ABR MEDICAL PHYSICS CERTIFICATION EXAMINATION PRACTICE QUESTIONS
AND CORRECT ANSWERS (VERIFIED ANSWERS) PLUS RATIONALES 2026 Q&A
Q1. The primary purpose of a parallel-plate ionization chamber is to:
A. Measure beam energy
B. Measure dose in a buildup region
C. Determine beam symmetry
D. Calibrate a thermoluminescent dosimeter
Correct Answer: B
Rationale: Parallel-plate ionization chambers have a shallow depth of maximum response, making
them ideal for measuring surface and near-surface doses, particularly in the buildup region .
Q2. The half-value layer (HVL) of an X-ray beam is defined as:
A. The thickness of a material that absorbs all the photons
B. The distance at which the beam intensity doubles
C. The thickness of a material that reduces the beam intensity by 50%
D. The time required for half of the radioactive source to decay
Correct Answer: C
Rationale: HVL is the thickness of a specified material that attenuates the beam to half its original
intensity, characterizing beam quality .
Q3. The primary advantage of flattening-filter-free (FFF) photon beams in radiotherapy is:
A. Increased dose homogeneity
B. Reduced penetration depth
C. Higher dose rate and reduced treatment time
,D. Lower skin dose
Correct Answer: C
Rationale: FFF beams remove the flattening filter, increasing dose rate and reducing treatment time,
though dose distribution becomes less uniform .
Q4. In MRI, T1 relaxation refers to:
A. The time constant for longitudinal magnetization recovery
B. The time constant for transverse magnetization decay
C. The frequency of precession of nuclei
D. The gradient strength required for slice selection
Correct Answer: A
Rationale: T1 relaxation describes the recovery of longitudinal magnetization along the main magnetic
field after an RF pulse .
Q5. The activity of a radioactive source decreases from 1600 MBq to 200 MBq in 12 hours. The half-life
of the radionuclide is:
A. 2 h
B. 3 h
C. 4 h
D. 6 h
Correct Answer: C
Rationale: Activity decreases by 50% each half-life. 1600 → 800 → 400 → 200 MBq requires 3 half-
lives. Time per half-life = 12 h / 3 = 4 h .
Q6. In a linear accelerator, bending magnets are used to:
,A. Accelerate electrons
B. Direct electrons along the circular path
C. Produce X-rays
D. Focus the photon beam
Correct Answer: B
Rationale: Bending magnets steer the electron beam along the curved trajectory in a circular
accelerator structure .
Q7. The principal limitation of film dosimetry is:
A. Low spatial resolution
B. Energy dependence and non-linear response
C. Poor tissue equivalence
D. Inability to measure large doses
Correct Answer: B
Rationale: Film dosimetry can have energy-dependent response and non-linear optical density-to-dose
conversion, especially at high doses .
Q8. A treatment planning system uses convolution-superposition algorithm primarily to:
A. Reduce patient setup errors
B. Generate MRI images
C. Accurately calculate dose distribution
D. Measure linac output
Correct Answer: C
, Rationale: Convolution-superposition algorithms model photon and electron scatter accurately to
predict 3D dose distributions in heterogeneous media .
Q9. In brachytherapy, the advantage of using high-dose-rate (HDR) sources over low-dose-rate (LDR)
sources is:
A. Lower shielding requirements
B. Reduced treatment precision
C. Shorter treatment times and outpatient delivery
D. Decreased radiation safety measures
Correct Answer: C
Rationale: HDR sources allow higher doses per fraction, reducing treatment times and allowing
outpatient therapy, while maintaining precision .
Q10. The quantity that best describes the energy deposited per unit mass in a medium is:
A. Exposure
B. Absorbed dose
C. Kerma
D. Equivalent dose
Correct Answer: B
Rationale: Absorbed dose is the energy deposited per unit mass in a medium .
Q11. Which interaction predominates for diagnostic X-rays in soft tissue (20-150 keV)?
