ARRT exam prep
ARRT Brachytherapy Practice Questions: Free 20-Question Set
Brachytherapy questions reward precision: dose rate definitions, isotope half-lives, implant systems, afterloader workflow, and the NRC rules that govern every administration. These 20 questions cover all of it, and every answer includes a full explanation of the reasoning.
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Section A — Dose Rates & Techniques
Questions 1–4 · answers and explanations included
Low dose rate (LDR) brachytherapy is defined by dose rates of approximately:
- A.0.5–2 Gy/hourCorrect
- B.2–10 Gy/hour
- C.Greater than 12 Gy/hour
- D.Less than 0.1 Gy/hour
Answer: A
LDR delivers 0.5–2 Gy per hour, historically through temporary implants and today through permanent seed implants such as I-125. HDR, by contrast, exceeds 12 Gy per hour through a remote afterloader.
High dose rate (HDR) brachytherapy delivers dose at rates:
- A.Below 0.5 Gy/hour
- B.Of 0.5–2 Gy/hour
- C.Greater than 12 Gy/hourCorrect
- D.Exactly 5 Gy/hour
Answer: C
HDR is defined as greater than 12 Gy per hour, typically delivered in minutes by a stepping Ir-192 source in a remote afterloader. The LDR band (0.5–2 Gy/hour) is the standard distractor on this question.
Radioactive sources placed directly within tissue using hollow needles or catheters describe which type of brachytherapy?
- A.Intracavitary
- B.InterstitialCorrect
- C.Surface mold
- D.Intraluminal only
Answer: B
Interstitial brachytherapy places sources inside the tissue itself (needles, seeds, catheters); intracavitary places applicators in a body cavity such as the uterus or vagina; and surface or mold techniques rest sources on the skin surface.
Pulsed dose rate (PDR) brachytherapy is designed to:
- A.Replace surgery in gynecologic cancer
- B.Use an HDR source stepping in short pulses to mimic the radiobiology of continuous LDRCorrect
- C.Deliver a single fraction at a very low dose rate
- D.Reduce total treatment time below that of standard HDR
Answer: B
PDR delivers small hourly pulses from a single HDR source, reproducing the radiobiological effect of continuous low dose rate irradiation while keeping the safety and controls of a remote afterloader.
Section B — Isotopes & Decay
Questions 5–8 · answers and explanations included
The isotope used for permanent prostate seed implants, with a 59.4-day half-life and approximately 28 keV photon energy, is:
- A.Cs-137
- B.Ir-192
- C.I-125Correct
- D.Co-60
Answer: C
Iodine-125 (59.4-day half-life, ~28 keV) is the classic permanent prostate seed isotope; Pd-103 (17 days) is the other permanent option. Ir-192 and Cs-137 are temporary implant sources.
The workhorse isotope of HDR remote afterloaders, with a 74-day half-life and approximately 380 keV average energy, is:
- A.I-125
- B.Ir-192Correct
- C.Cs-137
- D.Ra-226
Answer: B
Iridium-192 combines a high specific activity with a manageable 74-day half-life, making it the standard HDR stepping source. Its replacement cycle is driven by that half-life.
Cesium-137 is characterized by:
- A.A 74-day half-life
- B.A 30-year half-life, used historically for low dose rate gynecologic implantsCorrect
- C.A 17-day half-life
- D.An average energy of 28 keV
Answer: B
Cs-137 decays with a roughly 30-year half-life at about 0.662 MeV and was the LDR workhorse for gynecologic implants before Ir-192 HDR afterloaders became standard. The 74-day half-life belongs to Ir-192.
The decay constant λ is related to half-life (T½) by:
- A.λ = ln(2)/T½, so the decay factor over time t is exp(−λt)Correct
- B.λ = T½/ln(2)
- C.λ = 2 × T½
- D.λ = exp(−T½)
Answer: A
Decay is exponential: λ = 0.693/T½, and source strength at time t equals the initial strength times exp(−λt). This is the calculation behind every decay-correction question.
Section C — Applicators & Implant Systems
Questions 9–12 · answers and explanations included
The classic intracavitary applicator for cervical cancer, consisting of a central tube and lateral ovoids, is the:
- A.Fletcher-Suit tandem and ovoidsCorrect
- B.Paris applicator
- C.Manchester needle system
- D.MUPIT template
Answer: A
The Fletcher-Suit (Fletcher-Suit-Delclos) applicator pairs a central tandem with lateral ovoids to dose the cervix while shielding the bladder and rectum. The Paris system, by contrast, is a rules-based interstitial system.
Point A is defined relative to the cervical tandem as:
- A.2 cm superior to the cervical os and 2 cm lateral to the tandemCorrect
- B.5 cm lateral to the midline at the level of the pubis
- C.1 cm inferior to the vaginal fornix
- D.At the geometric center of the bladder
Answer: A
Point A sits 2 cm above the external os and 2 cm lateral to the midline of the tandem — the classic prescription point in gynecologic intracavitary implants. Point B, 3 cm further lateral, tracks the parametrial and nodal region.
The Paterson-Parker (Manchester) system is characterized by:
- A.Source arrangement rules designed to produce a uniform dose across the implant volumeCorrect
- B.A dose specification based only on the midpoint between sources
- C.Exclusively intraluminal source placement
- D.Use of unfiltered cobalt sources
Answer: A
The Manchester system prescribes spacing and arrangement rules for needles or seeds so the implant delivers a homogeneous dose across the treated volume — uniformity is its defining goal.
The Paris system for interstitial implants specifies dose based on:
- A.The maximum dose at each source surface
- B.The basal dose — the midpoint dose in the central plane of the sources — with linear source arrangement rulesCorrect
- C.The skin surface dose
- D.A single point 2 cm from each source line
Answer: B
The Paris system defines a basal dose from the midpoints between source lines in the central plane and prescribes a percentage of it, using strict rules on line length, spacing, and crossing. It is the counterpart to Manchester's uniformity rules.
