ABR RADIATION ONCOLOGY CERTIFICATION EXAMINATION PRACTICE
QUESTIONS AND CORRECT ANSWERS (VERIFIED ANSWERS) PLUS RATIONALES
2026 Q&A
Q1. A 65-year-old male with T2N1M0 squamous cell carcinoma of the tonsil is undergoing definitive
chemoradiation. Which radiation modality provides the most significant sparing of the contralateral
parotid gland while maintaining target coverage?
A. 3D-Conformal Radiation Therapy (3D-CRT)
B. Intensity-Modulated Radiation Therapy (IMRT)
C. Lateral-Opposed Photons
D. Single-Field Electrons
Correct Answer: B
Rationale: IMRT allows for highly conformal dose distributions, creating concave dose profiles that
wrap around critical structures like the parotid glands. Studies demonstrate that IMRT significantly
reduces the risk of long-term xerostomia compared to 3D-CRT or lateral fields in head and neck cancers .
Q2. In the treatment of early-stage (T1N0) glottic cancer, what is the standard daily fractionation
schedule typically utilized for a total dose of 63 Gy?
A. 1.8 Gy per fraction
B. 2.0 Gy per fraction
C. 2.25 Gy per fraction
D. 3.0 Gy per fraction
Correct Answer: C
Rationale: Standard curative fractionation for early-stage glottic cancer often utilizes
hypofractionation, such as 2.25 Gy per fraction to a total of 63 Gy, as clinical outcomes show high local
control rates with acceptable toxicity profiles in these small-volume targets .
Q3. According to the Linear-Quadratic model, the α/β ratio for most early-responding tissues (e.g., skin,
mucosa) is approximately:
,A. 2 Gy
B. 3 Gy
C. 10 Gy
D. 20 Gy
Correct Answer: C
Rationale: Early-responding tissues and most tumors generally have high α/β ratios (around 10 Gy),
meaning they are less sensitive to changes in fractionation size than late-responding tissues, which have
low α/β ratios (around 2-3 Gy) .
Q4. Which of the following is the primary mechanism of cell death following exposure to therapeutic
doses of ionizing radiation in most solid tumors?
A. Apoptosis
B. Mitotic catastrophe
C. Necrosis
D. Autophagy
Correct Answer: B
Rationale: While apoptosis occurs in some cell types (like lymphocytes), mitotic catastrophe—death
occurring during or as a result of aberrant mitosis—is the dominant mode of cell death for the majority
of epithelial-derived solid tumors treated with radiation .
Q5. A patient with Stage IIIA Non-Small Cell Lung Cancer (NSCLC) is receiving concurrent
chemoradiation. What is the standard dose-volume constraint for the lungs to minimize the risk of
Grade 2+ radiation pneumonitis?
A. V20 < 35%
B. V20 < 20%
C. V5 < 80%
,D. Mean Lung Dose < 30 Gy
Correct Answer: A
Rationale: Clinical guidelines, including those from QUANTEC, suggest keeping the V20 (volume of lung
receiving 20 Gy or more) below 30-35% and the Mean Lung Dose below 20 Gy to keep the risk of
symptomatic pneumonitis under 20% .
Q6. Which radioactive isotope is most commonly used in high-dose-rate (HDR) brachytherapy units
today?
A. Cesium-137
B. Cobalt-60
C. Iridium-192
D. Iodine-125
Correct Answer: C
Rationale: Ir-192 is preferred for HDR due to its high specific activity, which allows for very small source
sizes, and its manageable half-life of approximately 74 days, requiring source replacement every few
months .
Q7. A patient with prostate cancer is undergoing Stereotactic Body Radiation Therapy (SBRT). What is
the primary radiobiological rationale for using high doses per fraction in prostate cancer?
A. High α/β ratio of prostate cancer
B. Low α/β ratio of prostate cancer
C. Reoxygenation
D. Accelerated repopulation
Correct Answer: B
, Rationale: Prostate cancer typically has a low α/β ratio (approximately 1.5-3 Gy), making it unusually
sensitive to hypofractionation. This allows higher doses per fraction to be delivered safely with
potentially improved tumor control .
Q8. Which of the following best describes the "Percent Depth Dose" (PDD) as the depth increases
beyond Dmax?
A. It increases linearly.
B. It remains constant.
C. It decreases exponentially.
D. It increases for the first 10 cm and then drops.
Correct Answer: C
Rationale: Beyond the build-up region (Dmax), the dose decreases with depth due to beam attenuation
(absorption and scattering) and the inverse square law .
Q9. In the treatment of soft tissue sarcoma of the extremity, what is the primary advantage of
preoperative radiation over postoperative radiation?
A. Lower risk of long-term fibrosis and joint stiffness
B. Lower risk of acute wound healing complications
C. Higher total dose can be delivered
D. Preoperative radiation is always more expensive
Correct Answer: A
Rationale: Preoperative RT uses a lower dose (50 Gy vs 60-66 Gy) and treats a smaller volume, leading
to better long-term functional outcomes, though it carries a higher risk of acute wound complications
compared to postoperative RT .
