Ahmed GroupLecture 9 Modifiers of Cell Survival: Oxygen Effect LECTURE 9

Ahmed GroupLecture 9

Modifiers of Cell Survival:

Oxygen Effect

LECTURE 9


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


Definition of OER


• 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.

• 1921 - Holthusen: Ascaris eggs were resistant to radiation in the absence of oxygen; wrongly attributed to the absence of cell division under these conditions.

• 1923 - 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

Discovery of Oxygen Effect


The nature of the oxygen effect

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


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.


Effect of dose

OER has a value closeto 3 at high doses but may have a lower valueof about 2 at X-ray doses below about 2 Gy


The OER for various types of radiation

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


From Sinclair W.K., Radiat Res. 33:620-643, 1968. The broken line is a calculated curve expected to apply to mitotic cells under hypoxia.

Cell-survival curves for Chinese hamster cells at various stages of the cell cycle


OER as a function of 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.

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-rayswith an assumed LET of 1.3 keV/µm.



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.


Impact of O2 concentration


The concentration of oxygen required to potentiate the effect of radiation is strikingly similar betweenthe 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.


Survival curves for CHO cells exposed to X-rays in the presence of various oxygen concentrations. Oxygen is introduced graduallyinto the biologic system.

The introduction ofa very small quantityof oxygen, 100 ppm,is readily noticeablein a change in theslope of the survivalcurves. A concentration of2,200 ppm, which isabout 0.25% oxygen,moves the survivalcurve halfway toward the fullyaerated condition.


Variation of radiosensitivity withO2 concentration


Impact of O2 concentration

Very small amounts of oxygen are necessary to produce thedramatic and important oxygen effect observed with X-rays.Oxygen tension between different tissues may vary over a widerange from 1 to 100 mm Hg. Many tissues are bordering hypoxic and contain a small proportion of cells that areradiobiologically hypoxic. This is particularly true of, forexample, the liver and skeletal muscles. That shows up as a change of slope if the survival curve is pushed to lowsurvival levels.


Time scale of oxygen effect


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


Mechanisms of oxygen effect


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 asfollows:

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


The oxygen fixation hypothesisThe damage produced by free radicals in DNA can be repairedunder hypoxia, but may be “fixed” if oxygen is present



Summary

• The OER is the ratio of doses without and with oxygen toproduce the same biologic effect;

• The OER for X-rays is about 3 at high doses and ispossibly lower (about 2) at doses below about 2 Gy (200 rad);

• OER decreases as LET increases. The OER approachesunity (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;


• To produce its effect, oxygen must be present during the radiation exposure or at least during the lifetime of thefree radicals;

• Oxygen “fixes” (i.e. makes permanent) the damageproduced 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 forradiosensitization; 0.5% oxygen results in asensitivity halfway between hypoxia and full oxygenation.

Summary

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Ahmed GroupLecture 9 Modifiers of Cell Survival: Oxygen Effect LECTURE 9