CELL DIVISION, MORPHOLOGY, VIABILITY 



793 



sity treatment will have had approximately the same long period for 

 recovery, while only the first of the primary effects produced by low- 

 intensity treatment will have had a correspondingly long period for 

 recovery. The cells given a brief, high-intensity treatment may have 

 recovered completely, while those subjected to the prolonged, low- 

 intensity treatment, even though they received the same dose, will 

 exhibit a decrease in the number of cells in mitosis. Probably the most 

 accurate method of comparing the mitotic depression after short and 

 very long exposures to the same dose is to obtain a series of mitotic 

 counts at short time intervals for a few hours immediately following each 

 treatment. The mitotic picture during the long exposure can be obtained 

 by running other material at the same dosage rate and making a single 

 count immediately after any desired length of exposure to determine the 



2 4 6 8 10 12 14 16 



HOURS AFTER IRRADIATION 



Fig. 11-10. Relation of mitotic rate to time after y irradiation for different doses given 



at 0.25 r/min; Choriophaga neuroblasts at 38°C: open circle, 8 r; solid circle, 32 r; 



divided circle, 128 r (from Carlson, 1950). 



mitotic activity at that time. A curve illustrating the mitotic picture 

 from the beginning to several hours after the termination of a prolonged, 

 low-intensity treatment can thus be obtained. It can be seen, for 

 example, that the 128-r curve in Fig. 11-10, if projected to the left, would 

 parallel closely the zero ordinate to a point to the left of the first 32-r 

 count and then upward through the first 8-r count. 



Fractionated Treatment. A given dose of radiation may be delivered 

 not only in different periods of time and at different dosage rates but also 

 in two or more portions with intervening nonirradiation intervals. 

 Fractionated or spaced irradiation resembles low-intensity treatment in 

 that it reduces the possibility of interaction between primary effects by 

 giving time for recovery from one group of effects before the next group 

 is produced. Because mitosis is a cyclical process, however, with some 

 phases of the cycle more highly radiosensitive than others, the interpreta- 

 tion of the effects of fractionated treatment must take into account not 