Q10. What is the tolerance dose for the optic chiasm in standard fractionation?
QUESTIONS AND CORRECT ANSWERS (VERIFIED ANSWERS) PLUS RATIONALES
2026 Q&A
Q1. A 65-year-old male with T2N1M0 squamous cell carcinoma of the tonsil is undergoing definitive
chemoradiation. Which radiation modality provides the most significant sparing of the contralateral
parotid gland while maintaining target coverage?
A. 3D-Conformal Radiation Therapy (3D-CRT)
B. Intensity-Modulated Radiation Therapy (IMRT)
C. Lateral-Opposed Photons
D. Single-Field Electrons
Correct Answer: B
Rationale: IMRT allows for highly conformal dose distributions, creating concave dose profiles that
wrap around critical structures like the parotid glands. Studies demonstrate that IMRT significantly
reduces the risk of long-term xerostomia compared to 3D-CRT or lateral fields in head and neck cancers .
Q2. In the treatment of early-stage (T1N0) glottic cancer, what is the standard daily fractionation
schedule typically utilized for a total dose of 63 Gy?
A. 1.8 Gy per fraction
B. 2.0 Gy per fraction
C. 2.25 Gy per fraction
D. 3.0 Gy per fraction
Correct Answer: C
Rationale: Standard curative fractionation for early-stage glottic cancer often utilizes
hypofractionation, such as 2.25 Gy per fraction to a total of 63 Gy, as clinical outcomes show high local
control rates with acceptable toxicity profiles in these small-volume targets .
Q3. According to the Linear-Quadratic model, the α/β ratio for most early-responding tissues (e.g., skin,
mucosa) is approximately:
,A. 2 Gy
B. 3 Gy
C. 10 Gy
D. 20 Gy
Correct Answer: C
Rationale: Early-responding tissues and most tumors generally have high α/β ratios (around 10 Gy),
meaning they are less sensitive to changes in fractionation size than late-responding tissues, which have
low α/β ratios (around 2-3 Gy) .
Q4. Which of the following is the primary mechanism of cell death following exposure to therapeutic
doses of ionizing radiation in most solid tumors?
A. Apoptosis
B. Mitotic catastrophe
C. Necrosis
D. Autophagy
Correct Answer: B
Rationale: While apoptosis occurs in some cell types (like lymphocytes), mitotic catastrophe—death
occurring during or as a result of aberrant mitosis—is the dominant mode of cell death for the majority
of epithelial-derived solid tumors treated with radiation .
Q5. A patient with Stage IIIA Non-Small Cell Lung Cancer (NSCLC) is receiving concurrent
chemoradiation. What is the standard dose-volume constraint for the lungs to minimize the risk of
Grade 2+ radiation pneumonitis?
A. V20 < 35%
B. V20 < 20%
C. V5 < 80%
,D. Mean Lung Dose < 30 Gy
Correct Answer: A
Rationale: Clinical guidelines, including those from QUANTEC, suggest keeping the V20 (volume of lung
receiving 20 Gy or more) below 30-35% and the Mean Lung Dose below 20 Gy to keep the risk of
symptomatic pneumonitis under 20% .
Q6. Which radioactive isotope is most commonly used in high-dose-rate (HDR) brachytherapy units
today?
A. Cesium-137
B. Cobalt-60
C. Iridium-192
D. Iodine-125
Correct Answer: C
Rationale: Ir-192 is preferred for HDR due to its high specific activity, which allows for very small source
sizes, and its manageable half-life of approximately 74 days, requiring source replacement every few
months .
Q7. A patient with prostate cancer is undergoing Stereotactic Body Radiation Therapy (SBRT). What is
the primary radiobiological rationale for using high doses per fraction in prostate cancer?
A. High α/β ratio of prostate cancer
B. Low α/β ratio of prostate cancer
C. Reoxygenation
D. Accelerated repopulation
Correct Answer: B
, Rationale: Prostate cancer typically has a low α/β ratio (approximately 1.5-3 Gy), making it unusually
sensitive to hypofractionation. This allows higher doses per fraction to be delivered safely with
potentially improved tumor control .
Q8. Which of the following best describes the "Percent Depth Dose" (PDD) as the depth increases
beyond Dmax?
A. It increases linearly.
B. It remains constant.
C. It decreases exponentially.
D. It increases for the first 10 cm and then drops.
Correct Answer: C
Rationale: Beyond the build-up region (Dmax), the dose decreases with depth due to beam attenuation
(absorption and scattering) and the inverse square law .
Q9. In the treatment of soft tissue sarcoma of the extremity, what is the primary advantage of
preoperative radiation over postoperative radiation?
A. Lower risk of long-term fibrosis and joint stiffness
B. Lower risk of acute wound healing complications
C. Higher total dose can be delivered
D. Preoperative radiation is always more expensive
Correct Answer: A
Rationale: Preoperative RT uses a lower dose (50 Gy vs 60-66 Gy) and treats a smaller volume, leading
to better long-term functional outcomes, though it carries a higher risk of acute wound complications
compared to postoperative RT .
Q10. What is the tolerance dose for the optic chiasm in standard fractionation?
