Page 1 of 49
ABR RADIATION ONCOLOGY 2026
CERTIFICATION EXAMINATION COMPLETE
(100) CURRENT TESTING QUESTIONS AND
CORRECT ANSWERS WITH DETAILED
EXPLANATIONS|GUARANTEED PASS.
ABR
Prepare with confidence using this ABR Radiation Oncology
Certification Examination, designed to assess advanced knowledge
in radiation therapy and cancer treatment management. It focuses
on radiation physics, treatment planning, radiobiology, oncology
principles, patient safety, and clinical decision-making. The exam
strengthens technical expertise and evidence-based practice in
radiation oncology care. Suitable for radiation oncology residents
and professionals preparing for ABR board certification.
MULTIPLE CHOICE.
Subsection 1: Radiobiology & Radiation Physics (Questions
1–10)
1. What is the oxygen enhancement ratio (OER) for high linear
energy transfer (LET) radiation such as carbon ions?
A) 0.5 – 1.0 (OER approaches 1.0 at very high LET, e.g., >100
keV/μm)
B) 1.0 – 1.5
C) 2.5 – 3.0 (typical for low LET radiation such as photons)
, Page 2 of 49
D) 5.0 – 6.0
Answer: A. 0.5 – 1.0 (OER approaches 1.0 at very high LET,
e.g., >100 keV/μm)
Explanation: The OER is the ratio of the dose needed under
hypoxic conditions to achieve the same effect as under
normoxic conditions. For low-LET radiation (e.g., photons), OER
≈ 2.5 – 3.0. For high-LET radiation (e.g., carbon ions, neutrons),
OER approaches 1.0, meaning hypoxic cells are almost as
sensitive as oxygenated cells.
2. The parameter α/β (alpha-beta) in the linear-quadratic
model is the dose at which the linear and quadratic
contributions to cell killing are equal. For late-responding
normal tissues (e.g., spinal cord, lung), the α/β ratio is:
A) High (≥ 10 Gy)
B) Low (approximately 2 – 3 Gy)
C) Zero
D) Infinite
Answer: B. Low (approximately 2 – 3 Gy)
Explanation: Late-responding tissues (spinal cord, lung, kidney,
optic nerve) have a low α/β ratio (typically 2 – 3 Gy). This makes
them more sensitive to fraction size (large fractions cause
disproportionately more late damage). Tumors and
early-responding tissues typically have a higher α/β ratio (≥ 10
Gy).
3. Which of the following statements about the four Rs of
radiobiology is correct?
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A) Repair – sublethal damage repair is more efficient in
late-responding tissues than in tumors
B) Repopulation – accelerated repopulation in tumors may begin
3 – 4 weeks after starting radiotherapy
C) Redistribution – cells in S phase are most radiosensitive
(most resistant)
D) Reoxygenation – improves radiosensitivity of hypoxic cells
after the initial dose
Answer: A. Repair – sublethal damage repair is more efficient
in late-responding tissues than in tumors (and also B, C, D
with corrections?)
Explanation: In the four Rs:
• Repair: Late-responding normal tissues have greater repair
capacity for sublethal damage, which leads to fractionation
sensitivity.
• Repopulation: Accelerated repopulation in tumors may
occur after approximately 4 weeks of treatment.
• Redistribution: Cells in late S phase are most
radioresistant; cells in G2-M are most radiosensitive.
• Reoxygenation: After the first doses, hypoxic tumor cells
may become reoxygenated, increasing their sensitivity.
4. The spread-out Bragg peak (SOBP) used in proton therapy
is created by:
A) A single high-energy proton beam
B) A range modulator wheel or superposition of multiple beams
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of different energies (passive scattering) or scanning (active)
C) A flattening filter
D) A magnetic field
Answer: B. A range modulator wheel or superposition of
multiple beams of different energies (passive scattering) or
scanning (active)
Explanation: Protons have a characteristic Bragg peak (sharp
dose maximum at the end of range). To cover the target
thickness, a SOBP is created by spreading the Bragg peak (using
a modulator wheel or by stacking energy layers in scanning). No
flattening filter is used.
5. A linear accelerator (linac) operating at 6 MV produces a
photon beam. The depth of dose maximum (dmax) for a
10×10 cm² field at 100 cm SSD is approximately:
A) 1.5 cm
B) 3.0 cm
C) 0.5 cm
D) 5.0 cm
Answer: A. 1.5 cm
Explanation: For 6 MV photons, dmax ≈ 1.5 cm. For higher
energies, dmax increases (e.g., 18 MV → dmax ≈ 3.5 cm). The
dmax is the depth at which the maximum dose occurs due to
electronic equilibrium.
