888 GENETIC AND BIOLOGICAL EFFECTS OF RADIATION 
TO CAROTI D ARTERY 
Figure 1. — Schematic representation of extracorporeal irradiation of blood (ECIB) in calves. (Reprinted from 
Sipe et al., 1965.) 
a pumping system is required to propel lymph 
through the irradiator and back into the venous 
system. Thoracic duct lymph contains very few 
red blood cells thus ECIL can be given contin- 
uously for long periods of time (weeks to 
months) without complication. As with ECIB, 
the primary objective is to deplete the body of 
lymphocytes. One fundamental difference in the 
two methods is that, whereas ECIB destroys 
a certain fraction of all lymphocytes entering 
the blood irrespective or origin, ECIL destroys 
only those lymphocytes which either enter the 
thoracic through recirculation from blood to 
lymph or are produced in the regional lymph 
nodes. 
LYMPHOCYTE DEPLETION BY ECIB AND ECIL 
Prolonged continuous ECIB or daily repeti- 
tive ECIB produces a marked lymphopenia 
(Figure 2a). Transit doses between 15 and 900 
rads are effective, however, lymphopenia devel- 
ops more rapidly with higher doses, i.e., above 
200 rads. Daily repetitive ECIB also results in 
lymphopenia (Figure 2b) and is, in fact, more 
efficient than continuous ECIB in terms of the 
total radiation exposure to the blood. Increased 
efficiency with repetitive ECIB no doubt results 
from the establishment of new equilibria be- 
tween tissue and blood pools of lymphocytes 
during intervals when no ECIB is given. Since 
Ijonphocytes have a short intravascular transit 
time,* thereby reducing the probability of mak- 
ing multiple passes through the irradiator, the 
most efficient schedule of ECIB should be daily 
repetitive sessions in which 5-10 blood volumes 
are irradiated at a transit dose sufficiently high 
to kill the majority of lymphocytes on one pas- 
sage through the radiation field. 
ECIB also results in a substantial reduction 
in the output of lymphocytes in the thoracic 
duct.'' This finding provides direct confirmation 
that lymphocytes circulate from blood to 
lymph 1* and that the lymphocyte concentration 
in efferent lymph is directly dependent upon the 
lymphocyte concentration in the blood. 
Morphological classification of blood lympho- 
cytes into large and small categories suggested 
that ECIB had a greater effect on small lympho- 
* Based on the number of lymphocytes emerging in thoracic duct 
lymph and thymic vein blood alone, the mean intravascular time of 
lymphocytes would be less than 3 hours in calves. Transfusion of 
labeled lymphocytes indicates the minimum transit time to be between 
15 and 30 minutes. 
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