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RADIATION SAFETY AND RADIOBIOLOGY IN MEDICAL
IMAGING EXAMINATION EXAM WITH VERIFIED ANSWERS
DETAILED RATIONALES GRADED A+
Radiation Safety and Radiobiology in Medical Imaging — Test Bank
SECTION 1: RADIATION PHYSICS & INTERACTIONS (Questions 1–35)
1. Which of the following has the highest linear energy transfer (LET)?
A. Alpha particle
B. Gamma ray
C. X-ray
D. Beta particle
Correct Answer: A
Rationale: Alpha particles have high LET due to their relatively large mass and higher charge,
depositing large amounts of energy over a short path length. Gamma rays, X-rays, and beta
particles are low-LET radiations.
2. In which phase of the reproductive cycle are cells most sensitive to the damaging effects of
radiation?
A. G1 phase
B. S phase
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C. G2 phase
D. M phase
Correct Answer: D
Rationale: Cells are generally most sensitive to radiation damage when they are in mitosis (M
phase). Mitosis is the most sensitive phase because of lack of checkpoint prior to DNA
duplication and fewer resistive mechanisms available as compared to the other phases of
reproduction.
3. Most radiation-induced injury is due to damage to which type of molecules?
A. Deoxyribonucleic acid
B. Ribonucleic acid
C. DNA polymerase
D. Hemoglobin
Correct Answer: A
Rationale: There is strong evidence that the biologic effects of radiation damage—including cell
killing, carcinogenesis, and mutation—result from double-strand breaks (DSB) in the double
helical structure of DNA.
4. Which of the following is a stochastic effect of radiation?
A. Hair loss
B. Skin erythema
C. Cataract
D. Carcinogenesis
Correct Answer: D
Rationale: Risk is calculated as a stochastic or statistical probability, so increased risk of cancer is
a non-deterministic (stochastic) effect. All other effects are tissue (deterministic) effects that
have a threshold dose.
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5. What is approximately the LD₅₀/₆₀ for humans?
A. 3–5 mGy
B. 30–50 mGy
C. 300–500 mGy
D. 3000–5000 mGy
Correct Answer: D
Rationale: The LD₅₀/₆₀ for humans is approximately 3500–4000 mGy (3.5–4 Gy). Lethal dose
50/60 is the dose of radiation to the whole body that causes 50% of irradiated subjects to die
within 60 days without medical intervention.
6. What is a potential risk to the fetus from a pelvic CT exam acquired during the 30th week of
gestation?
A. Fetal malformation
B. Prenatal death
C. Leukemia
D. Cataracts
Correct Answer: C
Rationale: The risk to the fetus from low levels of radiation (below 50 mGy) is negligible. At 30
weeks of pregnancy, the woman is well into the third trimester, and during this stage of
gestation, the only potential risk is from a stochastic effect, which is childhood cancer (e.g.,
leukemia). The dose from an abdomen/pelvic CT scan is well below the thresholds necessary for
deterministic effects.
7. In radiation therapy with 6MV photons, which interaction process is dominant?
A. Photoelectric effect
B. Compton scattering
C. Pair production
D. Coherent scattering
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Correct Answer: B
Rationale: In radiation therapy with 6MV photons, Compton scattering is the dominant
interaction process. The photoelectric effect is dominant at lower energies (diagnostic range),
and pair production occurs at energies above 1.02 MeV but is not dominant at 6MV.
8. Which of the following is NOT a relevant damage to DNA that occurs during radiation
therapy?
A. Double-strand breaks
B. Single-strand breaks
C. Heat damage
D. DNA adducts
Correct Answer: C
Rationale: During radiation therapy, the relevant damages to DNA include double-strand breaks,
single-strand breaks, and DNA adducts. Heat damage is not a relevant damage that occurs
during radiation therapy; it may occur in hyperthermia therapy but is not a consequence of
radiation therapy.
9. In which phase of the cell cycle are cells most radioresistant?
A. End of S phase
B. End of G1 phase
C. Beginning of S phase
D. M phase
Correct Answer: A
Rationale: Cells are most radioresistant in the late S phase (end of S phase). This is because DNA
repair mechanisms are most active and the cell has duplicated its DNA, providing a template for
repair.
