916 
GENETIC AND BIOLOGICAL EFFECTS OF RADIATION 
Figure 5. — Chromosomal pattern of a dividing lymphocyte. Arrow indicates a chromosomal break, (oil 
immersion field x 1000 Giemsa stain). 
ample — can increase the frequency of chromo- 
somal aberrations. ^° 
The peripheral leukocytes that divide in re- 
sponse to phytohemagglutinin in vitro do not 
normally divide vi^hile in the circulation. They 
are, in fact, pre-DNA-synthesis cells in which 
onl^ chromosome-type aberrations are induced 
by irradiation^ — an advantage for the purpose 
of radiation diagnosis. The problem of aberra- 
tions arising in culture is minimal because most 
such aberrations will be chromatid types and 
easily distinguished (with the exception of the 
non-sister-union isochromatid deletions). 
Prolonged exposure to low doses of ionizing 
radiation would not ordinarily produce an ap- 
preciable number of multiple-break aberrations, 
but when the source is implanted intracor- 
poreally, the irradiation may actually produce 
more multiple-break aberrations than single- 
break aberrations because of the irregular dis- 
tribution of the radioactive material within the 
body. In other words, some cells may be ex- 
posed to high dose rates, even though the aver- 
age dose rate for the entire body may be very 
low. Thus, circulating cells may be irradiated 
in transit near the source and then recirculated 
to more distant fixed populations. It has been 
estimated that the true number of small lympho- 
cytes might be forty times the number of cir- 
culating lymphocytes and that for each lympho- 
cyte entering the blood through the thoracic 
duct there are 4-12 lymphocytes from other 
sources." Other factors are lymphocyte life 
span and kinetics, the degree of fibrous tissue 
formation and cellular degeneration around the 
radioactive capsule, leading to relative ischemia 
of the area most exposed to radiation. 
Tough et al. suggested that x-ray induced 
chromosomal aberrations in the peripheral 
blood may decrease within a few months. ^- 
Bender and Grooch found chromosomal aber- 
rations persisting for three years in eight pa- 
tients who had been accidentally over-exposed 
to radioactive material. Those cases, however, 
