ARRT exam prep
ARRT Radiation Physics Practice Questions: Free 20-Question Set
Physics is where mock-exam scores die — especially calculation questions under time pressure. These 20 questions span the ARRT physics weighting: inverse square and beam geometry, depth dose and energy, attenuation, wedges and fields, and monitor unit math, each with a worked explanation.
Want timed physics drills with instant rationales? Rad Therapy Guru packs mock exams, adaptive quizzes, flashcards, and mastery tracking into one free trial, and it was built by a working therapist and instructor you can read about here.
Section A — Inverse Square & Beam Geometry
Questions 1–4 · answers and explanations included
The intensity measured 100 cm from a point source is 400 R/min. What is the intensity at 200 cm from the source?
- A.200 R/min
- B.100 R/min
- C.50 R/minCorrect
- D.25 R/min
Answer: C
By the inverse square law, doubling the distance cuts intensity to one quarter: 400 × (100/200)² = 400 × 0.25 = 50 R/min. Intensity varies as 1/d² from a point source.
Which change widens the penumbra of a megavoltage photon field?
- A.Using a smaller radiation source
- B.Increasing the source-to-surface distance
- C.Using a larger radiation sourceCorrect
- D.Measuring at a shallower depth
Answer: C
Geometric penumbra grows with the source size and with greater source-to-surface distance, and it also widens with depth as scattered electrons accumulate. A smaller source narrows the penumbra.
Compared with a point source, dose falloff near a linear (line) brachytherapy source:
- A.Falls off faster than inverse square at all distances
- B.Falls off less steeply than inverse square close to the sourceCorrect
- C.Follows inverse square exactly at every distance
- D.Does not decrease with distance at all
Answer: B
A line source is a series of point sources spread along its length, so close to the source the contributions overlap and dose falls off more gently than 1/d². Far from the source it approaches point-source behavior.
A 10×10 cm field is defined at isocenter, 100 cm from the source. At a plane 200 cm from the source, the projected field size is:
- A.5×5 cm
- B.10×10 cm
- C.20×20 cmCorrect
- D.40×40 cm
Answer: C
Beam divergence makes field size scale linearly with distance from the source: 10 cm × (200/100) = 20 cm, so the projected field is 20×20 cm. Farther from the source means a larger projected field.
Section B — Depth Dose, Energy & Electrons
Questions 5–8 · answers and explanations included
Which statement correctly distinguishes PDD from TMR?
- A.PDD is independent of SSD, while TMR varies with SSD
- B.PDD depends on SSD, while TMR is measured at isocenter and is independent of SSDCorrect
- C.PDD and TMR are identical for all geometries
- D.TMR applies only to electron beams
Answer: B
Percentage depth dose is defined for a fixed SSD and varies as the SSD changes. Tissue-maximum ratio is measured at a fixed source-to-axis (isocentric) geometry, making it independent of SSD — which is why it belongs to isocentric calculations.
The depth of maximum dose (dmax) for a 6 MV photon beam in water is approximately:
- A.0.5 cm
- B.1.5 cmCorrect
- C.2.5 cm
- D.5.0 cm
Answer: B
A 6 MV beam reaches charged particle equilibrium at about 1.5 cm depth in water. This buildup depth is what produces the skin-sparing advantage of megavoltage beams.
As photon beam energy increases from 6 MV to 18 MV:
- A.dmax moves toward the surface and skin sparing decreases
- B.dmax moves deeper and skin sparing increasesCorrect
- C.dmax is unchanged but surface dose rises
- D.The beam becomes less penetrating
Answer: B
Higher-energy photons set secondary electrons in motion farther from the entrance surface, pushing dmax deeper — about 3.5 cm for 18 MV — and lowering the relative surface dose, which improves skin sparing.
The useful treatment depth of an electron beam is approximately:
- A.E/3 to E/4 cm, where E is the energy in MeV (practical range ≈ E/2 cm)Correct
- B.E cm in water
- C.2 × E cm in water
- D.Independent of the beam energy
Answer: A
Clinically useful electron dose is delivered to roughly E/3 to E/4 cm — a 12 MeV electron beam treats about 3–4 cm — with the practical range near E/2 cm. This energy-to-depth relationship drives electron energy selection.
