ABR RADIATION ONCOLOGY CERTIFICATION
EXAMINATION 300 Practice Questions with Verified
Answers & Expert Rationales Latest 2026 Q&A –
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SECTION 1: RADIATION BIOLOGY (Questions 1–70)
1. The target theory of cell survival assumes that cell death occurs when a cell
sustains damage to which critical structure?
A. Mitochondria
B. Cytoplasm
C. Cell membrane
D. DNA
Rationale: The target theory posits that DNA is the critical radiosensitive target.
Irreparable DNA damage leads to cell death. The nucleus is the primary target.
2. Which of the following describes the linear-quadratic (LQ) model?
A. Survival = e^(-αD)
B. Survival = e^(-αD - βD²)
C. Survival = αD + βD²
D. Survival = e^(-βD²)
Rationale: The LQ model equation: S = e^(-αD - βD²). The linear (α) component
represents non-repairable damage; the quadratic (β) component represents repairable
damage that requires two sublesions to interact.
3. The α/β ratio for early-responding tissues (e.g., skin, mucosa) is typically:
A. 10 Gy
B. 3 Gy
C. 2 Gy
D. 0.5 Gy
Rationale: Early-responding tissues have high α/β (≈10 Gy), indicating high sensitivity
to fractionation changes. Late-responding tissues (spinal cord, kidney) have low α/β
(≈2-3 Gy).
,4. Which of the following is the most radiosensitive organ?
A. Muscle
B. Bone
C. Bone marrow
D. Nerve tissue
Rationale: Rapidly dividing cells (bone marrow stem cells, germinal epithelium,
intestinal crypt cells) are most radiosensitive. Muscle, bone, and nerve are radioresistant.
5. The oxygen enhancement ratio (OER) for low-LET radiation (e.g., photons) is
approximately:
A. 1.0
B. 2.5-3.0
C. 1.5
D. 5.0
Rationale: For low-LET radiation, OER is about 2.5-3.0, indicating hypoxic cells are 2.5-3
times more resistant. For high-LET (neutrons, alpha), OER approaches 1.
6. The “five Rs” of radiobiology include all of the following EXCEPT:
A. Repair
B. Repopulation
C. Reoxygenation
D. Fracture
Rationale: The classic 5 Rs: Repair, Repopulation, Reoxygenation, Redistribution,
Radiosensitivity. Fracture is not part of radiobiology.
7. The phenomenon in which cells become more sensitive to radiation as they
progress through the cell cycle is most pronounced in which phase?
A. S phase
B. G2-M phase
C. G0 phase
D. Early G1
Rationale: Cells are most radiosensitive in M and G2 phases, and most resistant in S
phase (due to high glutathione and DNA repair capacity).
8. Which of the following best describes the “bystander effect”?
A. Radiation-induced cell death due to direct DNA damage
B. Biological effects in non-irradiated cells caused by signals from irradiated cells
C. The effect of fractionation on normal tissues
D. The increased sensitivity of hypoxic cells
Rationale: The bystander effect refers to cellular changes (e.g., damage, apoptosis,
,mutations) in cells that receive no direct radiation exposure but are adjacent or
communicate with irradiated cells.
9. The term “apoptosis” in radiobiology refers to:
A. Necrotic cell death
B. Programmed cell death (type I)
C. Mitotic catastrophe
D. Senescence
Rationale: Apoptosis is a genetically controlled form of cell death triggered by radiation
in certain sensitive cell types (e.g., lymphocytes, intestinal crypt cells). It involves
caspases and characteristic morphological changes.
10. The “therapeutic ratio” is defined as:
A. Tumor control probability / normal tissue complication probability
B. The ratio of tumor effect to normal tissue damage; higher ratio indicates wider
therapeutic window
C. Dose prescribed / dose delivered
D. α/β ratio of tumor / α/β ratio of normal tissue
Rationale: Therapeutic ratio = benefit (tumor control) / harm (normal tissue
complication). A higher ratio means better separation between tumor and normal tissue
response.
11. D10 is the dose required to reduce survival by 10%. Which organism has the
highest D10?
A. Bacterial spores
B. Gram-negative bacteria
C. Gram-positive bacteria
D. Viruses
Rationale: Bacterial spores (e.g., Bacillus stearothermophilus) are extremely resistant,
with D10 values many times higher than vegetative bacteria. This is why spore logs are
used to validate sterilization.
