ABR Radiation Oncology Certification
Examination – Practice Exam| WITH MUST
KNOW QUESTIONS AND ANSWER 2026|
2027
1. A patient receives 2 Gy of X-rays. The oxygen enhancement ratio (OER) for mammalian cells at this
dose is approximately:
A. 1.0
B. 1.5
C. 2.5
D. 3.0
Correct Answer: B
Rationale: OER for sparsely ionizing radiation (X-rays, gamma rays) at high dose per fraction (~2 Gy) is
about 1.5–2.0, typically 1.5 for most cell lines. It decreases at higher LET.
2. Which of the following radiation effects is not a deterministic effect?
A. Cataracts
B. Skin erythema
C. Leukemia
D. Sterility
Correct Answer: C
Rationale: Leukemia is a stochastic effect (probability increases with dose, no threshold).
Deterministic effects (cataracts, erythema, sterility) have a threshold dose.
3. In the linear-quadratic (LQ) model, the α/β ratio for early-responding tissues (e.g., skin, intestinal
mucosa) is typically:
A. 1–2 Gy
,B. 3–5 Gy
C. 8–12 Gy
D. >20 Gy
Correct Answer: C
Rationale: Early-responding tissues have high α/β (~10 Gy), meaning they are more sensitive to
fraction size changes. Late-responding tissues have low α/β (~2–3 Gy).
4. A 65-year-old man with T2N0M0 squamous cell carcinoma of the glottic larynx is treated with
definitive radiation. The most appropriate fractionation schedule is:
A. 35 fractions of 2 Gy (70 Gy)
B. 28 fractions of 2.5 Gy (70 Gy)
C. 20 fractions of 3 Gy (60 Gy)
D. 15 fractions of 4 Gy (60 Gy)
Correct Answer: A
Rationale: Standard definitive RT for early glottic cancer is 66–70 Gy in 2 Gy/fraction. Larger fractions
increase late toxicity (laryngeal necrosis). Hypofractionation is not standard for T2.
5. Which brachytherapy source has the highest average photon energy?
A. Iodine-125
B. Palladium-103
C. Cesium-131
D. Iridium-192
Correct Answer: D
Rationale: Average photon energies: I-125 (~28 keV), Pd-103 (~21 keV), Cs-131 (~30 keV), Ir-192 (~380
keV). Ir-192 is used for high-dose-rate (HDR) brachytherapy.
6. The four R’s of radiobiology include all of the following except:
,A. Repair
B. Repopulation
C. Reoxygenation
D. Recombination
Correct Answer: D
Rationale: The four R’s: Repair, Repopulation, Reoxygenation, Redistribution (cell cycle).
Recombination is a DNA repair mechanism but not one of the four R’s.
7. A patient with locally advanced prostate cancer receives 78 Gy in 39 fractions. The planning target
volume (PTV) margin around the clinical target volume (CTV) should account for:
A. Microscopic extension only
B. Set-up error and organ motion
C. Only set-up error
D. Only organ motion
Correct Answer: B
Rationale: PTV = CTV + margins for set-up uncertainties and internal motion (prostate motion from
rectum/bladder filling).
8. Which of the following is the most common late effect after mediastinal radiation for Hodgkin
lymphoma?
A. Radiation pneumonitis
B. Pericardial effusion
C. Breast cancer
D. Esophageal stricture
Correct Answer: C
Rationale: Breast cancer risk is significantly increased in young women after mantle irradiation.
Pericardial disease is also late but breast cancer is the most common solid cancer late effect.
, 9. The dose limiting toxicity for conventionally fractionated radiation to the spinal cord is
approximately:
A. 30 Gy
B. 45 Gy
C. 60 Gy
D. 70 Gy
Correct Answer: B
Rationale: Spinal cord tolerance (TD 5/5) is ~45–50 Gy in 1.8–2 Gy fractions. Above this, risk of
myelopathy rises sharply.
10. A physicist measures the percentage depth dose (PDD) for a 6 MV photon beam at 10 cm depth,
100 cm SSD, 10×10 cm² field. Which factor would increase PDD at 10 cm?
A. Increase field size to 20×20 cm²
B. Decrease SSD to 90 cm
C. Increase energy to 18 MV
D. Decrease depth to 5 cm
Correct Answer: C
Rationale: Higher energy increases penetration and PDD at deeper depths. Increased field size slightly
increases PDD due to scatter but less than energy change.
