those of stable tissues. A cell in mitosis is much more radiosensitive than the same cell
at rest. Blood-forming cells and immature gonadal cells are examples of types of cells
that are particularly sensitive to radiation.
b. Nerve, brain, and muscle cells are least sensitive. The reduced sensitivity of
nerve, brain, and muscle is due to the fact that a person is born with all the nerve, brain,
and muscle cells he will have. Therefore, according to the Bergonie-Tribondeau law,
these cells are neither immature nor rapidly dividing and should have a greater
4-26. RADIOSENSITIVITY OF SPECIES
Species show an even greater variation in radiosensitivity, as shown in the
following examples. The LD (lethal dose) 50/30 varies for humans, sheep, poultry, and
bacteria. LD50/30 is the amount of radiation required to produce death in 50 percent of
the exposed animals or humans within 30 days.
3,000 to 5,000 R
Section VI. RADIATION--RESIDUAL DOSAGE
a. It is often said that "x-rays accumulate in the body." This statement is not
technically accurate; it is more accurate to say that the damaging effects of radiation are
cumulative. The basis for this statement is the fact that damage caused by radiation
has two components: one component is irreparable or permanent and is often referred
to as residual dose; the other component is reparable in that it is possible to recover
from its effects. It is believed that about 90 percent of the damage caused by any
exposure to radiation can be repaired, leaving about a 10 percent residual component
that will not be repaired. It follows, then, that multiple exposures can result in an
accumulation of radiation injury.
b. Although the word "safe" is commonly used in referring to radiation exposure,
it should be clearly understood that no amount of radiation, regardless of the amount,
can be considered 100 percent safe. "Biologically acceptable" is more descriptive in
that no individual receiving up to that amount would be expected to develop
manifestations of radiation injury.