ACUTE RADIATION EFFECTS ON MAN
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marked reduction in the mitotic index of the heavily irradiated victims
(236-365 rad) (Fig. 2). There was a significant difference between this
group and the 3 workers who were exposed to less than 100 rad: in the
latter group the mitotic index wasessentially within normallimits; in the
high-exposure group, the mitotic index remained low for about 3 weeks,
but 4 weeks after exposure was again within normallimits.
The explanation of the low mitotic index of the bone marrow after
radiation injury is complex. The mitotic index of the marrow asdefined by
Fliedneret al. will depend on several factors:
1. Mitotic time: other things being equal, any increase in mitotic time
will increase the mitotic index, and any decrease in mitotic time will lower
the mitotic index.
2. The relative numbers of red and white cell precursors in the marrow at
any time: in normal marrows, red cell precursors contribute about 2/3 of
all mitoses, whereas neutrophil precursors—in spite of their larger numbers
—only account for somewhat less than 1/3 of all mitoses in the marrow
(Killmannetal., 1961 a).
3, The relative numberofcells that have the capacity to divide and the
relative number that have matured andlost this capacity. The mitotic index
decreases after radiation injury because cells are prevented from going into
mitosis and because there is a decrease in the relative numberof red cell precursors which contribute the bulk of the mitoses (Brucer, 1959). On the
other hand, the prolonged mitotic time and the disappearance from the
marrow ofcells incapable of mitosis (e.g., segmented neutrophils) would
tend to increase the mitotic index. With these multiple variables it is
virutally impossible to predict what would happen to the mitotic index
after irradiation of such a mixed cell population as is found in the bone
marrow. The data collected by Fliedner et al. clearly demonstrate, however, that the mitotic index falls: with doses of the order of 236-365 rad,
mitoses were virtually absent on the 4th day. In spite of the theoretical
objections referred to, we believe that the mitotic index will be useful as a
biological dosimeter for doses up to approximately 250-350 rad.
The
chances are that the course will be uneventful if the mitotic index on the
4th day is essentially normal. A severe depression of the mitotic index to
around 0.1% on day 4 indicates an exposure to 250-350 rad or more. Thus,
a mitotic index of 0.1% on day 4 is compatible with survival—as in the
Y-12 cases—but does not necessarily guarantee a favourable prognosis.
Reliable determinations of the mitotic index require some experience on
the part of the investigator, but specialized equipment is not necessary.
Serial mitotic indices may well prove to be helpful in evaluating the exposure
of irradiated human beings.
It is important to note in this context that
these counts can be completed within approximately 12 hours after the
marrow has beenaspirated.
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