12. A cell survival curve with a broad shoulder (large Dq) indicates:
A. High radiosensitivity
B. High repair capacity and sublethal damage recovery
C. Hypoxic cells
D. Lack of repair
Rationale: The shoulder (Dq) represents the dose over which sublethal damage is
accumulated and repaired. A broad shoulder indicates robust repair capacity.
, 13. The “relative biological effectiveness” (RBE) is defined as:
A. The ratio of a reference radiation dose to the dose of test radiation producing
the same biological effect
B. The ratio of tumor dose to normal tissue dose
C. The oxygen enhancement ratio
D. The α/β ratio
Rationale: RBE = (dose of 250 kVp X-rays to produce effect) / (dose of test radiation to
produce same effect). Higher RBE means more effective per unit dose.
14. The RBE of high-LET neutrons relative to 250 kVp X-rays for cell killing is
approximately:
A. 1
B. 2-5 (depending on endpoint)
C. 10
D. 20
Rationale: For cell killing endpoints, neutron RBE ranges from 2-5. For late effects, RBE
can be higher. The high RBE results from dense ionization tracks causing irreparable
DNA damage.
15. The oxygen effect is minimal for which type of radiation?
A. X-rays
B. Gamma rays
C. Neutrons
D. Electrons
Rationale: High-LET radiation (neutrons, alpha particles) produces dense ionization
independent of oxygen, so OER ~1. Low-LET radiation requires oxygen to “fix” free
radical damage.
16. The term “sublethal damage” (SLD) repair halftime is typically:
A. 1-2 hours
B. 10 minutes
C. 12 hours
D. 24 hours
Rationale: SLD repair is rapid, with half-times of 0.5-2 hours. This is why fractionation
intervals are usually ≥4-6 hours to allow nearly complete repair in late-responding
tissues.
17. Potentially lethal damage (PLD) repair is defined as:
A. Damage that is always lethal
B. Damage that can be repaired if post-irradiation conditions are optimal
C. Damage repaired only in S phase
EXAMINATION 300 Practice Questions with Verified
Answers & Expert Rationales Latest 2026 Q&A –
Instant Download
SECTION 1: RADIATION BIOLOGY (Questions 1–70)
1. The target theory of cell survival assumes that cell death occurs when a cell
sustains damage to which critical structure?
A. Mitochondria
B. Cytoplasm
C. Cell membrane
D. DNA
Rationale: The target theory posits that DNA is the critical radiosensitive target.
Irreparable DNA damage leads to cell death. The nucleus is the primary target.
2. Which of the following describes the linear-quadratic (LQ) model?
A. Survival = e^(-αD)
B. Survival = e^(-αD - βD²)
C. Survival = αD + βD²
D. Survival = e^(-βD²)
Rationale: The LQ model equation: S = e^(-αD - βD²). The linear (α) component
represents non-repairable damage; the quadratic (β) component represents repairable
damage that requires two sublesions to interact.
3. The α/β ratio for early-responding tissues (e.g., skin, mucosa) is typically:
A. 10 Gy
B. 3 Gy
C. 2 Gy
D. 0.5 Gy
Rationale: Early-responding tissues have high α/β (≈10 Gy), indicating high sensitivity
to fractionation changes. Late-responding tissues (spinal cord, kidney) have low α/β
(≈2-3 Gy).
,4. Which of the following is the most radiosensitive organ?
A. Muscle
B. Bone
C. Bone marrow
D. Nerve tissue
Rationale: Rapidly dividing cells (bone marrow stem cells, germinal epithelium,
intestinal crypt cells) are most radiosensitive. Muscle, bone, and nerve are radioresistant.
5. The oxygen enhancement ratio (OER) for low-LET radiation (e.g., photons) is
approximately:
A. 1.0
B. 2.5-3.0
C. 1.5
D. 5.0
Rationale: For low-LET radiation, OER is about 2.5-3.0, indicating hypoxic cells are 2.5-3
times more resistant. For high-LET (neutrons, alpha), OER approaches 1.
