156 BENSTED, BLACKETTj COURTENAY, AND LAMERTON 



with a larger number of smaller fractions, together with histological studies of 

 regeneration, would be useful. 



The present experiments have not been of a large enough size to determme 

 whether there is any significant diiference in favoured sites of tumour pro- 

 duction in bone between the single and fractionated treatments with the same 

 radiation source. Anatomical and metabolic differences between different 

 sites are probably of importance in determining the carcinogenic efficiency of 

 a given radiation dose distribution and ideally comparisons should only be 

 made for tumours arismg at the same site. If subsequent work were to show 

 significant differences m timiour sites between single and fractionated treat- 

 ments the conclusions drawn here regarding the efi^ects of fractionation might 

 have to be revised. 



Work with bone tumours has the advantage that, using the bone-seeking 

 isotopes, high tumour yields can be obtained but the heterogeneous nature of 

 bone and the changes with age of histological and metabolic pattern greatly 

 complicate the interpretation of experimental data. It may be possible to 

 design experiments on the mechanisms of bone-tumour production by 

 radiation which will avoid many of these difficulties. Besides fractionation, 

 the quality of the radiation is a factor that can be altered. A study of the 

 variation of the comparative effectiveness of an a- and a j8-emitting bone- 

 seeker over a wide range of injected amounts could be a very useful approach. 

 Some comparative data are already available from the work of Finkel (1959) 

 and studies are in progress in other laboratories, but for the purpose of study- 

 ing mechanisms it would be necessary to extend to low levels of isotope 

 concentration, which would require the use of very large groups of animals, 



ACKNOWLEDGMENTS 



The authors wish to express their appreciation to many colleagues for 

 help and advice and particularly to Dr. D. M. Taylor, to Miss K. Adams, 

 Mr. J. Blackmore, Mr. E. J. Perry, Miss M. Shreeve and Mr. L. Smith and to 

 Professor W. V. Mayneord, Director of the Physics Department, Institute of 

 Cancer Research, Royal Cancer Hospital. 



REFERENCES 



Bensted, J. P. M., Blackett, N. M., and Lamerton, L. F. (1961). Brit. J. Radiol. 34, 399. 

 Blackett, N. M. (1962). Council for International Organisation of Sciences Symposium 



"Radioactive Isotopes and Bone" 1960. Blackwell Scientific Publications (in press). 

 Finkel, M. P. (1959). "Radiation Biology and Cancer", p. 322. University of Texas Press. 

 Jee, W. S. S., Ottosen, p., Mical, R., and Lowe, M. (1957). Amiual Progress Report 



Radiobiology Laboratory, University of Utah College of Medicine, Salt Lake City. 



Utah. p. 86. 



