“200”, Aptitude Test Questions and Answers for the Medical Physicists II – Ocean Road Cancer Institute (ORCI).
ABSTRACT
This material provides 200 multiple-choice
questions for candidates preparing for the Medical Physicist II position at the
Ocean Road Cancer Institute (ORCI) through the Public Service Recruitment
Secretariat. The questions focus on core medical physics areas such as
radiotherapy, dosimetry, treatment planning, and quality assurance, with a
strong emphasis on real clinical scenarios and problem-solving. Each question
is structured to reflect the challenging nature of aptitude tests, where answer
choices are closely related and require clear understanding rather than
guessing. The set also includes calculation-based problems and QA
interpretation to help candidates build accuracy, speed, and confidence.
Overall, this is a practical and high-level preparation tool for candidates
aiming to perform strongly in competitive recruitment exams and clinical
practice.
Prepared
by: Medical Physicists II
Compiled
by Johnson Yesaya.
An
author based in Dar-es-salaam.
0628729934.
Date:
May 02, 2026
Dear
applicants,
This
collection of questions and answers has been prepared to help all of you to understand
the key areas tested during the interview. The goal is to provide a useful, and
practical study guide so you can all perform confidently and fairly in the
selection process. I wish you the best of luck, and may this resource support
you in achieving success!
Warm
regards,
Johnson
Yesaya Mgelwa
For Personal Use by Applicants Preparing
for Medical Physicists II – Ocean Road Cancer
Institute (ORCI).
ALL QUESTIONS ARE COMPILED
TOGETHER.
1. A treatment plan requires delivering 200 cGy at a depth of 5 cm
using a 6 MV photon beam. If the Percentage Depth Dose (PDD) at 5 cm is 80% and
the machine is calibrated to deliver 1 cGy/MU at dmax, what MU is required?
A. 200 MU | B. 225 MU | C. 240 MU | D.
250 MU
Answer: D
Rationale: PDD is defined relative to dose at dmax
under the same geometry, so when using PDD-based calculations, inverse square
correction is inherently accounted for under standard SSD setup assumptions.
Therefore, MU = 200 / 0.80 = 250 MU.
2. During routine QA, a LINAC output is found to be 3% higher than
baseline. What is the MOST appropriate immediate action?
A. Continue treatment and adjust later |
B. Stop treatment and recalibrate immediately | C. Ignore if within ±5%
tolerance | D. Document and review next QA cycle
Answer: B
Rationale: Radiotherapy tolerances are strict;
typically ±2% for daily output constancy. A 3% deviation exceeds acceptable
limits and could result in systematic overdose. Immediate recalibration ensures
patient safety and prevents propagation of error across multiple patients.
3. Which factor MOST directly influences the build-up region in
megavoltage photon beams?
A. Photon attenuation coefficient | B. Scatter
from collimators | C. Beam flatness | D. Secondary electron equilibrium
Answer: D
Rationale: The build-up region arises because
secondary electrons set into motion by photons need a finite distance to reach
equilibrium. Initially, fewer electrons deposit energy, but as depth increases,
equilibrium is achieved, leading to maximum dose.
4. A wedge filter is introduced in a photon beam. What is its
PRIMARY effect?
A. Modify dose distribution gradient | B.
Increase beam energy | C. Reduce scatter radiation | D. Increase penetration
depth
Answer: A
Rationale: Wedges create a tilt in isodose lines,
compensating for irregular patient anatomy or beam angles. They do not change
photon energy or penetration significantly but redistribute dose spatially.
5. Which dosimeter is MOST suitable for absolute dose calibration
in radiotherapy?
A. Film dosimeter | B. TLD | C.
Ionization chamber | D. Semiconductor diode
Answer: C
Rationale: Ionization chambers are the reference
standard for absolute dosimetry due to their stability, accuracy, and
traceability to calibration protocols such as TRS-398. Other detectors are
typically used for relative or in vivo measurements.
6. A shielding barrier reduces radiation intensity by 75%. What is
the transmission factor?
