172 Applied Biophysics 



efficiency of therapy. As with surgery, radiation treatment of 

 cancers aims at the complete ehmination or steriHzation of viable 

 tumor cells, and a 90% success of therapy is ultimately a failure. 

 The histological-reaction charts (figures 2 to 5) are measures of 

 tumor activity, and bear no relation to the actual size of the 

 tumor at the time of the biopsy excision. 



The persistence of active microscopic tumor foci in appar- 

 ently restored sites is the reason why, shortly after treatment, 

 the histological findings may be at variance with the results of 

 clinical examinations. Agreement becomes, however, closer with 

 the lapse of time. For example, in the series of 150 cases referred 

 to,^- there was agreement between histological and clinical find- 

 ings in only 50% of the cases after 4 months and in 80% of 

 the cases 2 years after treatment. 



Apart from showing within 3 weeks of beginning treatment 

 whether or not the aim of therapy is being realized, the histo- 

 logical analysis gives some useful information about the way in 

 which the therapeutic results are obtained. In cases like that 

 of figure 2, the successful treatment is due in particular to the 

 "mitotic'' eflFect of radiation, i.e., the mitotic inhibition and to 

 disintegration of dividing cells ; this prevents the further forma- 

 tion of resting cells which consequently age and, having reached 

 the limits of their short span of life, die. Some resting cells 

 are also killed immediately by the radiation and others fall 

 victims to unfavorable conditions in the tumor bed induced by 

 radiation. 



In epitheliomata like that of figure 3, the mitotic and vas- 

 cular effect of radiation is supplemented by the "diflferentiation" 

 effect, i.e., resting cells are forced (either directly or secondarily 

 to mitotic inhibition) into differentiation, and are thus sterilized. 

 This observation suggests that the capacity for differentiation in 

 resting malignant cells and its stimulation by radiation may be 

 one of the factors in the "radiosensitivity" of tumor tissue. 



An indication of the capacity for differentiation of the tumor 

 tissue — though not of its reaction to radiation — may be gained 

 from the presence or absence of differentiated foci in the pre- 

 radiation biopsy of the tumor. Histological classification as to 