Section D — Treatment Delivery & Afterloaders
Questions 13–16 · answers and explanations included
Uveal (intraocular) melanoma is commonly treated with:
- A.An episcleral plaque holding radioactive seeds, sutured to the eyeCorrect
- B.Permanent I-125 interstitial seeds throughout the orbit
- C.External beam photons alone
- D.An intracavitary tandem insertion
Answer: A
Eye plaques (episcleral brachytherapy, commonly I-125 or Ru-106 seeds) are sutured over the tumor base and removed after the prescribed dose — the standard eye-conserving treatment for uveal melanoma.
In a remote afterloader HDR treatment, the correct workflow is:
- A.The source stays connected to the patient all day
- B.Applicators are placed and dressed first; the patient connects to the afterloader only at treatment time, and staff leave the room during deliveryCorrect
- C.The physicist remains in the room for the entire HDR delivery
- D.The patient operates the afterloader remotely from the console
Answer: B
Applicator insertion happens without the source present; the transfer tubes connect at treatment time and everyone exits before the source steps out. This workflow is what keeps staff dose negligible in HDR.
The dose distribution very close to a single brachytherapy seed is dominated by:
- A.Tissue heterogeneity corrections
- B.Inverse square falloff, with anisotropy making the ends of the seed cooler than its sidesCorrect
- C.A buildup region identical to megavoltage photons
- D.Charged particle equilibrium at every point
Answer: B
At centimeter distances and below, the 1/d² term dominates dose gradients, and the seed's encapsulation and self-absorption create anisotropy — the ends receive less dose than the sides of the seed.
A seed strength deviation is discovered after a permanent implant. The correct response is to:
- A.Ignore it if only one seed is affected
- B.Evaluate against the reportable-event thresholds and notify the radiation safety officer (RSO), documenting the eventCorrect
- C.Return the patient to the operating room to remove the implanted seeds
- D.Re-implant immediately without notifying anyone
Answer: B
Regulations define thresholds (in total activity and dose deviation) above which a post-implant discrepancy is reportable. The response is evaluation against those thresholds, RSO notification, and documentation — never improvised correction.
Section E — Regulations & Source Safety
Questions 17–20 · answers and explanations included
An Ir-192 source was calibrated 30 days ago. To determine the source strength for today's treatment, the physicist:
- A.Uses the certificate value unchanged, since Ir-192 lasts 74 days
- B.Applies the decay factor exp(−λt), using the 74-day half-life to correct the certificate valueCorrect
- C.Adds a 10% safety margin to the certificate value
- D.Recalibrates the source inside the patient treatment room
Answer: B
Source strength between calibration and use is corrected by the exponential decay factor exp(−λt) with λ = ln(2)/T½. For Ir-192, 30 days of decay reduces strength by roughly 25% — never ignored in HDR calculations.
Under 10 CFR 35, a written directive for any brachytherapy administration must be:
- A.A verbal order documented within 30 days
- B.Dated and signed by the authorized user before administrationCorrect
- C.Completed by the radiation therapist after the procedure
- D.An unsigned physics calculation worksheet
Answer: B
NRC regulations require a written directive, dated and signed by the authorized user, before any brachytherapy dose — including seeds and ophthalmic plaques. A physicist's worksheet supports the directive but never replaces it.
The three pillars of occupational dose reduction around radioactive sources are:
- A.Time, distance, and shieldingCorrect
- B.Filtration, collimation, and monitoring
- C.Decay, containment, and burial
- D.Rotation, attenuation, and ventilation
Answer: A
Minimize time near the source, maximize distance (inverse square is powerful near small sources), and use shielding where practical. These same three principles anchor every radiation protection question.
An HDR source becomes stuck in an applicator during a treatment. The first response is to:
- A.Withdraw the source by hand using improvised tools
- B.Move the patient to a shielded or control area, follow the facility's emergency procedures, and notify the RSO and medical physicistCorrect
- C.Restart the treatment unit immediately
- D.Send the patient home and call the vendor the next morning
Answer: B
A stuck source is a radiological emergency handled by procedure: get the patient away from the source into a shielded area, follow the emergency steps, and bring in the RSO and physicist. Never improvise with tools — that risks both dose and source integrity.
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 — dose rates and techniques, isotopes and decay, applicator systems, afterloader workflow, and regulations — and study one category at a time, starting with your weakest.
Get the printable Dosimetry Cheat Sheet (PDF)
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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 Brachytherapy Practice Questions FAQ
What is brachytherapy?
Brachytherapy places a sealed radioactive source in or next to the tumor, delivering a very high dose to a small volume while sparing surrounding healthy tissue. Common sites include the prostate (permanent seeds), gynecologic tumors (tandem and ovoid applicators), and skin or eye lesions.
Does brachytherapy hurt?
Applicator or needle placement can be uncomfortable and is usually done with anesthesia or sedation; the radiation itself is painless. HDR treatments through a temporary applicator take minutes and feel like nothing more than lying still.
What is the difference between LDR and HDR brachytherapy?
LDR delivers 0.5–2 Gy per hour, often with permanent seeds or low dose rate implants, while HDR delivers more than 12 Gy per hour through a remote afterloader in a few short sessions. The two differ in isotopes, workflow, shielding requirements, and radiobiological trade-offs.
Is the patient radioactive after brachytherapy?
With permanent seed implants (I-125 or Pd-103), small amounts of low-energy radiation remain in the body, with exposure concerns limited to very close, prolonged contact — patients receive written precautions. Temporary implants (Ir-192, Cs-137) are removed before discharge, so no radioactive material stays in the body.
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