Section C — Attenuation, HVL & Isotopes
Questions 9–12 · answers and explanations included
Half-value layer (HVL) is defined as:
- A.The thickness of material that reduces beam intensity by 90%
- B.The thickness of material that reduces beam intensity to one halfCorrect
- C.The depth in water at which dose falls to 50%
- D.The thickness that doubles the beam energy
Answer: B
The HVL is the absorber thickness that halves the beam intensity. Each additional HVL halves the intensity again — an exponential, not linear, reduction: after 2 HVLs, 25% remains; after 3, 12.5%.
Adding filtration to a kilovoltage photon beam:
- A.Removes low-energy photons, raising the average energy and sparing the skinCorrect
- B.Removes the high-energy photons and softens the beam
- C.Lowers the average energy of the beam
- D.Has no effect on the beam spectrum
Answer: A
Beam hardening: the filter preferentially absorbs the abundant low-energy photons that would only add to skin dose, raising the average energy and increasing the penetrating fraction of the beam.
If the linear attenuation coefficient of a material is 0.3465 cm⁻¹, the half-value layer is:
- A.0.2 cm
- B.2.0 cmCorrect
- C.0.693 cm
- D.5.0 cm
Answer: B
μ = 0.693/HVL, so HVL = 0.693/0.3465 = 2.0 cm. This relationship lets you move between attenuation coefficients and HVLs in either direction.
The average photon energy of a cobalt-60 beam is:
- A.0.511 MeV
- B.About 1.25 MeV (1.17 and 1.33 MeV rays)Correct
- C.4–6 MeV
- D.18 MeV
Answer: B
Co-60 emits two gamma rays at 1.17 and 1.33 MeV, averaging 1.25 MeV. This places Co-60 teletherapy roughly between a 2–4 MV linear accelerator beam.
Section D — Wedges, Fields & Isodose
Questions 13–16 · answers and explanations included
In a wedged photon field, the thick end of the wedge (the heel):
- A.Attenuates the beam less, tilting isodose lines toward the heel
- B.Attenuates the beam more, tilting the isodose lines toward the thin end (toe)Correct
- C.Has no effect on the isodose tilt
- D.Increases the dose beyond the thin end of the field
Answer: B
The heel is the thick, most attenuating end, so dose is reduced there and the isodose lines tilt toward the thin end. The wedge angle describes that isodose tilt at a reference depth.
Two wedged beams form a hinge angle of 90°. The wedge angle required for each beam is approximately:
- A.15°
- B.30°
- C.45°Correct
- D.60°
Answer: C
The wedge angle is approximately half the hinge angle: 90° ÷ 2 = 45°. The wedges tilt the individual beam distributions so their composite flattens across the target volume.
Increasing the field size at a given depth generally:
- A.Decreases scatter and lowers the output factor
- B.Increases scatter, raising the surface dose and the output factorCorrect
- C.Has no effect on scatter contribution
- D.Reduces the dose everywhere in the field
Answer: B
Larger fields irradiate more tissue, adding scattered radiation. That raises both the depth-dose curve (more scatter) and the output factor (more dose per MU at the reference point).
For parallel-opposed fields at low photon energy, the isodose distribution across the midline:
- A.Is flat across the entire field with no surface variation
- B.Shows a saddle shape, with higher-dose horns near the entrance surfacesCorrect
- C.Has its maximum at the midline for all energies
- D.Splits into two disconnected beam regions
Answer: B
Opposed low-energy beams produce a saddle-shaped distribution: dose builds to horns near each surface while the midline — where the beams overlap through more tissue — sits lower. Higher energies flatten the distribution toward the midline.
