Chapter 6
The oxygen effect and reoxygenation



1. Which of the following statements is false?

a. Cells are more sensitive to x-rays in the presence of oxygen than in its absence (i.e. under hypoxia).

b. By "oxygen enhancement ratio" (OER) is meant the ratio of hypoxic to aerated doses needed to achieve the same biological effect.

c. For sparsely ionizing radiation, the OER is about 3 at high doses and about 2 at low doses (i.e. at doses of the order of the daily dose per fraction in radiotherapy).

d. The OER does not vary with the phase of the cell cycle.



2. The OER for alpha particles is equal to:

a. 3.

b. 2.5.

c. 1.6.

d. 1.0.



3. Sensitization by oxygen takes place if oxygen is present:

a. during irradiation.

b. as late as 5 ms after irradiation.

c. as late as 1 h after irradiation.



4. The following sequence of statements explains the "oxygen fixation hypothesis". One of the statements is false. Find which.

a. The absorption of radiation causes the production of fast charged particles.

b. The charged particles produce ion pairs.

c. The ion pairs produce free radicals (these molecules have an unpaired electron and are thus highly reactive).

d. The free radicals break chemical bonds, thus initiating the chain of events that results in biologic damage.

e. All biologic damage is caused by free radicals.

f. A part of the free radicals, produced as explained above, are located on the target molecules (DNA).

g. When oxygen is present, it reacts with the free radicals in the target to produce organic peroxides. These peroxides constitute a nonrestorable form of the target material (i.e. if they were not formed, the damaged target molecules would still have the capability to repair themselves).



5. Which of the following statements is false?

a. The oxygen effect can be visualized as a change in the slope of the survival curve (on a log-linear plot) as oxygen is gradually introduced into the biologic system (i.e. the greater the concentration of oxygen, the greater the change in the slope).

b. The most sensitive cells are those which are fully aerated ("equilibrated with air"). The most resistant cells are those that are under the lowest level of hypoxia (i.e. those that have the smallest concentration of oxygen).

c. The slope of the survival curve increases significantly as the oxygen concentration is increased between the value corresponding to air and 100%.

d. In the case of x-rays, very small amounts of oxygen are enough to induce dramatic changes in the radiosensitivity of the irradiated cells.



6. The radiosensitivity of cells under various oxygen concentrations can be expressed by a quantity proportional to the reciprocal of the D0 value of the curve (i.e. by a quantity which is proportional to the final slope of the survival curve).

a. True.

b. False.



7. The radiosensitivity of cells under full oxygenation is a factor of 10 greater than the radiosensitivity of cells under anoxia.

a. True.

b. False.



8. Which of the following statements is false?

a. Rapid increase in radiosensitivity is observed when the partial pressure of oxygen is increased from 0 to 30 mm Hg.

b. Further increase in oxygen tension from 30 mm Hg to 760 mm Hg (i.e. 100% oxygen) has a significant effect in cell radiosensitivity.

c. An oxygen tension of 3 mm Hg results in a radiosensitivity halfway between anoxia and full oxygenation.

d. Oxygen tension in different tissues varies between 1 mm Hg and 100 mm Hg.



9. Which of the following statements is true?

a. Hypoxia in tumours can result from two different mechanisms: chronic hypoxia and acute hypoxia.

b. Chronic hypoxia is caused by a temporary closing of a blood vessel in the tumour.

c. Acute hypoxia is caused by the limited diffusion distance of oxygen through tissue that is respiring.

d. Oxygen does not affect the radiosensitivity of tumours.



10. Which of the following statements referring to chronic hypoxia is true?

a. The centers of large tumours are necrotic and surrounded by intact tumour cells.

b. As the tumour grows, the necrotic center remains unchanged.

c. Both large and small tumours have a necrotic center.

d. The necrosis observed in the center of large tumours is not caused by the absence of oxygen.



11. The distance to which oxygen can diffuse in tumour cells:

a. is the same as in normal cells.

b. is limited in comparison to normal cells because oxygen is metabolized faster in tumour cells.

c. is approximately 70 micrometers at the venous end of a capillary and less at the arterial end.



12. Tumour cells at which the oxygen tension is high enough for them to be clonogenic but low enough for them to be radioresistant are called:

a. Normoxic.

b. Anoxic (necrotic).

c. Hypoxic viable.



13. Which of the following statements referring to acute hypoxia in tumour cells is false:

a. Tumour blood vessels open and close in a random fashion.

b. Acute hypoxia is due to blood-flow instability.

c. Most cells with acute hypoxia eventually die.



14. The difference between chronic hypoxia and acute hypoxia is that in the latter case the cells return to the normoxic state.

a. True.

b. False.



15. The probability to kill tumour cells which are in an acute hypoxic state increases when the dose is fractionated (in comparison to the single dose case).

a. True.

b. False.



16. The fraction of hypoxic cells in human tumours has been estimated to be:

a. 5%.

b. 10-15 %.

c. 20-25%.

d. 50%.



17. Which of the following statements is false?

a. Oxygen probes are electrodes implanted directly into tumours to measure oxygen concentration.

b. Oxygen probes can be used before treatment as a predictive assay.

c. Oxygen probe measurements have been shown to correlate with local control of certain tumour types.

d. Hypoxia is not considered to be an indication of tumour aggression.



18. The fraction of hypoxic cells in a tumour is approximately the same before and after a single dose has been delivered to the tumour if sufficient time is allowed to elapse.

a. True.

b. False.



19. "Reoxygenation" is a phenomenon by which:

a. acute hypoxic cells become normoxic.

b. oxygen contained in a high pressure chamber is allowed to "explode" onto a layer of irradiated tissue at given time intervals after irradiation.

c. oxygen diffuses to the necrotic center of the tumour.

d. hypoxic cells become oxygenated after a dose of radiation.



20. If reoxygenation did not take place, the fraction of hypoxic cells after a fractionated radiotherapy regimen would:

a. increase.

b. decrease.

c. remain constant.



21. The extent and rapidity of reoxygenation in various types of tumours are:

a. quantitatively similar.

b. very variable and impossible to predict.

c. approximately the same.