1. Modifiers of Cell Survival: Oxygen Effect
LECTURE 9
Modifiers of Cell Survival:
Oxygen Effect

2. Modifiers of Cell Survival: Oxygen Effect
Definition of OER
Effects of dose, dose rate, cell type
OER as a function of LET
Impact of O2 concentration
Time scale of oxygen effect
Mechanism of oxygen effect

3. Definition of OER

4. Discovery of Oxygen Effect
The oxygen effect was observed in 1912 by Swartz in Germany, who noted that the skin reaction to a radium applicator wasreduced if the applicator was pressed hard onto the skin.
Holthusen: Ascaris eggs were resistant to radiation in the absence of oxygen; wrongly attributed to the absence of cell division under these conditions.
Petry: correlation between radiosensitivity and the presence of oxygen based on the study of the effects of radiation on vegetable seeds.
1930s - Crabtree and Cramer: survival of tumor slices irradiated in the presence or absence of oxygen.
1930s - Mottram, Gray and Read: quantitative measurement of the oxygen effect

5. The nature of the oxygen effect
Survival curves for mammalian cells exposed to X-rays in the
presence (lower curve) and absence (top curve) of oxygen.
The ratio of hypoxic to aerated doses needed to achieve the
same biological effect is called oxygen enhancement ratio (OER)

6. Effect of dose, dose rate, cell type
There is some evidence that for rapidly growing cells cultured in vitro the OER has a smaller value of about 2 at lower doses.
This results from the variation of OER with the phase of the cell cycle:
Cells in G1 have a lower OER than those in S. Because G1 cells are more radiosensitive they dominate the low-dose region of the survival curve. For this reason the OER of an asynchronous population is slightly smaller at low doses than at high doses.

7. Effect of dose OER has a value close to 3 at high doses but
may have a lower value
of about 2 at X-ray
doses below about 2 Gy

8. The OER for various types of radiation
The OER for alpha particles is unity. X-rays exhibit a larger OER
of 2.5. Neutrons are between these extremes, with an OER of 1.6.

9. Cell-survival curves for Chinese hamster cells at various stages of the cell cycle
From Sinclair W.K., Radiat Res. 33: , The broken line is
a calculated curve expected to apply to mitotic cells under hypoxia.

10. OER as a function of LET

11. The oxygen effect and LET
At low LET, the OER is between 2.5 and 3.
As LET increases, the OER falls until the LET reaches 60 keV/m.
When LET exceeds 60 keV/m, the OER falls rapidly and reaches unity by the time the LET has reached about 200 keV/m.

12. The oxygen effect and LET
Measurements were made with cultured cells of
human origin. Closed circles refer to monoenergetic
charged particles, the open triangle to 250 kVp X-rays
with an assumed LET of 1.3 keV/µm.

13. The oxygen effect and LET
Variation of the OER and the relative biologic effectiveness as
a function of the linear energy transfer of the radiation involved.
The data were obtained by using T1 kidney cells of human origin,
irradiated with alpha particles or deuterons.

14. Impact of O2 concentration

15. The concentration of oxygen required to potentiate
the effect of radiation is strikingly similar between
the bacteria and mammalian cells.
The next slide shows the simple way to visualize the
effect of oxygen is by considering the change of slope
of the mammalian cell-survival curve.

16. Survival curves for CHO cells exposed to X-rays in the presence
of various oxygen concentrations. Oxygen is introduced gradually
into the biologic system.
The introduction of
a very small quantity
of oxygen, 100 ppm,
is readily noticeable
in a change in the
slope of the survival
curves.
A concentration of
2,200 ppm, which is
about 0.25% oxygen,
moves the survival
curve halfway
toward the fully
aerated condition.

17. Variation of radiosensitivity with
O2 concentration

18. Impact of O2 concentration
Very small amounts of oxygen are necessary to produce the
dramatic and important oxygen effect observed with X-rays.
Oxygen tension between different tissues may vary over a wide
range from 1 to 100 mm Hg. Many tissues are bordering
hypoxic and contain a small proportion of cells that are
radiobiologically hypoxic. This is particularly true of, for
example, the liver and skeletal muscles. That shows up as a
change of slope if the survival curve is pushed to low
survival levels.

19. Time scale of oxygen effect

20. For the oxygen effect to be observed, oxygen must be
present during the radiation exposure or, to be
precise, during or within microseconds after the
radiation exposure.
In a number of experiments it has been shown that
oxygen need not be present during the irradiation to
sensitize but could be added afterward, provided
the delay was not too long.
Some sensitization occurred with oxygen added as late
as 5 ms after irradiation.

21. Mechanisms of oxygen effect

22. There is general agreement that oxygen acts at the levels of
the free radicals.
The chain of events from absorption of
radiation to the final expression of biologic damage is as
follows:
Absorption of radiation
Production of fast charged particles
Production of ion pairs
Production of free radicals
Breakage of chemical bonds, chemical changes,
initiation of the chain of events that result in
biological damage

23. The oxygen fixation hypothesis
The damage produced by free radicals in DNA can be repaired
under hypoxia, but may be “fixed” if oxygen is present

25. Summary The OER is the ratio of doses without and with oxygen to
produce the same biologic effect;
The OER for X-rays is about 3 at high doses and is
possibly lower (about 2) at doses below about 2 Gy (200 rad);
OER decreases as LET increases. The OER approaches
unity (i.e., no oxygen effect) at a LET of about 160 keV/µm.
For neutrons, the OER has an intermediate value of about 1.6;

26. Summary To produce its effect, oxygen must be present during the
radiation exposure or at least during the lifetime of the
free radicals;
Oxygen “fixes” (i.e. makes permanent) the damage
produced by free radicals. In the absence of oxygen,
damage produced by the indirect action may be repaired;
Only a small quantity of oxygen is required for
radiosensitization; 0.5% oxygen results in a
sensitivity halfway between hypoxia and full oxygenation.