A. 0.25 | B. 0.50 | C. 0.75 | D. 0.90
Answer: A
Rationale: Transmission factor is the fraction of
radiation that passes through the barrier. If 75% is attenuated, 25% remains,
giving a transmission factor of 0.25.
7. In radiotherapy planning, what does “isocenter” refer to?
A. Surface entry point | B. Beam origin
inside machine | C. Point where beams intersect | D. Maximum dose depth
Answer: C
Rationale: The isocenter is the central reference
point in space where all treatment beams converge. Accurate targeting ensures
the tumor receives the intended cumulative dose from all beam angles.
8. Which quantity is measured in Gray (Gy)?
A. Exposure | B. Absorbed dose | C.
Activity | D. Equivalent dose
Answer: B
Rationale: Gray (Gy) is the SI unit of absorbed
dose, defined as energy deposited per unit mass (J/kg). It directly reflects
the physical dose delivered to tissue.
9. A patient receives radiation from multiple beam angles. The main
advantage is:
A. Increased skin dose | B. Reduced tumor
dose | C. Dose concentration at target | D. Increased scatter to organs
Answer: C
Rationale: Multiple beams intersect at the tumor,
summing doses at the target while spreading lower doses across normal tissues.
This improves tumor control while minimizing toxicity.
10. What is the PRIMARY purpose of quality assurance in
radiotherapy?
A. Increase machine speed | B. Simplify
planning | C. Reduce treatment cost | D. Ensure treatment accuracy and safety
Answer: D
Rationale: QA ensures that equipment performance
and treatment delivery match planned parameters. It prevents systematic errors
that could affect many patients, making it essential for safe clinical
practice.
11. Which interaction dominates in soft tissue for 6 MV photons?
A. Photoelectric effect | B. Compton
scattering | C. Pair production | D. Coherent scattering
Answer: B
Rationale: At megavoltage energies, Compton
scattering dominates because it depends primarily on electron density rather
than atomic number, making it the main interaction in soft tissue.
12. What is the HALF-VALUE LAYER (HVL)?
A. Thickness reducing intensity by half |
B. Depth of maximum dose | C. Dose at 50% depth | D. Beam divergence measure
Answer: A
Rationale: HVL is the thickness of material
required to reduce radiation intensity to 50% of its original value. It is
widely used in shielding and beam quality characterization.
13. Which device is used to immobilize patients during treatment?
A. Collimator | B. Bolus | C. Fixation
device | D. Detector
Answer: C
Rationale: Immobilization devices (e.g., masks,
vacuum cushions) ensure reproducible positioning across treatment sessions,
which is critical for accurate dose delivery.
14. A bolus is used to:
A. Shift dose to surface | B. Increase
beam energy | C. Reduce scatter | D. Improve imaging
Answer: A
Rationale: A bolus mimics tissue and effectively
reduces the depth of maximum dose, allowing higher dose delivery to superficial
tumors.
15. Which parameter defines beam quality?
A. Field size | B. Energy spectrum | C.
Treatment time | D. Patient position
Answer: B
Rationale: Beam quality relates to photon energy
distribution, which determines penetration and attenuation characteristics
within tissue.
16. If dose rate doubles while prescribed dose remains constant,
treatment time will:
A. Double | B. Increase slightly | C.
Remain constant | D. Halve
Answer: D
Rationale: Dose = Dose rate × Time. For a fixed
dose, increasing dose rate reduces the required time proportionally.
17. What is the PRIMARY role of a multileaf collimator (MLC)?
A. Shape radiation beam | B. Measure dose
| C. Increase energy | D. Reduce noise
Answer: A
Rationale: MLCs dynamically shape the radiation
beam to match tumor contours, enabling conformal and intensity-modulated
treatments.
18. Which concept ensures no net electron flow in a volume?
A. Beam attenuation | B. Electronic
equilibrium | C. Dose gradient | D. Scatter balance
Answer: B
Rationale: Electronic equilibrium occurs when the
number of electrons entering a volume equals those leaving, resulting in stable
dose deposition.
