Biomedical Engineering Reference
In-Depth Information
Fig. 13.17
Dependence of LD50 on dose rate for mice
irradiated with
60
Cogammarays.[BasedonJ.F.Thomsonand
W. W. Tourtellotte,
Am. J. Roentg. Rad. Ther. Nucl. Med.
69
, 826
(1953).]
Oxygen Enhancement Ratio
Dissolved oxygen in tissue acts as a radio-sensitizing agent. This so-called oxy-
gen effect, which is invariably observed in radiobiology, is illustrated in Fig. 13.18.
The curves show the survival of cells irradiated under identical conditions, except
that one culture contains dissolved O
2
(e.g., from the air) and the other is purged
with N
2
. The effect of oxygen can be expressed quantitatively by means of the oxy-
gen enhancement ratio (OER), defined as the ratio of the dose required under con-
ditions of hypoxia and that under conditions in air to produce the same level of
effect. According to this definition, one would obtain the OER from Fig. 13.18 by
taking the ratio of doses at a given survival level. OER values are typically 2-3 for
X rays, gamma rays, and fast electrons; around 1.7 for fast neutrons; and close to
unity for alpha particles.
The existence of the oxygen effect provides strong evidence of the importance
of indirect action in producing biological lesions (Section 13.6). Dissolved oxygen
is most effective with low- rather than high-LET radiation, because intratrack reac-
tions compete to a lesser extent for the initial reaction product.
Chemical Modifiers
Chemicals which, like oxygen, have a strong affinity for electrons can make cells
more sensitive to radiation. A number of radiosensitizing chemicals and drugs
are known. Some sensitize hypoxic cells, but have little or no effect on normally
aerated cells. Other agents act as radioprotectors, reducing biological effectiveness.
