CMD exam prep
MDCB Practice Questions: Free CMD Exam Prep
Struggling to find realistic practice questions for the MDCB exam? While the ARRT has dozens of free prep resources, the Certified Medical Dosimetrist exam is a different story. These 20 practice questions match the style and content weighting of the actual MDCB exam, and every question includes a detailed explanation.
Want the same depth for the ARRT exam? Rad Therapy Guru packs mock exams, question banks with rationales, 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 — Treatment Planning
Questions 1–6 · answers and explanations included
A patient has a target volume that wraps around the left kidney, which must be limited to 20 Gy. Which technique is most appropriate to spare the kidney while covering the concave target?
- A.Forward-planned 3D conformal fields with wedges
- B.IMRT planned with inverse optimizationCorrect
- C.Opposed AP/PA fields
- D.A single en-face photon field
Answer: B
Inverse planning creates the concave, dose-sculpted distributions needed to wrap a target around an organ at risk. Forward-planned 3D techniques cannot practically produce a concave distribution around the kidney, and AP/PA or en-face fields would treat the kidney to full dose.
Electron arc therapy is most useful for treating which of the following?
- A.A prostate boost
- B.The post-mastectomy chest wallCorrect
- C.Whole-brain irradiation
- D.A central lung tumor
Answer: B
Electron arc therapy conforms dose to long, curved surfaces — most commonly the post-mastectomy chest wall — while the shallow electron range spares the underlying lung. It is not suited to deep-seated targets like the prostate.
In inverse planning, the treatment plan is produced by:
- A.Manually adjusting beam weights after each dose calculation
- B.An optimization algorithm that adjusts beam fluence to meet dose objectivesCorrect
- C.Copying a template plan from a similar patient
- D.Selecting standard field arrangements before any dose objectives are entered
Answer: B
In inverse planning the planner enters dose objectives and constraints first, and the optimizer adjusts the beam fluence to meet them. That is the opposite of forward planning, where the planner manually picks fields and weights and iterates on calculated doses.
Increasing the size of the dose calculation grid will:
- A.Increase dose computation speed while reducing spatial resolutionCorrect
- B.Slow the calculation but improve spatial resolution
- C.Have no effect on calculation speed
- D.Improve dose accuracy at tissue interfaces
Answer: A
A coarser grid computes fewer voxels, so dose calculation runs faster, but steep dose gradients are resolved less accurately — especially near heterogeneity interfaces. Fine grids trade speed for resolution.
A thoracic plan is computed without applying heterogeneity corrections for lung tissue. The reported dose distal to the lung is likely to be:
- A.Overestimated by about 10%
- B.UnderestimatedCorrect
- C.Accurate — correction only affects dose inside the lung
- D.Underestimated proximal to the lung, but accurate distal to it
Answer: B
Lung tissue has a much lower density than water and attenuates less. A water-equivalent calculation assumes too much attenuation, so it underestimates the dose delivered beyond (distal to) the lung.
A wedged pair of beams has a hinge angle of 90°. The wedge angle required for each beam is:
- 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 combination flattens across the target.
Section B — Dosimetry Calculations & Physics
Questions 7–13 · answers and explanations included
A dose of 200 cGy is prescribed at a point located at 120 cm SSD. Calibration gives 1 MU = 1 cGy at dmax for a 10×10 cm field at 100 cm SSD. Ignoring other correction factors, how many monitor units are required?
- A.167 MU
- B.200 MU
- C.240 MU
- D.288 MUCorrect
Answer: D
Apply the inverse square law: (120/100)² = 1.44. Dose per MU at 120 cm is 1/1.44 of the calibration rate, so 200 × 1.44 = 288 MU. Extended SSDs always require more MU.
For an isocentric (SAD) treatment technique, dose calculations are based on:
- A.PDD
- B.TMRCorrect
- C.Backscatter factor
- D.Peak scatter factor at extended SSD
Answer: B
PDD varies with SSD, so it belongs to fixed-SSD (SSD) techniques. Isocentric setups hold the source-to-axis distance constant, and tissue-maximum ratios (TMR) are measured and used for exactly that geometry.
As megavoltage photon beam energy increases, the surface dose:
- A.Increases
- B.Decreases, improving skin sparingCorrect
- C.Stays the same
- D.Becomes equal to the dose at dmax
Answer: B
Higher-energy beams set charged particles in motion farther from the surface, so dose builds up more slowly and dmax lies deeper. Surface dose therefore falls and relative skin sparing improves as energy increases.
A prescription reads 70 Gy delivered in 35 fractions. What is the dose per fraction?
- A.1.8 Gy
- B.2.0 GyCorrect
- C.2.12 Gy
- D.2.5 Gy
Answer: B
70 Gy ÷ 35 fractions = 2.0 Gy per fraction — the standard conventional fractionation for most definitive treatments.
dmax is best defined as:
- A.The depth at which dose falls to 50% of maximum
- B.The depth of maximum dose, where charged particle equilibrium is reachedCorrect
- C.The maximum range of secondary electrons in water
- D.The depth at which PDD equals TMR
Answer: B
Dose builds up with depth until charged particle equilibrium is reached at dmax — about 1.5 cm for a 6 MV beam. Beyond dmax, photon attenuation makes dose fall off.
