Download - Chap6 oxygen effect and reoxygenation
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Chap 6. Oxygen Effect and Reoxygenation
2012.05.23Dahoon Jung
Korea Cancer Center Hospital
Radiobiology for the Radiologist, Hall, 7th ed
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Overview• The Nature of the Oxygen Effect• The Time at which Oxygen Acts and the Mechanism of the Oxygen Effect• The Concentration of Oxygen Required• Chronic and Acute Hypoxia
– Chronic Hypoxia– Acute Hypoxia
• The First Experimental Demonstration of Hypoxic Cells in a Tumor• Proportion of Hypoxic Cells in Various Animal Tumors• Evidence for Hypoxia in Human Tumors• Techniques to Measure Tumor Oxygenation
– Oxygen Probe Measurements– Markers of Hypoxia
• Reoxygenation• Time Sequence of Reoxygenation• Mechanism of Reoxygenation• The Importance of Reoxygenation in Radiotherapy• Hypoxia and Chemoresistance• Hypoxia and Tumor Progression
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Preface
• No other pharmacologic agent is simpler than oxy-gen, none produces such a dramatic effect, none has such obvious practical implications.
• Swartz, 1912, Germany ; The Oxygen effect.– Skin reaction change by pressure
• Holthusen, 1921 ; Ascaris eggs more radioresistant in the absence of oxygen.
• Petry, 1923 ; The correlation between radiosensitiv-ity and the presence of oxygen.
• Mottram, 1930s; on work of Crabtree and Cramer on the survival of tumor slices in the presence or ab-sence of oxygen.
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The Nature of the Oxygen Effect
• Survival curves for mam-malian cells exposed to x-rays in the presence and ab-sence of oxygen.
• Oxygen enhancement ratio (OER)– The ratio of doses adminis-
tered under hypoxic to aer-ated conditions needed to achieve the same biologic ef-fect.
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• For sparsely ionizing radiations(x- or r-rays), the OER at high doses : 2.5 – 3.5
• For rapidly growing cells cultured in vitro, the OER has a smaller value of about 2.5 at lower doses (in RT area).–May be due to the phase of the cell cy-
cle.• G1 have lower OER than S, because G1 cells
are more radiosensitive, they dominate the low-dose region of the survival curve.
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• There is no oxygen effect in a-particle.
• For radiations of interme-diate ionizing density, such as neutrons - much reduced shoulder.
• The oxygen effect is large and important in the case of sparsely ionizing radia-tions, such as x-rays.
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The Time at which Oxygen Acts and the Mechanism of the Oxygen
Effect• For the oxygen effect to be observed,
oxygen must be present during the radi-ation exposure or, to be precise, during or within microseconds after the radia-tion exposure.
• Oxygen sensitization occurred with oxy-gen added as late as 5 milliseconds after irradiation.
• Oxygen acts at the level of the free radi-cals.
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• The absorption of radiation.• The production of fast-charged particles.• The charged particles produce several ion
pairs(about 10-10 second of life span).• Produce free radicals(highly reactive
molecules, 10-5 second).• The free radicals break chemical bonds.• The extent of the damage depends on
the presence or absence of oxygen.
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The Oxygen fixation hypothesis.
- The formation of RO2, an or-ganic peroxide, represents a nonrestorable form of the target material; that is, the reaction results in a change in the chem-ical composition of the material exposed to the radiation.
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The Concentration of Oxygen Re-quired
• The concentration of oxygen required to potentiate the ef-fect of radiation.
• Curve A is characteristic of the response under conditions of equilibration with air.
• Curve B is a survival curve for irradiation in as low a level of hypoxia.
• The introduction of a very small quantity of oxygen, 100 ppm, is readily noticeable in a change in the slope of the survival curve.
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• By the time a concentration of oxygen corresponding to 2% has been reached, the survival curve is virtually indistinguishable from that obtained under conditions of normal aeration.
• Increasing the amount of oxygen present from that characteristic of air to 100% oxygen does not further af-fect the slope of the curve.
3 times
Rapid change of radiosensitivity occurs from zero to about 30 mmHg(5% oxygen)
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• It is evident that very small amounts of oxygen are necessary to produce the dramatic and important oxygen effect observed with x-rays.
• Many tissues are borderline hypoxic and contain a small proportion of cells that are radiobiologically hy-poxic.
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Chronic and Acute Hypoxia
• Hypoxia in tumors can result from two quite different mechanisms.– Chronic hypoxia : results from the lim-
ited diffusion distance of oxygen through tissue that is respiring.
– Acute hypoxia : the result of the tempo-rary closing of a tumor blood vessel ow-ing to the malformed vasculature of the tumor.
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• Chronic hypoxia• Thomlinson and Gray, 1955
– Triggered the tremendous inter-est in oxygen as a factor in ra-diotherapy.
• Fig.A shows a transverse sec-tion of a tumor cord and is typ-ical of areas of a tumor in which necrosis is not far ad-vanced.
• Fig B shows large areas of necrosis separated from stroma by a narrow band of tumor cells about 100 um wide.
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• As the tumor cord grows larger, the necrotic center also enlarges, so that the thickness of the sheath of viable tumor cells re-mains essentially con-stant.
• The oxygen can diffuse in respiring tissue is about 70 um.
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Cells would be at an oxy-gen tension high enough for cells to be clonogenic but low enough to render the cells protected from the effect of ionizing radia-tion.
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• Acute hypoxia
• Regions of acute hypoxia develop in tumors as a result of the temporary closing or blockage of a particular blood vessel.
• Results from transient fluc-tuations in blood flow be-cause of the malformed vas-culature.
