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

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

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. 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

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)

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 close to 3 at high doses but may have a lower value of 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 OER of 2.5. Neutrons are between these extremes, with an OER of 1.6.

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

OER as a function of LET

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.

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.

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.

Impact of O2 concentration

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.

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.

Variation of radiosensitivity with O2 concentration

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.

Time scale of oxygen effect

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.

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 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

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

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;

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.