A compensator is generally preferred over bolus for megavoltage photon beams because it:
- A.Raises the surface dose
- B.Corrects for missing tissue while preserving skin sparingCorrect
- C.Is easier to fabricate than bolus
- D.Eliminates the need for heterogeneity correction
Answer: B
Bolus deliberately pulls the buildup region to the surface, sacrificing skin sparing. A compensator modifies the beam intensity in the beam path to compensate for missing tissue while the buildup region and skin sparing remain intact.
A 10×10 cm field is set at isocenter, 100 cm from the source. The field size projected on the patient surface at 90 cm SAD is:
- A.8×8 cm
- B.9×9 cmCorrect
- C.10×10 cm
- D.11×11 cm
Answer: B
Field size scales linearly with distance from the source: 10 cm × (90/100) = 9 cm, so the projected field is 9×9 cm. Closer to the source means a smaller projected field.
Section C — Brachytherapy & Safety
Questions 14–17 · answers and explanations included
A permanent prostate seed implant uses an isotope with a half-life of approximately 60 days. Which isotope is it?
- A.Cs-137
- B.Ir-192
- C.I-125Correct
- D.Co-60
Answer: C
I-125 has a 59.4-day half-life and ~28 keV average photon energy — the classic permanent prostate seed isotope. (Pd-103, at 17 days, is the other permanent seed option; Ir-192 and Cs-137 are temporary HDR/LDR sources.)
HDR brachytherapy is defined as a dose rate of:
- A.0.5–2 Gy/hour
- B.Greater than 12 Gy/hourCorrect
- C.Less than 0.5 Gy/hour
- D.Exactly 2 Gy/hour
Answer: B
HDR (high dose rate) brachytherapy delivers more than 12 Gy/hour, typically through a remote afterloader. LDR brachytherapy sits at 0.5–2 Gy/hour.
Which dosimeter is best suited to monitoring individual low-energy brachytherapy seeds?
- A.Parallel-plate ionization chamber
- B.Radiographic film
- C.TLDCorrect
- D.Calorimeter
Answer: C
Thermoluminescent dosimeters (TLDs) are sensitive enough to measure the low energies of I-125 and Pd-103 seeds (~21–28 keV) and integrate dose over a tiny volume, making them the standard for individual seed monitoring.
Under 10 CFR 35, before any brachytherapy administration — including permanent seed implants and ophthalmic plaques — there must be:
- A.A verbal order from any physician
- B.A written directive dated and signed by the authorized userCorrect
- C.A nursing documentation entry
- D.Only a physicist's calculation on file
Answer: B
NRC regulations (10 CFR 35) require a written directive, dated and signed by the authorized user, before administration of any brachytherapy dose. A physicist's calculation supports the directive but does not replace it.
Section D — QA & Imaging
Questions 18–20 · answers and explanations included
The TG-51 protocol is used for:
- A.Commissioning a treatment planning system
- B.Absorbed dose to water calibration of megavoltage photon and electron beamsCorrect
- C.Daily output constancy checks with diodes
- D.Brachytherapy source strength specification
Answer: B
AAPM Task Group 51 (TG-51) is the protocol for calibrating megavoltage photon and electron beams — absorbed dose to water — using a calibrated ionization chamber in a water phantom.
On a lung dose-volume histogram, V20 represents:
- A.20% of the prescribed dose delivered to lung
- B.The percentage of lung volume receiving at least 20 Gy — a pneumonitis risk metricCorrect
- C.A 20 cm³ volume of irradiated lung
- D.A mean lung dose of 20 Gy
Answer: B
V20 is the percentage of the total lung volume receiving at least 20 Gy. Higher V20 values correlate strongly with the risk of radiation pneumonitis, which is why it is a standard lung plan-evaluation metric.
A commonly accepted maximum dose constraint for the spinal cord is approximately:
- A.20 Gy
- B.30 Gy
- C.45 GyCorrect
- D.70 Gy
Answer: C
The TD5/5 tolerance for the spinal cord is roughly 45–50 Gy, and myelopathy risk rises sharply above 50 Gy, so planners commonly hold the cord maximum near 45 Gy.
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 its MDCB content outline category and study one category at a time, starting with dosimetry calculations — they carry the most exam weight.
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 and MDCB topic.
Start your free trialGet the printable Dosimetry Cheat Sheet (PDF)
The same free printable dosimetry cheat sheet our students use for monitor-unit math, isotope tables, and plan checks — grab it with your email and it downloads instantly.
MDCB Practice Questions FAQ
How many questions are on the MDCB exam?
The MDCB certification exam contains 155 multiple-choice questions, plus unscored embedded pretest items that are being evaluated for use on future exams. Questions are weighted across content categories — treatment planning, dosimetry calculations, brachytherapy, and QA — just like the practice set on this page.
What is the passing score for the MDCB exam?
The MDCB uses scaled scoring rather than a fixed percentage, so the exact cut score varies by exam form. The best predictor of passing is practicing with questions weighted across the exam categories — start with the 20 questions above, then keep drilling your weakest section.
How long should I study for the MDCB exam?
Most candidates do best with 3–6 months of structured review plus regular timed practice, adjusted for their clinical dosimetry experience. Start your free trial to build the routine around mock exams and mastery tracking.
Is the MDCB exam harder than the ARRT exam?
They test different skill sets rather than one being strictly harder. The ARRT radiation therapy exam is broader across therapy practice, while the MDCB is more calculation- and planning-heavy — expect more monitor-unit math, plan evaluation, and brachytherapy physics. The instructor behind this study suite explains the difference on the about page.
Content reviewed by Kate, a working radiation therapist and instructor — see the full story on the about page.