are highly resistant to radiation. The tissue or organ that contains more
radiosensitive cells will be highly radiosensitive and vice versa. For example,
bone marrow containing radiosensitive erythroblasts is very radiosensitive,
whereas nerves and muscles containing radioresistant cells are less
radiosensitive. Following irradiation of blood, depressed blood counts are
observed as follows: lymphocytes on the same day, granulocytes in 3 days,
platelets in 6 days, and RBCs in 10 days.
Cell Survival Curves
When mammalian cells are irradiated, not all cells are affected to the same
extent. Different factors such as the total dose, the dose rate, the LET of
the radiation, the particular stage of the cell cycle (M, G 1 ,S,or G 2 ) and the
type of cell will affect the radiation-induced damage. Some cells may die
and some will survive. The cellular response to radiation is illustrated by
what is called the cell survival curve. It is obtained by plotting the dose along
the linear X-axis and the surviving fraction along the logarithmic Y-axis.
Surviving cells are those cells that retain all reproductive as well as func-
tional activities after irradiation, whereas the death of cells is indicated by
the loss of their function in differentiated cells and by the loss of repro-
238 15. Radiation Biology
Table 15.1. Different types of cells and their
radiosensitivity.
Types of cells* Radiosensitivity
VIM Mature lymphocytes Highly sensitive
Erythroblasts
Spermatogonia
DIM Myelocytes Relatively sensitive
Intestinal crypt cells
Basal cells of epidermis
MCT Osteoblasts Intermediate sensitivity
Spermatocytes
Chondroblasts
Endothelial cells
RPM Spermatozoa Relatively resistant
Granulocytes
Erythrocytes
Osteocytes
FPM Nerve cells Highly resistant
Muscle cells
Fibrocytes
Adapted from Casarett AP.Radiation Biology.Englewood
Cliffs, NJ: Prentice-Hall; 1968:168–169.
* VIM—vegetative intermitotic; DIM—Differentiating inter-
mitotic; MCT—multipotential connective tissue; RPM—
reverting postmitotic; FPM—fixed postmitotic.