6. According to QUANTEC, the maximum point dose to the
optic chiasm in conventional fractionation (1.8 – 2.0 Gy/fx) to
avoid optic neuropathy should be limited to:
ABR RADIATION ONCOLOGY 2026
CERTIFICATION EXAMINATION COMPLETE
(100) CURRENT TESTING QUESTIONS AND
CORRECT ANSWERS WITH DETAILED
EXPLANATIONS|GUARANTEED PASS.
ABR
Prepare with confidence using this ABR Radiation Oncology
Certification Examination, designed to assess advanced knowledge
in radiation therapy and cancer treatment management. It focuses
on radiation physics, treatment planning, radiobiology, oncology
principles, patient safety, and clinical decision-making. The exam
strengthens technical expertise and evidence-based practice in
radiation oncology care. Suitable for radiation oncology residents
and professionals preparing for ABR board certification.
MULTIPLE CHOICE.
Subsection 1: Radiobiology & Radiation Physics (Questions
1–10)
1. What is the oxygen enhancement ratio (OER) for high linear
energy transfer (LET) radiation such as carbon ions?
A) 0.5 – 1.0 (OER approaches 1.0 at very high LET, e.g., >100
keV/μm)
B) 1.0 – 1.5
C) 2.5 – 3.0 (typical for low LET radiation such as photons)
, Page 2 of 49
D) 5.0 – 6.0
Answer: A. 0.5 – 1.0 (OER approaches 1.0 at very high LET,
e.g., >100 keV/μm)
Explanation: The OER is the ratio of the dose needed under
hypoxic conditions to achieve the same effect as under
normoxic conditions. For low-LET radiation (e.g., photons), OER
≈ 2.5 – 3.0. For high-LET radiation (e.g., carbon ions, neutrons),
OER approaches 1.0, meaning hypoxic cells are almost as
sensitive as oxygenated cells.
2. The parameter α/β (alpha-beta) in the linear-quadratic
model is the dose at which the linear and quadratic
contributions to cell killing are equal. For late-responding
normal tissues (e.g., spinal cord, lung), the α/β ratio is:
A) High (≥ 10 Gy)
B) Low (approximately 2 – 3 Gy)
C) Zero
D) Infinite
Answer: B. Low (approximately 2 – 3 Gy)
Explanation: Late-responding tissues (spinal cord, lung, kidney,
optic nerve) have a low α/β ratio (typically 2 – 3 Gy). This makes
them more sensitive to fraction size (large fractions cause
disproportionately more late damage). Tumors and
early-responding tissues typically have a higher α/β ratio (≥ 10
Gy).
3. Which of the following statements about the four Rs of
radiobiology is correct?
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A) Repair – sublethal damage repair is more efficient in
late-responding tissues than in tumors
B) Repopulation – accelerated repopulation in tumors may begin
3 – 4 weeks after starting radiotherapy
C) Redistribution – cells in S phase are most radiosensitive
(most resistant)
D) Reoxygenation – improves radiosensitivity of hypoxic cells
after the initial dose
Answer: A. Repair – sublethal damage repair is more efficient
in late-responding tissues than in tumors (and also B, C, D
with corrections?)
Explanation: In the four Rs:
• Repair: Late-responding normal tissues have greater repair
capacity for sublethal damage, which leads to fractionation
sensitivity.
• Repopulation: Accelerated repopulation in tumors may
occur after approximately 4 weeks of treatment.
• Redistribution: Cells in late S phase are most
radioresistant; cells in G2-M are most radiosensitive.
• Reoxygenation: After the first doses, hypoxic tumor cells
may become reoxygenated, increasing their sensitivity.
4. The spread-out Bragg peak (SOBP) used in proton therapy
is created by:
A) A single high-energy proton beam
B) A range modulator wheel or superposition of multiple beams
, Page 4 of 49
of different energies (passive scattering) or scanning (active)
C) A flattening filter
D) A magnetic field
Answer: B. A range modulator wheel or superposition of
multiple beams of different energies (passive scattering) or
scanning (active)
Explanation: Protons have a characteristic Bragg peak (sharp
dose maximum at the end of range). To cover the target
thickness, a SOBP is created by spreading the Bragg peak (using
a modulator wheel or by stacking energy layers in scanning). No
flattening filter is used.
5. A linear accelerator (linac) operating at 6 MV produces a
photon beam. The depth of dose maximum (dmax) for a
10×10 cm² field at 100 cm SSD is approximately:
A) 1.5 cm
B) 3.0 cm
C) 0.5 cm
D) 5.0 cm
Answer: A. 1.5 cm
Explanation: For 6 MV photons, dmax ≈ 1.5 cm. For higher
energies, dmax increases (e.g., 18 MV → dmax ≈ 3.5 cm). The
dmax is the depth at which the maximum dose occurs due to
electronic equilibrium.
6. According to QUANTEC, the maximum point dose to the
optic chiasm in conventional fractionation (1.8 – 2.0 Gy/fx) to
avoid optic neuropathy should be limited to:
