the lower broken line the incidence rate excluding leukaemia, in those 

 exposed between 500-1,400 metres from the hypo-centre. The interrupted line gives the inci- 

 dence rate of all malignant neoplasms in the non-exposed (beyond 10,000 metres). 



If the first kind of event is produced by radiation (as well as occurring 

 spontaneously) then irradiated individuals will carry a larger number of 

 different clones than unirradiated people of the same age and will show the 

 cancer incidence to be expected of older unirradiated individuals with the 

 same number of clones. The age-specific cancer rate of the particular group of 

 irradiated Japanese recorded by Harada and Ishida (1960) is in fact equivalent 

 at all ages to that of unirradiated Japanese 7 to 8 years older (Fig. 2). Since 



