166 • Applied Biophysics 



potentialities, or they may disintegrate immediately after ex- 

 posure. Enlargement of resting cells often follows an irradia- 

 tion. 



After a period .of mitotic inhibition, cell divison may be 

 resumed with varying degrees of abnormality. A sufficiently 

 high dose of radiation delivered at a high intensity may cause 

 the immediate disintegration of mitotic cells. The direct effects 

 of radiation thus cause a diminution in number of resting and 

 dividing cells and promote the "aging" of cells and foci. Apart 

 from some increase in cell size, the efifect of radiations on cells 

 in the early stages of differentiation has not yet been precisely 

 determined. 



The indirect effects of radiation are due to the interference 

 with the vascular and connective-tissue system of the tumor, 

 and to the induction or exacerbation of inflammatory reactions. 

 Insufficient blood supply affects the process and the incidence of 

 cell division, and may cause the disintegration of cells. The 

 inflammatory reaction leads to the infiltration and the breaking 

 up of tumor strands by round cells, followed by the formation 

 of fibrotic scars. 



The aim of radiotherapy in malignant disease is to convert 

 viable into nonviable cells, i.e., to induce the breakdown of 

 dividing cells and to prevent cell division, to cause the immediate 

 disintegration of resting cells, or their permanent sterilization by 

 differentiation. The observed radiation changes in malignant 

 growths vary according to the tumor type and the dose, dose 

 rate, and time interval between a given dose and the biopsy 

 excision. Some types of reaction of young foci to radiotherapy 

 are illustrated in figures 2 to 5. 



Figure 2 represents the reaction chart of a basal-celled car- 

 cinoma of tile temple treated by a dose of 3,200 r of X-rays 

 given in 13 days. Cell counts made in selected young foci of 

 serial biopsies show a diminution and finally a disappearance of 

 mitotic cells and an initially slow and later rapid disintegration 

 of resting cells. Clinically, the lesion responded well to treat- 

 ment and remains healed. This case illustrates the response of 

 undifferentiated tumor cells to radiotherapy by mitotic inhibition. 