19. What is the SI unit of radioactivity?
A. Gray | B. Sievert | C. Becquerel | D.
Coulomb
Answer: C
Rationale: The Becquerel (Bq) measures radioactive
decay rate, defined as one disintegration per second.
20. Which QA test specifically verifies beam symmetry?
A. Output constancy | B. Flatness check |
C. Energy test | D. Symmetry test
Answer: D
Rationale: Symmetry tests assess whether the dose
distribution is equal on both sides of the central axis. This is distinct from
flatness, which measures uniformity across the field.
21. Which imaging modality is MOST commonly used for radiotherapy
treatment planning?
A. MRI only | B. CT scan | C. Ultrasound
| D. X-ray film
Answer: B
Rationale: CT imaging provides electron density
information necessary for accurate dose calculations, making it the standard
for treatment planning.
22. Which factor MOST affects scatter radiation in a patient?
A. Field size | B. Beam energy | C.
Patient age | D. Treatment time
Answer: A
Rationale: Larger field sizes irradiate more tissue
volume, increasing the probability of scatter interactions and thus increasing
scattered radiation.
23. What is the main function of a linear accelerator (LINAC)?
A. Detect radiation | B. Generate
high-energy beams | C. Store isotopes | D. Measure dose
Answer: B
Rationale: A LINAC accelerates electrons to high
energies, which are then used directly or converted into high-energy photons
for cancer treatment.
24. Which technique ensures accurate daily patient positioning?
A. QA protocol | B. Dose recalculation |
C. Beam energy adjustment | D. Image guidance
Answer: D
Rationale: Image-Guided Radiotherapy (IGRT) uses
imaging before or during treatment to confirm and correct patient positioning.
25. If two identical beams deliver equal dose from opposite
directions, what happens to skin dose?
A. Doubles | B. Remains unchanged | C. Halves
| D. Becomes negligible
Answer: C
Rationale: Each beam contributes dose at its own
entry surface. With opposing beams, each skin point receives dose from only one
beam rather than two, while doses add at depth. This reduces the relative skin
dose compared to a single-beam arrangement delivering the same tumor dose.
26. A LINAC delivers 300 cGy using 150 MU under reference
conditions. What is the calibrated output?
A. 1.5 cGy/MU | B. 2.0 cGy/MU | C. 0.5
cGy/MU | D. 3.0 cGy/MU
Answer: B
Rationale: Output is defined as dose per MU.
Therefore, 300 ÷ 150 = 2.0 cGy/MU. This represents machine calibration under
reference conditions.
27. A wedge filter reduces beam intensity. What is the MOST
appropriate compensation?
A. Increase beam energy | B. Decrease
field size | C. Reduce SSD | D. Increase MU
Answer: D
Rationale: A wedge attenuates the beam, reducing
dose rate. To maintain the prescribed dose, MU must be increased.
28. If the SSD increases while all other parameters remain constant,
the dose at a fixed point will:
A. Increase linearly | B. Decrease due to
inverse square law | C. Remain unchanged | D. Increase slightly
Answer: B
Rationale: Dose follows the inverse square law,
meaning intensity decreases as distance from the source increases. Increasing
SSD spreads the beam, reducing dose at the point.
29. Which factor MOST directly determines penetration depth of a
photon beam?
A. Beam energy | B. Field size | C. Dose
rate | D. Patient position
Answer: A
Rationale: Beam energy dictates how deeply photons
penetrate tissue. Higher energy beams deposit dose deeper, shifting the depth
dose curve inward.
30. During QA, beam symmetry is acceptable but flatness fails. What
does this imply?
A. Beam energy is too high | B. Detector
error occurred | C. Uneven dose distribution across field | D. Incorrect MU
calculation
Answer: C
Rationale: Symmetry compares left vs right balance,
while flatness evaluates uniformity across the entire field. A flatness failure
indicates dose variation across the beam profile despite symmetry.
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