ABR Radiation Oncology Certification Exam
Practice Questions and Correct Answers
1: Which AAPM report established the current clinical protocol for reference dosimetry of high-
energy photon and electron beams in radiation therapy?*
A. TG-21
B. TG-51
C. TG-142
D. TG-100
CORRECT ANSWER: B. TG-51
Rationale: TG-51 replaced TG-21 as the standard for reference dosimetry. It requires calibration
in terms of absorbed dose to water under reference conditions, uses ionization chambers with
N_D,w calibration factors, and specifies beam quality specifiers (TPR20,10 or %dd(10)x). TG-142
covers linac QA tolerances, and TG-100 addresses risk-based QA.
2: A patient with intermediate-risk prostate cancer is planned for definitive IMRT to 78 Gy in 2
Gy fractions. According to QUANTEC guidelines, which rectal dose-volume constraint is most
strongly associated with reduced risk of grade ≥2 late rectal toxicity?*
A. V70 Gy < 20%
B. V75 Gy < 25%
C. V60 Gy < 50%
D. V50 Gy < 70%
CORRECT ANSWER: A. V70 Gy < 20%
Rationale: QUANTEC recommends V70 < 20% and V75 < 15% to minimize late rectal bleeding
and toxicity. RTOG 0126 and subsequent trials validated that limiting high-dose rectal volume
significantly reduces GI complications. V60/V50 constraints are less predictive of severe late
toxicity.
,3: In the Linear-Quadratic (LQ) model, what is the biological effective dose (BED) for 70 Gy
delivered in 2 Gy fractions for a tissue with an α/β ratio of 3 Gy?*
A. 105 Gy3
B. 116.7 Gy3
C. 140 Gy3
D. 84 Gy3
CORRECT ANSWER: B. 116.7 Gy3
Rationale: BED = n × d × [1 + d/(α/β)]. Here, n=35, d=2 Gy, α/β=3. BED = 35 × 2 × [1 + 2/3] = 70 ×
(5/3) = 116.7 Gy3. This calculation is essential for comparing fractionation schemes and
predicting late tissue effects.
4: Which radiobiological phenomenon best explains why fractionated radiotherapy is more
effective at tumor control than a single large dose, particularly for hypoxic tumors?*
A. Reassortment
B. Repair
C. Reoxygenation
D. Repopulation
CORRECT ANSWER: C. Reoxygenation
Rationale: Reoxygenation occurs between fractions as tumor vasculature reperfuses and
previously hypoxic, radioresistant cells become oxygenated and radiosensitive. Repair,
reassortment, and repopulation also contribute to fractionation effects, but reoxygenation
specifically addresses the hypoxia challenge.
5: According to AAPM TG-142, what is the recommended monthly tolerance for linear
accelerator output constancy?*
A. ±3%
B. ±2%
C. ±1%
,D. ±5%
CORRECT ANSWER: B. ±2%
Rationale: TG-142 specifies output constancy tolerances of ±3% daily, ±2% monthly, and ±2%
annually. These tolerances ensure consistent dose delivery across treatment courses. Daily
checks catch acute drifts; monthly/annual verify long-term stability.
6: For a patient with oropharyngeal squamous cell carcinoma treated with definitive IMRT,
which mean dose constraint to the parotid glands is recommended to preserve salivary
function?*
A. Mean dose < 35 Gy to at least one parotid
B. Mean dose < 26 Gy to at least one parotid
C. Mean dose < 20 Gy to both parotids
D. V30 Gy < 10% to the contralateral parotid
CORRECT ANSWER: B. Mean dose < 26 Gy to at least one parotid
Rationale: QUANTEC and ASTRO guidelines recommend maintaining a mean dose < 26 Gy to at
least one parotid gland to significantly reduce the risk of clinically significant xerostomia.
Sparing both is ideal but often anatomically constrained.
