H 2 O +energy →H 2 O++e−
H 2 O +e−→HOH−
H 2 O+→H++OH• (free radical)
HOH−→OH−+H• (free radical)
H++OH−→H 2 O
H• +OH• →H 2 O
OH• +OH• →H 2 O 2
H• +O 2 →HO 2 • (perhydroxyl radical)
Free radicals are characterized by an unpaired electron in the structure,
which is very reactive, and therefore reacts with DNA molecules to cause
a molecular structural damage (Fig. 15.10). Hydrogen peroxide, H 2 O 2 , is also
toxic to the DNA molecule. The result of indirect action of radiation on
DNA molecules is the impairment of function or death of the cell. The
number of free radicals produced by ionizing radiation depends on the total
dose but not on the dose rate. It has been found that the majority of
radiation-induced damage results from the indirect action mechanism be-
cause water constitutes nearly 70% of the composition of the cell.
Radiosensitivity of Cells
In living matter, there are two types of cells: differentiated and undifferen-
tiated. Undifferentiated cells do not have any specific physiologic function
except to develop into mature cells. They undergo mitosis and serve as the
precursors for mature cells. In contrast, all mature cells are differentiated
and perform specific functions in the living body. For example, red blood
cells (RBCs) are mature and differentiated cells performing the function of
oxygen carriers, whereas erythroblasts are undifferentiated cells that
develop into RBCs through mitosis.
According to the law of Bergonié and Tribondeau, undifferentiated cells
that are undergoing active mitosis are most sensitive to radiation, and dif-
ferentiated or mature cells are least affected by radiation. For example, in
a sample of mixed RBCs, erythroblasts are most damaged and mature
RBCs are least affected by radiation. Undifferentiated cells that are killed
by radiation may be replaced by new cells, but those that survive with defec-
tive DNAs can induce late effects, such as cancer (see later). In contrast,
the S phase of DNA synthesis in the cell cycle is least radiosensitive.
Radiosensitivity is best assessed by cell death. For differentiated cells, it
means loss of cellular function, whereas for undifferentiated cells it means
loss of reproductivity.
Groups of cells and their relative radiosensitivity are listed in Table 15.1.
As can be seen, lymphocytes, though mature cells, are most sensitive to radi-
ation, owing to a large nucleus; nuclear material is more radiosensitive.
Nerve cells and muscle cells are totally differentiated cells and therefore
Radiosensitivity of Cells 237