6. The “five Rs” of radiobiology include all of the following EXCEPT:
A. Repair
B. Repopulation
C. Reoxygenation
D. Fracture
Rationale: The classic 5 Rs: Repair, Repopulation, Reoxygenation, Redistribution,
Radiosensitivity. Fracture is not part of radiobiology.
7. The phenomenon in which cells become more sensitive to radiation as they
progress through the cell cycle is most pronounced in which phase?
A. S phase
B. G2-M phase
C. G0 phase
D. Early G1
Rationale: Cells are most radiosensitive in M and G2 phases, and most resistant in S
phase (due to high glutathione and DNA repair capacity).
8. Which of the following best describes the “bystander effect”?
A. Radiation-induced cell death due to direct DNA damage
B. Biological effects in non-irradiated cells caused by signals from irradiated cells
C. The effect of fractionation on normal tissues
D. The increased sensitivity of hypoxic cells
Rationale: The bystander effect refers to cellular changes (e.g., damage, apoptosis,
,mutations) in cells that receive no direct radiation exposure but are adjacent or
communicate with irradiated cells.
9. The term “apoptosis” in radiobiology refers to:
A. Necrotic cell death
B. Programmed cell death (type I)
C. Mitotic catastrophe
D. Senescence
Rationale: Apoptosis is a genetically controlled form of cell death triggered by radiation
in certain sensitive cell types (e.g., lymphocytes, intestinal crypt cells). It involves
caspases and characteristic morphological changes.
10. The “therapeutic ratio” is defined as:
A. Tumor control probability / normal tissue complication probability
B. The ratio of tumor effect to normal tissue damage; higher ratio indicates wider
therapeutic window
C. Dose prescribed / dose delivered
D. α/β ratio of tumor / α/β ratio of normal tissue
Rationale: Therapeutic ratio = benefit (tumor control) / harm (normal tissue
complication). A higher ratio means better separation between tumor and normal tissue
response.
11. D10 is the dose required to reduce survival by 10%. Which organism has the
highest D10?
A. Bacterial spores
B. Gram-negative bacteria
C. Gram-positive bacteria
D. Viruses
Rationale: Bacterial spores (e.g., Bacillus stearothermophilus) are extremely resistant,
with D10 values many times higher than vegetative bacteria. This is why spore logs are
used to validate sterilization.
12. A cell survival curve with a broad shoulder (large Dq) indicates:
A. High radiosensitivity
B. High repair capacity and sublethal damage recovery
C. Hypoxic cells
D. Lack of repair
Rationale: The shoulder (Dq) represents the dose over which sublethal damage is
accumulated and repaired. A broad shoulder indicates robust repair capacity.
, 13. The “relative biological effectiveness” (RBE) is defined as:
A. The ratio of a reference radiation dose to the dose of test radiation producing
the same biological effect
B. The ratio of tumor dose to normal tissue dose
C. The oxygen enhancement ratio
D. The α/β ratio
Rationale: RBE = (dose of 250 kVp X-rays to produce effect) / (dose of test radiation to
produce same effect). Higher RBE means more effective per unit dose.
14. The RBE of high-LET neutrons relative to 250 kVp X-rays for cell killing is
approximately:
A. 1
B. 2-5 (depending on endpoint)
C. 10
D. 20
Rationale: For cell killing endpoints, neutron RBE ranges from 2-5. For late effects, RBE
can be higher. The high RBE results from dense ionization tracks causing irreparable
DNA damage.
15. The oxygen effect is minimal for which type of radiation?
A. X-rays
B. Gamma rays
C. Neutrons
D. Electrons
Rationale: High-LET radiation (neutrons, alpha particles) produces dense ionization
independent of oxygen, so OER ~1. Low-LET radiation requires oxygen to “fix” free
radical damage.
16. The term “sublethal damage” (SLD) repair halftime is typically:
A. 1-2 hours
B. 10 minutes
C. 12 hours
D. 24 hours
Rationale: SLD repair is rapid, with half-times of 0.5-2 hours. This is why fractionation
intervals are usually ≥4-6 hours to allow nearly complete repair in late-responding
tissues.
17. Potentially lethal damage (PLD) repair is defined as:
A. Damage that is always lethal
B. Damage that can be repaired if post-irradiation conditions are optimal
C. Damage repaired only in S phase
