lar components to repair the cell, RO• 2 is an altered chemical entity and
cannot help in cell repair. The oxygen effect is most predominant for g- and
x-rays, and is practically absent for high-LET radiations (e.g.,a-particles).
Because tumor cells are hypoxic, treatment of tumors with radiation under
high-oxygen pressure has been advocated.
It has been found experimentally that the proportion of hypoxic cells in
a tumor remains the same before and after fractionated radiation therapy.
Logically, radiotherapy should have killed more oxygenated cells and thus
raised the proportion of hypoxic cells. Instead, it remains the same and has
brought in the argument of reoxygenation of the tumor cells during frac-
tional radiation therapy, provided sufficient time is allowed for this to
happen. This phenomenon has an important implication in radiation
therapy in that even though the proportion of hypoxic cells remains the
same, the total number of hypoxic tumor cells will be killed by radiation
over time, thus leading to a successful treatment. The degree of reoxy-
genation varies with tumor types. The mechanism of reoxygenation has
been attributed to the fact that as the tumor shrinks in size, surviving cells
that were previously deprived of oxygen diffusion due to distal location
Factors Affecting Radiosensitivity 243Fig. 15.14. The cell survival curve illustrating the effect of oxygen. In the presence
of oxygen, the curve becomes steeper, indicating effective killing of the cells by
radiation.