(1) At cell level, the changes are produced when energy imparted by
whole animal, direct effects can be produced in given tissue by virtue of its life span and
function. For example, bone marrow continuously produces blood cells. These
immature cells are highly susceptible to radiation injury. A high enough dose of
radiation received by bone could result in bone marrow depression with a subsequent
drop in the blood count of an individual.
(2) One of the possibilities of direct effects is a chromosome break. Such a
break would require 20 ion pairs and could occur as the result of secondary ionization.
Gene mutations, on the other hand, can be produced by as little as one ion pair.
Chromosome breaks do not necessarily cause immediate cell death. One result of a
chromosome break is the possibility of an abnormal recombination. Despite this, the
whole cell may still be "operating" normally for the present. There is a definite delay
before the injurious effects of radiation are observed. Cell death may occur during
attempts at cell division (mitotic-linked death). The sooner the division following
radiation, the greater the chances for cell death. Cell death may also occur during
subsequent divisions due to loss of chromosome material.
(3) First and second generations may appear perfectly normal and be able
to function normally. It should be noted that chromosome damage generally does not
bring about cell death unless and until the cell enters division after irradiation.
b. Indirect Damage Theory. The indirect damage theory covers all other
effects, which are generally produced in the fluid environment and in neighboring cells.
These effects may be found in one part of the body due to irradiation of another part.
Indirect effects may be the result of circulating toxic substances, histamine imbalance,
or autointoxication by tissue breakdown products. The resulting mechanical/chemical
injury to cells leads to cell death. It occurs in both man and animal, but animals have
provided most of the experimental data. Keep in mind that a given amount of radiation
will not produce the same biological effects in a man as it does in a rat. In fact, it will not
produce the same injury in different cells in the same man. It appears likely that both
direct and indirect actions contribute to the chemical changes that lead to radiation
injury in a biological system.
4-25. RADIOSENSITIVITY OF CELLS
Two important factors influencing the varying responses to radiation or
radiosensitivity are cell variety and species variety.
a. These factors are explained by the Bergonie-Tribondeau law, which states,
"the more undifferentiated physiologically and morphologically, and active mitotically,
the longer the cell requires to undergo active mitosis; and the more division it has yet to
go through, then the more radiosensitive is the cell." Stated generally, this means
immature and rapidly dividing cells are more radiosensitive than mature ones and/or