Radiobiology 5 MCQ

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Questions and Answers

What happens to the oxygen enhancement ratio (OER) as linear energy transfer (LET) increases beyond 60 keV/µm?

OER becomes unaffected by LET and stabilizes.

OER falls slowly until it reaches unity at 200 keV/µm. (correct)

OER increases rapidly until it reaches 5.

OER remains constant around 2.5.

What is the maximum distance oxygen can diffuse from a capillary in the context provided?

70 μm at the arterial end. (correct)

100 μm at both ends.

150 μm at the venous end.

50 μm regardless of the type of capillary.

What is the possible consequence of acute hypoxia in tumor tissues as described?

Permanent damage to blood vessels.

Increased smooth muscle in tumor blood vessels.

Temporary resistance to radiation therapy. (correct)

Improved oxygenation in all tumor cells.

How does the re-oxygenation process affect tumoral cells after radiotherapy?

It increases the sensitivity of previously hypoxic tumoral cells to radiation. Signup and view all the answers

Which statement best describes the relationship between OER and RBE as a function of LET?

Both curves exhibit a similar pattern, indicating they are mirror images. Signup and view all the answers

What role does hypoxia play in malignant progression according to the information provided?

It may contribute to increased radioresistance and metastatic potential. Signup and view all the answers

Why do tumor cells become hypoxic intermittently?

Because of the disorganized structure of tumor blood vessels. Signup and view all the answers

At what LET value is the optimal RBE and rapid fall of OER observed?

100 keV/µm. Signup and view all the answers

What can be inferred about the relationship between tumor cell oxygenation and radiation therapy effectiveness?

Effective radiation therapy may rely on the sequence of cellular oxygenation. Signup and view all the answers

What is the oxygen enhancement ratio (OER) primarily used to measure?

The modulation of biological effects due to oxygen presence Signup and view all the answers

Which factor is NOT classified as a biological factor affecting radiation response?

Linear energy transfer Signup and view all the answers

At what partial oxygen pressure (pO2) does relative radio-sensitivity reach midway between anoxia and full oxygenation?

3 mm Hg Signup and view all the answers

What effect does increased oxygen concentration have on radiation sensitivity up to 30 mm Hg?

Most change in sensitivity occurs Signup and view all the answers

Why are hypoxic cells considered more radio-resistant compared to aerated cells?

Lack of oxygen impairs DNA damage response Signup and view all the answers

Which of the following best describes the reciprocal of the oxygen enhancement ratio (OER)?

Relative radiosensitivity Signup and view all the answers

What is the primary biological effect of ionizing radiation at low LET in relation to oxygen?

Enhanced damage to DNA through oxygen Signup and view all the answers

Which method can be used to assess the impact of oxygen on radiation effectiveness?

Comparing survival rates under varying oxygen conditions Signup and view all the answers

Which of the following statements regarding oxygen concentration and relative radio-sensitivity is true?

Sensitivity markedly changes from 0 to 30 mm Hg Signup and view all the answers

What is the primary reason for the oxygen effect in radiobiology?

Oxygen facilitates the formation of free radicals Signup and view all the answers

What is the term used to describe the ratio of doses required under hypoxic versus aerated conditions to achieve the same level of cell killing?

Oxygen enhancement ratio Signup and view all the answers

Which statement about the presence of molecular oxygen during radiation exposure is true?

Oxygen enhances the permanence of radiation damage Signup and view all the answers

What is the oxygen effect on free radical damage in the absence of oxygen?

It repairs the damage Signup and view all the answers

Which type of radiation sees a more pronounced oxygen effect?

Low linear energy transfer (LET) radiation Signup and view all the answers

What oxygen enhancement ratio (OER) is observed for high-LET radiations?

OER = 1 Signup and view all the answers

What occurs to the indirect damage caused by radiation when oxygen is present?

It is fixed as an organic peroxide Signup and view all the answers

How does the oxygen effect vary with radiation dose?

Increases with dose Signup and view all the answers

Which of the following statements regarding oxygen and ionizing radiation is false?

Oxygen increases the amount of free radicals produced Signup and view all the answers

What role does molecular oxygen play in the context of indirect radiation damage?

It stabilizes the indirect damage Signup and view all the answers

In which scenario is the oxygen enhancement ratio expected to be closest to 1?

Under normal aerated conditions with high LET Signup and view all the answers

What happens to the percentage of hypoxic cells immediately after irradiation with x-rays?

It reaches approximately 100%. Signup and view all the answers

What is the primary reason that x-ray irradiation kills more aerated cells than hypoxic cells?

Aerated cells are more radiosensitive. Signup and view all the answers

What phenomenon occurs 6 hours after irradiation concerning hypoxic cells?

Their percentage falls to a level close to preirradiation. Signup and view all the answers

What effect does administering radiation in a series of fractions have on hypoxic cells?