11. A 55-year-old woman with early-stage breast cancer undergoes breast-conserving surgery. Whole
breast radiation is planned. The boost volume (lumpectomy cavity) is treated to:
A. 40 Gy in 15 fractions
B. 50 Gy in 25 fractions
C. 60 Gy in 30 fractions (includes 10 Gy boost)
D. 66 Gy in 33 fractions
Examination – Practice Exam| WITH MUST
KNOW QUESTIONS AND ANSWER 2026|
2027
1. A patient receives 2 Gy of X-rays. The oxygen enhancement ratio (OER) for mammalian cells at this
dose is approximately:
A. 1.0
B. 1.5
C. 2.5
D. 3.0
Correct Answer: B
Rationale: OER for sparsely ionizing radiation (X-rays, gamma rays) at high dose per fraction (~2 Gy) is
about 1.5–2.0, typically 1.5 for most cell lines. It decreases at higher LET.
2. Which of the following radiation effects is not a deterministic effect?
A. Cataracts
B. Skin erythema
C. Leukemia
D. Sterility
Correct Answer: C
Rationale: Leukemia is a stochastic effect (probability increases with dose, no threshold).
Deterministic effects (cataracts, erythema, sterility) have a threshold dose.
3. In the linear-quadratic (LQ) model, the α/β ratio for early-responding tissues (e.g., skin, intestinal
mucosa) is typically:
A. 1–2 Gy
,B. 3–5 Gy
C. 8–12 Gy
D. >20 Gy
Correct Answer: C
Rationale: Early-responding tissues have high α/β (~10 Gy), meaning they are more sensitive to
fraction size changes. Late-responding tissues have low α/β (~2–3 Gy).
4. A 65-year-old man with T2N0M0 squamous cell carcinoma of the glottic larynx is treated with
definitive radiation. The most appropriate fractionation schedule is:
A. 35 fractions of 2 Gy (70 Gy)
B. 28 fractions of 2.5 Gy (70 Gy)
C. 20 fractions of 3 Gy (60 Gy)
D. 15 fractions of 4 Gy (60 Gy)
Correct Answer: A
Rationale: Standard definitive RT for early glottic cancer is 66–70 Gy in 2 Gy/fraction. Larger fractions
increase late toxicity (laryngeal necrosis). Hypofractionation is not standard for T2.
5. Which brachytherapy source has the highest average photon energy?
A. Iodine-125
B. Palladium-103
C. Cesium-131
D. Iridium-192
Correct Answer: D
Rationale: Average photon energies: I-125 (~28 keV), Pd-103 (~21 keV), Cs-131 (~30 keV), Ir-192 (~380
keV). Ir-192 is used for high-dose-rate (HDR) brachytherapy.
6. The four R’s of radiobiology include all of the following except:
,A. Repair
B. Repopulation
C. Reoxygenation
D. Recombination
Correct Answer: D
Rationale: The four R’s: Repair, Repopulation, Reoxygenation, Redistribution (cell cycle).
Recombination is a DNA repair mechanism but not one of the four R’s.
7. A patient with locally advanced prostate cancer receives 78 Gy in 39 fractions. The planning target
volume (PTV) margin around the clinical target volume (CTV) should account for:
A. Microscopic extension only
B. Set-up error and organ motion
C. Only set-up error
D. Only organ motion
Correct Answer: B
Rationale: PTV = CTV + margins for set-up uncertainties and internal motion (prostate motion from
rectum/bladder filling).
8. Which of the following is the most common late effect after mediastinal radiation for Hodgkin
lymphoma?
A. Radiation pneumonitis
B. Pericardial effusion
C. Breast cancer
D. Esophageal stricture
Correct Answer: C
Rationale: Breast cancer risk is significantly increased in young women after mantle irradiation.
Pericardial disease is also late but breast cancer is the most common solid cancer late effect.
, 9. The dose limiting toxicity for conventionally fractionated radiation to the spinal cord is
approximately:
A. 30 Gy
B. 45 Gy
C. 60 Gy
D. 70 Gy
Correct Answer: B
Rationale: Spinal cord tolerance (TD 5/5) is ~45–50 Gy in 1.8–2 Gy fractions. Above this, risk of
myelopathy rises sharply.
10. A physicist measures the percentage depth dose (PDD) for a 6 MV photon beam at 10 cm depth,
100 cm SSD, 10×10 cm² field. Which factor would increase PDD at 10 cm?
A. Increase field size to 20×20 cm²
B. Decrease SSD to 90 cm
C. Increase energy to 18 MV
D. Decrease depth to 5 cm
Correct Answer: C
Rationale: Higher energy increases penetration and PDD at deeper depths. Increased field size slightly
increases PDD due to scatter but less than energy change.
11. A 55-year-old woman with early-stage breast cancer undergoes breast-conserving surgery. Whole
breast radiation is planned. The boost volume (lumpectomy cavity) is treated to:
A. 40 Gy in 15 fractions
B. 50 Gy in 25 fractions
C. 60 Gy in 30 fractions (includes 10 Gy boost)
D. 66 Gy in 33 fractions
