238 Applied Biophysics 



In most cases, this is due to the fact that growths are ill-defined 

 in their extent, and this being so, it is evident that unless irradia- 

 tion is extended well beyond the probable limits of the growth, 

 some of the malignant cells will escape. We are, in fact, dealing 

 largely with probabilities, not certainties, in the treatment of 

 malignant disease ; and an experienced radiotherapist is more 

 likely to discern these probabilities than an equally clever but 

 less experienced one. On this basis, it is evident that technical 

 methods are developments of ingenuity in the best means of 

 balancing the manifold considerations that are involved in the 

 irradiation of a malignant growth. 



There is indeed a wide difference in outlook between those 

 who, for instance, plan an extensive irradiation of a breast tumor 

 by the implantation of radium needles, and those who seek the 

 same end by the use of externally applied gamma radiation 

 which can be repeated at intervals determined by the day-to-day 

 response of the organism. It is the latter working philosophy 

 which originated in the French School, and which has been 

 given a rather different orientation by the work of Spear and his 

 colleagues of the Strangeways Laboratory, Cambridge ; here, 

 in fact, is a technical method which combines the virtues of 

 sound biological intuition with the asset of rigid physical control. 



If technical methods are to be improved, there must be a 

 happy balance between biological probabilities and physical cer- 

 tainties ; it is well, however, not to insist too much on the latter. 

 Isodose curves are usually derived from measurements upon 

 media having about the same density as the average of the 

 tissues concerned in treatment, but there need be no insistence 

 on the general crudeness of any such similarity. Any assessment 

 of the differential response of the various structures of the body 

 to irradiation is a matter not for the physicist ])ut for the 

 radiologist. It need not be emphasized that judgment upon this 

 crucial matter will depend not only upon the clinical sense of 

 the radiotherapist, but on his pathological knowledge. It is one 

 of the greatest claims to eminence in the field of radiotherapy, 

 that the French School, led by Regaud, and now by Lacassagne, 

 has so persistently maintained that this pathological knowledge, 