10. At which dose does hyperradiosensitivity occur?
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RADIATION SAFETY AND RADIOBIOLOGY IN MEDICAL
IMAGING EXAMINATION EXAM WITH VERIFIED ANSWERS
DETAILED RATIONALES GRADED A+
Radiation Safety and Radiobiology in Medical Imaging — Test Bank
SECTION 1: RADIATION PHYSICS & INTERACTIONS (Questions 1–35)
1. Which of the following has the highest linear energy transfer (LET)?
A. Alpha particle
B. Gamma ray
C. X-ray
D. Beta particle
Correct Answer: A
Rationale: Alpha particles have high LET due to their relatively large mass and higher charge,
depositing large amounts of energy over a short path length. Gamma rays, X-rays, and beta
particles are low-LET radiations.
2. In which phase of the reproductive cycle are cells most sensitive to the damaging effects of
radiation?
A. G1 phase
B. S phase
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C. G2 phase
D. M phase
Correct Answer: D
Rationale: Cells are generally most sensitive to radiation damage when they are in mitosis (M
phase). Mitosis is the most sensitive phase because of lack of checkpoint prior to DNA
duplication and fewer resistive mechanisms available as compared to the other phases of
reproduction.
3. Most radiation-induced injury is due to damage to which type of molecules?
A. Deoxyribonucleic acid
B. Ribonucleic acid
C. DNA polymerase
D. Hemoglobin
Correct Answer: A
Rationale: There is strong evidence that the biologic effects of radiation damage—including cell
killing, carcinogenesis, and mutation—result from double-strand breaks (DSB) in the double
helical structure of DNA.
4. Which of the following is a stochastic effect of radiation?
A. Hair loss
B. Skin erythema
C. Cataract
D. Carcinogenesis
Correct Answer: D
Rationale: Risk is calculated as a stochastic or statistical probability, so increased risk of cancer is
a non-deterministic (stochastic) effect. All other effects are tissue (deterministic) effects that
have a threshold dose.
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,Page 3 of 92
5. What is approximately the LD₅₀/₆₀ for humans?
A. 3–5 mGy
B. 30–50 mGy
C. 300–500 mGy
D. 3000–5000 mGy
Correct Answer: D
Rationale: The LD₅₀/₆₀ for humans is approximately 3500–4000 mGy (3.5–4 Gy). Lethal dose
50/60 is the dose of radiation to the whole body that causes 50% of irradiated subjects to die
within 60 days without medical intervention.
6. What is a potential risk to the fetus from a pelvic CT exam acquired during the 30th week of
gestation?
A. Fetal malformation
B. Prenatal death
C. Leukemia
D. Cataracts
Correct Answer: C
Rationale: The risk to the fetus from low levels of radiation (below 50 mGy) is negligible. At 30
weeks of pregnancy, the woman is well into the third trimester, and during this stage of
gestation, the only potential risk is from a stochastic effect, which is childhood cancer (e.g.,
leukemia). The dose from an abdomen/pelvic CT scan is well below the thresholds necessary for
deterministic effects.
7. In radiation therapy with 6MV photons, which interaction process is dominant?
A. Photoelectric effect
B. Compton scattering
C. Pair production
D. Coherent scattering
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Correct Answer: B
Rationale: In radiation therapy with 6MV photons, Compton scattering is the dominant
interaction process. The photoelectric effect is dominant at lower energies (diagnostic range),
and pair production occurs at energies above 1.02 MeV but is not dominant at 6MV.
8. Which of the following is NOT a relevant damage to DNA that occurs during radiation
therapy?
A. Double-strand breaks
B. Single-strand breaks
C. Heat damage
D. DNA adducts
Correct Answer: C
Rationale: During radiation therapy, the relevant damages to DNA include double-strand breaks,
single-strand breaks, and DNA adducts. Heat damage is not a relevant damage that occurs
during radiation therapy; it may occur in hyperthermia therapy but is not a consequence of
radiation therapy.
9. In which phase of the cell cycle are cells most radioresistant?
A. End of S phase
B. End of G1 phase
C. Beginning of S phase
D. M phase
Correct Answer: A
Rationale: Cells are most radioresistant in the late S phase (end of S phase). This is because DNA
repair mechanisms are most active and the cell has duplicated its DNA, providing a template for
repair.
10. At which dose does hyperradiosensitivity occur?
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