MOLOKHIA, ROBINSON, HUFFMAN AND NORMAN 
915 
Figure 4. — Chromosomal pattern of a dividing lymphocyte. Arrow indicates a fragmented chromosome. 
(oil immersion field x 1000 Giemsa stain). 
are affected more dramatically than other cells ; 
after about 500 days of irradiation, the lympho- 
cyte count will fall to between 20 and 50% of 
normal values.^ 
In our study, no significant hematologic 
changes occurred as a result of the prolonged 
intracorporeal irradiation. Because of the ap- 
preciable natural fluctuations in peripheral 
blood count, however, it would be difficult to 
demonstrate statistically significant changes in 
cell counts at these low dose rates. Furthermore, 
it is not possible to extrapolate to lower dose 
rates in order to estimate the possible effects 
of an implanted radioisotope-powered artificial 
human heart or pacemaker with perfected radia- 
tion shielding. 
Polyploidy, endoreduplication and the fre- 
quency of aneuploid cells, in addition to chro- 
mosomal aberration frequencies, have been sug- 
gested as cytogenetic indices of radiation 
exposure. Unfortunately, though, only chromo- 
somal aberration frequencies seem to be suf- 
ficiently stable, reproducible and predictable to 
be of practical value for radiation diagnosis." 
In our study, the only chromosomal changes 
that might have been caused by the irradiation 
were the fragmentation and the ring formation 
found soon after implantation. For several rea- 
sons, however, these findings must be inter- 
preted cautiously. 
For instance, even in the cells of normal, 
unirradiated people, chromosomal aberrations 
occur from time to time. These "spontaneous" 
aberrations constitute the background "noise" 
or base line above which increases caused by 
radiation exposure are measured, and they in- 
clude all the types of aberrations which may be 
induced by radiation, though in some cases at 
a much lower frequency. Spontaneous aberra- 
tion frequencies thus set a lower limit on the 
radiation dose that can be detected by cyto- 
genetic analysis." Furthermore, many environ- 
mental and health factors other than radiation 
exposure — drugs and viral infections, for ex- 