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The First Experimental Demonstra-tion of Hypoxic Cells in a tumor
The survival curve for this solid tumor clearly consists of two sepa-rate components.
- The tumor consists of two sepa-rate groups of cells. - About 1% of the clonogenic cells in the tumor were deficient in oxy-gen. - If 99% of the cells are well oxy-genated and 1% are hypoxic, the response to lower doses is domi-nated by the killing of the well-oxygenated cells.
D0 of 1.1 Gy
D0 of 2.6 Gy
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Proportion of Hypoxic Cells in Vari-ous Animal Tumors
The fraction of hypoxic cells in the tumor determines the dis-tance between the parallel ter-minal slopes of the dose-re-sponse curves.
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• The proportion of hy-poxic cells in a mouse tumor.
• The biphasic curve la-beled air curve repre-sents data for cells from tumors irradiated in mice breathing air, which are therefore a mixture of aerated and hypoxic cells.
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Evidences for Hypoxia in Human Tumors
• Various techniques have been used to de-termine the oxygenation of human tumors.– Measuring the distance between tumor cells and
vessels– Determining the oxygen saturation of Hb– Oxygen probes, hypoxia markers, the comet as-
say, noninvasive imaging.
• Hypoxia is a common feature of human solid tumors that can influence both the malig-nant progression and the response of tu-mors to therapy.
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Techniques to Measure Tumor Oxy-genation
• Oxygen Probe Measurements– Oxygen probes : electrodes implanted
directly into tumors to measure oxygen concentration by a polarographic tech-nique.
– Hypoxia affects local control of tumors and also predicts tumor aggressiveness.
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• Markers of Hypoxia• The advantages of hypoxia markers
over oxygen electrodes include– That they provide the relative oxygen
concentrations on an individual cell ba-sis,
– That they make is possible to distinguish between viable and necrotic tissue,
– That they make it possible to distinguish between chronic and acute hypoxia.
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From a patient with carcinoma of the cervix.
Green : pimonidazole binding(hypoxia)Red : HIF-1 alpha(in a region of decreas-ing oxygen tension)Blue : blood vessels expressing CD31.
There is a gradient in oxygen tension away from blood vessels.
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Reoxygenation
• Van Putten and Kallman– The proportion of hypoxic cells in the un-
treated tumor was about 14%.– The ratio did not change after giving frac-
tionated radiation.
• During the course of the treatment, some hypoxic cells become oxygenated.
• This phenomenon, by which hypoxic cells become oxygenated after a dose of radia-tion, is termed reoxygenation.
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If reoxygenation is efficient between dose fractions, the presence of hypoxic cells does not have a significant effect on the outcome of a multifraction reg-imen.
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Time Sequence of Reoxygenation
• Kallman and Blee-hen, 1968.
• In this particular tumor, the process of reoxygenation is very rapid indeed.
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The proportion of hypoxic cells as a function of time af-ter irradiation with a large dose of x-rays for five trans-planted tumors in experi-mental animals.
○: mouse mammary ca. that reoxygenates rapidly and well.▲: rat sarcoma that shows two waves of reoxygenation.△: mouse osteosarcoma that does not reoxygenate at all for several days and then only slowly.●: mouse fibrosarcoma that reoxygenates quickly but not as completely as the mam-mary carcinoma.◆: mouse fibrosarcoma that reoxygenates quickly and well.
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Mechanism of Reoxygena-tion
• The differences of timescale reflect the different types of hypoxia that are being reversed, chronic versus acute.
• Chronic : As the tumor shrinks in size, surviving cells that previously were beyond the range of oxygen diffusion are closer to a blood supply and so reoxygenate.
• Acute (complete within hours): the reoxygenation of acutely hypoxic cells; those cells that were hypoxic at the time of ir-radiation because they were in regions in which a blood vessel was temporarily closed quickly reoxygenate when that vessel is reopened.
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The Importance of Reoxygenation in Radiotherapy
• The reoxygenation studies with mouse mammary carci-noma indicate that by 2 to 3 days after a dose of radiation, the proportion of hypoxic cells is actually lower than in un-treated tumors.
• It was predicted that several large doses of x-rays given at 48-hour intervals would virtually eliminate the problem of hypoxic cells in this tumor.
• It is an attractive hypothesis that some of the human tu-mors that do not respond to conventional radiotherapy are those that do not reoxygenate quickly and efficiently.
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Hypoxia and Chemoresis-tance
• Hypoxia can also decrease the efficacy of some chemother-apeutic agents owing to fluctuating blood flow, drug diffu-sion distance, and decreased proliferation.
• Some chemotherapeutic agents that induce DNA damage, such as doxorubicin and bleomycin, are less efficient at killing hypoxic tumor cells in part because of decreased free-radical generation.
• Hypoxic tumor regions are frequently associated with a low pH that can also diminish the activity of some chemother-apy agents.
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Hypoxia and Tumor Progression
• A clinical study in Germany in the 1990s showed a correlation between local control in advanced carcinoma of the cervix treated by radiotherapy and oxygen-probe measurements.
• A Canadian study supported the concept that hy-poxia drove malignant progression of cervical carcinomas in node-negative patients, as most of the failures occurred in the poorly oxygenated tumors with distant tumor spread outside the pelvis.
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• Studies carried out in the United States on pa-tients receiving RT for soft tissue sarcoma.– 70% of those patients with pO2s less than 10mmHg de-
veloped distant metastases– 35% of those with pO2s greater than 10 mm Hg.
• In a Danish study in which 28 patients with soft tissue sarcoma exhibited an increased risk of metastatic spread if they possessed low tumor pO2 values.
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• Thank you for listening.