It has no significant influence on tumor response. Signup and view all the answers

What does the term OER refer to in the context of radiation?

The oxygen enhancement ratio in radiation therapy. Signup and view all the answers

How does the OER change with increasing LET (Linear Energy Transfer)?

OER decreases as LET increases. Signup and view all the answers

When is oxygen most effective during the irradiation process?

During irradiation or very shortly after (in microseconds). Signup and view all the answers

What percentage of oxygen is required to observe a significant effect during irradiation?

Under 5% Signup and view all the answers

Which of the following statements is false regarding the relationship between cell oxygenation and radiosensitivity?

Oxygen enhances the effects of hypoxia. Signup and view all the answers

What is the likelihood of cell viability immediately following irradiation with respect to hypoxia?

Almost all viable cells become hypoxic. Signup and view all the answers

Study Notes

Oxygen Enhancement Ratio (OER)

OER is the ratio of doses under hypoxic to aerated conditions producing the same biologic effect.
The presence or absence of molecular oxygen significantly affects the biological effects of X-rays.
Oxygen presence (aerated cells) increases radiation effectiveness in cell killing.
Lack of oxygen (hypoxic cells) leads to increased radio resistance.
Ionizing radiation at low LET is more effective in the presence of oxygen.
The reciprocal of OER is Relative Radio-sensitivity, defined as 1.0 at 0% O2.

Factors Affecting Radiation Response

Physical factors include linear energy transfer, relative biological effectiveness, fractionation, and protraction.
Biological factors include the oxygen effect, phase of the cell cycle, ability to repair, chemical agents, and hormesis.

Radio-Sensitivity and Oxygen Concentration

Most of the change in sensitivity occurs as the oxygen tension increases from 0 to 30 mmHg.
Further increase in oxygen content has little further effect.
Relative radio-sensitivity halfway between anoxia and full oxygenation occurs at a pO2 of about 3 mmHg, which corresponds to approximately 0.5% oxygen.

Oxygen Effect

Oxygen is required to "fix" damage caused by free radicals generated during radiation exposure.
In the absence of oxygen, damage caused by indirect action may be repaired.
Generally, free-radical reactions involve the addition of oxygen to functional groups, forming organic peroxides which fix the indirect damage.

OER Effect

The OER varies from 2-3, increasing with dose.
Low-LET radiations experience a more pronounced oxygen effect.
High-LET radiations have a non-existent oxygen effect (OER = 1)

Other Radiations and OER

For 15 MeV neutrons, OER = 1.6.
For alpha particles, OER = 1.0.

Relationship Between OER and LET

At low LET (e.g x- or γ-rays), the OER is between 2.5 and 3.
As LET increases, the OER gradually decreases, falling rapidly to unity when LET reaches approximately 200 keV/µm.

Relationship Between OER & RBE

OER and RBE are virtually mirror images of each other as a function of LET.
Optimal RBE and the rapid fall of OER occur around the same LET value (100 keV/µm).

Tumor Hypoxia and Necrosis

Necrosis occurs in tumor cords with radii exceeding 200 μm often lacking a central necrotic area.
Tumor cells at greater distances from a capillary show oxygen depletion and necrosis.
Tumor cells close to a capillary are well oxygenated.
The distance oxygen can diffuse is limited (approximately 70 μm at the arterial end of a capillary and less at the venous end).

Acute Hypoxia

Acute hypoxia arises from temporary closure of tumor blood vessels due to abnormal vasculature, missing smooth muscle and incomplete endothelial lining.
Tumor blood vessels open and close randomly, leading to intermittent hypoxic regions within the tumor.

Chronic Hypoxia

There is evidence that hypoxia plays a role in malignant progression, radioresistance, and metastasis.

Reoxygenation

Hypoxic tissues are more resistant to radiation effects than aerated tissues.
In radiotherapy, irradiation leads to better oxygenation of hypoxic tumor cells as a result of damaged surrounding cells and increased oxygen availability.
This reoxygenation process makes resistant tumor cells more sensitive to radiation.

Dose and Time Effect on Oxygen

Immediately after irradiation for a large dose, most tumor cells are hypoxic.
In 6 hours post irradiation, the percentage of hypoxic cells returns to the pre-radiation level.

Reoxygenation and Fractionation

If radiation is delivered in fractions with sufficient time between exposures for reoxygenation to occur, the presence of hypoxic cells doesn't significantly influence the tumor's response.

Conclusion on OER

OER is the ratio of hypoxic-to-aerated doses.
OER decreases as LET increases.
Oxygen must be present during or soon after irradiation.
Only a small amount of oxygen (less than 5%) is needed.

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Description

Explore the relationship between oxygen levels and the effectiveness of radiation therapy. This quiz covers key concepts such as Oxygen Enhancement Ratio (OER), factors affecting radiation response, and how oxygen concentration influences radio-sensitivity in cells. Test your understanding of the biological effects of X-rays under different oxygen conditions.