A. Pair production
B. Photoelectric effect
AND CORRECT ANSWERS (VERIFIED ANSWERS) PLUS RATIONALES 2026 Q&A
Q1. The primary purpose of a parallel-plate ionization chamber is to:
A. Measure beam energy
B. Measure dose in a buildup region
C. Determine beam symmetry
D. Calibrate a thermoluminescent dosimeter
Correct Answer: B
Rationale: Parallel-plate ionization chambers have a shallow depth of maximum response, making
them ideal for measuring surface and near-surface doses, particularly in the buildup region .
Q2. The half-value layer (HVL) of an X-ray beam is defined as:
A. The thickness of a material that absorbs all the photons
B. The distance at which the beam intensity doubles
C. The thickness of a material that reduces the beam intensity by 50%
D. The time required for half of the radioactive source to decay
Correct Answer: C
Rationale: HVL is the thickness of a specified material that attenuates the beam to half its original
intensity, characterizing beam quality .
Q3. The primary advantage of flattening-filter-free (FFF) photon beams in radiotherapy is:
A. Increased dose homogeneity
B. Reduced penetration depth
C. Higher dose rate and reduced treatment time
,D. Lower skin dose
Correct Answer: C
Rationale: FFF beams remove the flattening filter, increasing dose rate and reducing treatment time,
though dose distribution becomes less uniform .
Q4. In MRI, T1 relaxation refers to:
A. The time constant for longitudinal magnetization recovery
B. The time constant for transverse magnetization decay
C. The frequency of precession of nuclei
D. The gradient strength required for slice selection
Correct Answer: A
Rationale: T1 relaxation describes the recovery of longitudinal magnetization along the main magnetic
field after an RF pulse .
Q5. The activity of a radioactive source decreases from 1600 MBq to 200 MBq in 12 hours. The half-life
of the radionuclide is:
A. 2 h
B. 3 h
C. 4 h
D. 6 h
Correct Answer: C
Rationale: Activity decreases by 50% each half-life. 1600 → 800 → 400 → 200 MBq requires 3 half-
lives. Time per half-life = 12 h / 3 = 4 h .
Q6. In a linear accelerator, bending magnets are used to:
,A. Accelerate electrons
B. Direct electrons along the circular path
C. Produce X-rays
D. Focus the photon beam
Correct Answer: B
Rationale: Bending magnets steer the electron beam along the curved trajectory in a circular
accelerator structure .
Q7. The principal limitation of film dosimetry is:
A. Low spatial resolution
B. Energy dependence and non-linear response
C. Poor tissue equivalence
D. Inability to measure large doses
Correct Answer: B
Rationale: Film dosimetry can have energy-dependent response and non-linear optical density-to-dose
conversion, especially at high doses .
Q8. A treatment planning system uses convolution-superposition algorithm primarily to:
A. Reduce patient setup errors
B. Generate MRI images
C. Accurately calculate dose distribution
D. Measure linac output
Correct Answer: C
, Rationale: Convolution-superposition algorithms model photon and electron scatter accurately to
predict 3D dose distributions in heterogeneous media .
Q9. In brachytherapy, the advantage of using high-dose-rate (HDR) sources over low-dose-rate (LDR)
sources is:
A. Lower shielding requirements
B. Reduced treatment precision
C. Shorter treatment times and outpatient delivery
D. Decreased radiation safety measures
Correct Answer: C
Rationale: HDR sources allow higher doses per fraction, reducing treatment times and allowing
outpatient therapy, while maintaining precision .
Q10. The quantity that best describes the energy deposited per unit mass in a medium is:
A. Exposure
B. Absorbed dose
C. Kerma
D. Equivalent dose
Correct Answer: B
Rationale: Absorbed dose is the energy deposited per unit mass in a medium .
Q11. Which interaction predominates for diagnostic X-rays in soft tissue (20-150 keV)?
A. Pair production
B. Photoelectric effect