7: In brachytherapy dosimetry, which TG-43 parameter accounts for the non-isotropic dose
distribution caused by source self-absorption and encapsulation?*
A. Air-kerma strength (Sk)
B. Dose rate constant (Λ)
C. Radial dose function gL(r)
D. Anisotropy function F(r,θ)
CORRECT ANSWER: D. Anisotropy function F(r,θ)
Rationale: F(r,θ) corrects for dose variation with angle due to source geometry, encapsulation,
and self-absorption. Sk defines source strength, Λ converts Sk to dose rate in water, and gL(r)
accounts for attenuation/scatter in water along the transverse plane.
, 8: A patient with early-stage peripheral NSCLC is planned for SBRT (50 Gy in 4 fractions). Which
normal tissue constraint is most critical to prevent chest wall toxicity?*
A. Ipsilateral lung V20 < 10%
B. Chest wall V30 > 30 cc should be avoided
C. Spinal cord max dose < 18 Gy
D. Esophagus max dose < 30 Gy
CORRECT ANSWER: B. Chest wall V30 > 30 cc should be avoided
Rationale: RTOG 0813 and subsequent SBRT studies show chest wall V30 > 30 cc significantly
increases risk of rib fracture and pain. Spinal cord, esophagus, and lung constraints are also
critical, but chest wall volume constraint is uniquely predictive for peripheral lesions.
9: Which photon beam energy threshold requires additional vault shielding considerations due
to photoneutron production?*
A. 6 MV
B. 8 MV
C. 10 MV
D. 15 MV
CORRECT ANSWER: C. 10 MV
Rationale: Photoneutron production becomes clinically significant at beam energies >10 MV.
NCRP Report 151 requires neutron shielding calculations for maze design, door thickness, and
secondary barriers when energies exceed this threshold.
10: In radiation shielding design, what does the "use factor" (U) represent?*
A. The fraction of time the primary beam is directed toward a specific barrier
B. The occupancy of adjacent areas by staff or public
C. The total workload in Gy/week at 1 meter
D. The transmission factor of the barrier material
Practice Questions and Correct Answers
1: Which AAPM report established the current clinical protocol for reference dosimetry of high-
energy photon and electron beams in radiation therapy?*
A. TG-21
B. TG-51
C. TG-142
D. TG-100
CORRECT ANSWER: B. TG-51
Rationale: TG-51 replaced TG-21 as the standard for reference dosimetry. It requires calibration
in terms of absorbed dose to water under reference conditions, uses ionization chambers with
N_D,w calibration factors, and specifies beam quality specifiers (TPR20,10 or %dd(10)x). TG-142
covers linac QA tolerances, and TG-100 addresses risk-based QA.
2: A patient with intermediate-risk prostate cancer is planned for definitive IMRT to 78 Gy in 2
Gy fractions. According to QUANTEC guidelines, which rectal dose-volume constraint is most
strongly associated with reduced risk of grade ≥2 late rectal toxicity?*
A. V70 Gy < 20%
B. V75 Gy < 25%
C. V60 Gy < 50%
D. V50 Gy < 70%
CORRECT ANSWER: A. V70 Gy < 20%
Rationale: QUANTEC recommends V70 < 20% and V75 < 15% to minimize late rectal bleeding
and toxicity. RTOG 0126 and subsequent trials validated that limiting high-dose rectal volume
significantly reduces GI complications. V60/V50 constraints are less predictive of severe late
toxicity.
,3: In the Linear-Quadratic (LQ) model, what is the biological effective dose (BED) for 70 Gy
delivered in 2 Gy fractions for a tissue with an α/β ratio of 3 Gy?*
A. 105 Gy3
B. 116.7 Gy3
C. 140 Gy3
D. 84 Gy3
CORRECT ANSWER: B. 116.7 Gy3
Rationale: BED = n × d × [1 + d/(α/β)]. Here, n=35, d=2 Gy, α/β=3. BED = 35 × 2 × [1 + 2/3] = 70 ×
(5/3) = 116.7 Gy3. This calculation is essential for comparing fractionation schemes and
predicting late tissue effects.
4: Which radiobiological phenomenon best explains why fractionated radiotherapy is more
effective at tumor control than a single large dose, particularly for hypoxic tumors?*
A. Reassortment
B. Repair
C. Reoxygenation
D. Repopulation
CORRECT ANSWER: C. Reoxygenation
Rationale: Reoxygenation occurs between fractions as tumor vasculature reperfuses and
previously hypoxic, radioresistant cells become oxygenated and radiosensitive. Repair,
reassortment, and repopulation also contribute to fractionation effects, but reoxygenation
specifically addresses the hypoxia challenge.
5: According to AAPM TG-142, what is the recommended monthly tolerance for linear
accelerator output constancy?*
A. ±3%
B. ±2%
C. ±1%
,D. ±5%
CORRECT ANSWER: B. ±2%
Rationale: TG-142 specifies output constancy tolerances of ±3% daily, ±2% monthly, and ±2%
annually. These tolerances ensure consistent dose delivery across treatment courses. Daily
checks catch acute drifts; monthly/annual verify long-term stability.
6: For a patient with oropharyngeal squamous cell carcinoma treated with definitive IMRT,
which mean dose constraint to the parotid glands is recommended to preserve salivary
function?*
A. Mean dose < 35 Gy to at least one parotid
B. Mean dose < 26 Gy to at least one parotid
C. Mean dose < 20 Gy to both parotids
D. V30 Gy < 10% to the contralateral parotid
CORRECT ANSWER: B. Mean dose < 26 Gy to at least one parotid
Rationale: QUANTEC and ASTRO guidelines recommend maintaining a mean dose < 26 Gy to at
least one parotid gland to significantly reduce the risk of clinically significant xerostomia.
Sparing both is ideal but often anatomically constrained.
7: In brachytherapy dosimetry, which TG-43 parameter accounts for the non-isotropic dose
distribution caused by source self-absorption and encapsulation?*
A. Air-kerma strength (Sk)
B. Dose rate constant (Λ)
C. Radial dose function gL(r)
D. Anisotropy function F(r,θ)
CORRECT ANSWER: D. Anisotropy function F(r,θ)
Rationale: F(r,θ) corrects for dose variation with angle due to source geometry, encapsulation,
and self-absorption. Sk defines source strength, Λ converts Sk to dose rate in water, and gL(r)
accounts for attenuation/scatter in water along the transverse plane.
, 8: A patient with early-stage peripheral NSCLC is planned for SBRT (50 Gy in 4 fractions). Which
normal tissue constraint is most critical to prevent chest wall toxicity?*
A. Ipsilateral lung V20 < 10%
B. Chest wall V30 > 30 cc should be avoided
C. Spinal cord max dose < 18 Gy
D. Esophagus max dose < 30 Gy
CORRECT ANSWER: B. Chest wall V30 > 30 cc should be avoided
Rationale: RTOG 0813 and subsequent SBRT studies show chest wall V30 > 30 cc significantly
increases risk of rib fracture and pain. Spinal cord, esophagus, and lung constraints are also
critical, but chest wall volume constraint is uniquely predictive for peripheral lesions.
9: Which photon beam energy threshold requires additional vault shielding considerations due
to photoneutron production?*
A. 6 MV
B. 8 MV
C. 10 MV
D. 15 MV
CORRECT ANSWER: C. 10 MV
Rationale: Photoneutron production becomes clinically significant at beam energies >10 MV.
NCRP Report 151 requires neutron shielding calculations for maze design, door thickness, and
secondary barriers when energies exceed this threshold.
10: In radiation shielding design, what does the "use factor" (U) represent?*
A. The fraction of time the primary beam is directed toward a specific barrier
B. The occupancy of adjacent areas by staff or public
C. The total workload in Gy/week at 1 meter
D. The transmission factor of the barrier material
