Linear Energy Transfer
High-LET radiations do not exhibit a dose-rate effect on the survival curve.
Also at high-dose rates (above 100 rad/min) of low- and moderate-LET
radiations, no dose-rate effects are observed on the survival curve in con-
trast to low-dose rates. Thus, high-LET radiations exhibit no shoulder (i.e.,
no Dq) on the survival curve resulting in an extrapolation number of 1.
High-LET radiations are densely ionizing radiations causing more double-
strand breaks in the DNA molecules, and thus leading to more cell deaths
than low-LET radiations, which are sparsely ionizing radiations. Radiation
damage by high-LET radiations is so severe that the chances of repair are
minimal, and even if repair takes place, the cell is likely to be defective.
Chemicals
Several chemicals, if present during irradiation, have been found to
augment or diminish the effects of radiation on cells. Agents that enhance
the cell response to radiation are celled radiosensitizers, and those that
protect cells from radiation-induced damage are called radioprotectors.
Radiosensitizers
Oxygen
Oxygen is the best-known sensitizer encountered in radiation biology. It has
been found that hypoxic cells are very resistant to radiation, whereas oxy-
genated cells are highly radiosensitive. Such radiosensitization by oxygen is
called the oxygen effectand is measured by a quantity called the oxygen
enhancement ratio(OER). The OER is given by the ratio of the dose
required to produce a given radiation damage to cells in the absence of
oxygen to that required to produce the same damage in the presence of
oxygen. The oxygen effect occurs only when oxygen is administered simul-
taneously with radiation. It increases with O 2 tension up to 30 mm Hg, and
remains constant at higher O 2 tension. For mammalian cells, the oxygen
concentration required to produce a radiation response midway between
hypoxic and aerobic conditions is approximately 0.5%. The OER value
reaches a maximum of 3.0 for x- and g-radiations, whereas it is about unity
for high-LET radiations such as a-particles.
Figure 15.14 illustrates the effects of oxygen on the survival curve. The
presence of oxygen makes the curve much steeper, indicating the augmen-
tation of cellular damage at smaller doses relative to the situation of no
oxygen. The mechanism of the oxygen effect is not clearly understood but
is most likely related to DNA strand breaks. It has been postulated,
however, that oxygen combines with already formed free radicals, R•, to
produce peroxidyl group RO 2 •, which is more damaging to the DNA mol-
ecules. While normally R• could recombine with complementary molecu-
242 15. Radiation Biology