Section E — Monitor Unit Calculations
Questions 17–20 · answers and explanations included
In the fixed-SSD monitor unit equation MU = D / (K · Sc · Sp · PDD · TF · f_ISL), the term f_ISL accounts for:
- A.The wedge (or tray) transmission factor
- B.The inverse square correction between the calibration distance and the treatment distanceCorrect
- C.The patient's tissue-maximum ratio
- D.The output factor for the largest open field
Answer: B
f_ISL is the inverse square factor ((calibration SSD/treatment SSD)²) correcting for the difference between calibration and treatment distances. K is the calibrated dose per MU, Sc and Sp the collimator and phantom scatter factors, PDD the depth-dose factor, and TF any tray or wedge transmission.
A rectangular field measures 10×20 cm. Using eq ≈ 4A/P, its equivalent square is approximately:
- A.13×13 cmCorrect
- B.15×15 cm
- C.10×10 cm
- D.20×20 cm
Answer: A
Area A = 200 cm² and perimeter P = 60 cm, so eq = 4 × 200/60 ≈ 13.3 cm — about a 13×13 cm square. Equivalent squares let rectangular fields use standard square-field data tables.
Making the dose calculation grid coarser in a treatment planning system:
- A.Speeds the calculation but lowers spatial resolutionCorrect
- B.Improves accuracy at tissue interfaces
- C.Slows the calculation significantly
- D.Has no effect on spatial resolution
Answer: A
A coarser grid computes fewer voxels, so the calculation runs faster — but steep dose gradients, especially near heterogeneity interfaces, are resolved less accurately. Fine grids trade speed for resolution.
Calibration gives 1 MU = 1 cGy at dmax for a 10×10 cm field at 100 cm SSD. How many monitor units deliver 200 cGy at a point 100 cm from the source, before applying any other correction factors?
- A.100 MU
- B.173 MU
- C.200 MUCorrect
- D.288 MU
Answer: C
At the calibration SSD there is no inverse square correction, so 200 cGy ÷ 1 cGy/MU = 200 MU. In a real calculation, Sc, Sp, PDD, TF, and any inverse square factor would each modify that baseline — this is the arithmetic backbone every MU problem builds on.
How Did You Do?
Score yourself one point per correct answer, then act on your band:
17–20 correct
You are in strong shape.
You are close to exam-ready. Identify your weakest category above and drill it until every explanation feels routine — then keep up timed practice sets.
12–16 correct
Solid foundation.
You know the fundamentals but have gaps. Rebuild the categories where you missed the most questions, then retake this set in a week and confirm the score holds.
Under 12
Study by category.
Map every miss to the ARRT content outline — beam geometry, depth dose, attenuation, wedges and fields, and monitor unit calculations — and study one category at a time, starting with calculations, where the most exam weight lives.
Get the printable Dosimetry Cheat Sheet (PDF)
Every formula behind these calculation questions — monitor-unit math, inverse square, decay tables — in one free printable PDF. Grab it with your email and it downloads instantly.
Ready for the full study suite?
These 20 questions are a taste of the practice materials inside Rad Therapy Guru: timed mock exams, adaptive quizzes, flashcards, and mastery tracking across every ARRT topic.
ARRT Physics Practice Questions FAQ
How many physics questions are on the ARRT radiation therapy exam?
Radiation physics is one of the largest single content categories on the ARRT Radiation Therapy exam, so a weak physics section can sink an otherwise strong score. Weighted practice across all categories — like this 20-question set — is the fastest way to find where you stand.
Do I need to memorize formulas for the ARRT exam?
Only a handful. The inverse square law, equivalent square fields, radioactive decay, and the pieces of the monitor unit equation cover most calculation questions; the rest of the physics section tests concepts such as depth-dose behavior, attenuation, and isodose shapes.
Is the physics section hard?
It is the section students fear most, but it is also the most practice-able: the question styles repeat, and worked explanations convert each miss into a concept you can drill. Students who score poorly on their first physics mock typically gain the most from timed question sets.
How do I practice dose calculations?
Work problems with full solutions — like the ones above — then re-time yourself on similar problems until the arithmetic is automatic. Timed drills with instant rationales, plus mastery tracking per topic, are exactly what the mock exam mode inside Rad Therapy Guru is built for.
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Content reviewed by Kate, a working radiation therapist and instructor — see the full story on the about page.