Brachytherapy MCQ practice paper

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Brachytherapy

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Radioisotopes & Physics
1. Which radioisotope is most commonly used for High Dose Rate (HDR) brachytherapy?
  • A) Iodine-125
  • B) Cesium-137
  • C) Iridium-192
  • D) Palladium-103
Reveal Answer
Correct: C) Iridium-192 Ir-192 is the standard for HDR due to its high specific activity and relatively short half-life.
2. What is the approximate half-life of Iridium-192?
  • A) 17 days
  • B) 60 days
  • C) 74 days
  • D) 30 years
Reveal Answer
Correct: C) 74 days Specifically 73.8 days.
3. Which isotope is commonly used for permanent prostate seed implants (LDR) and has a half-life of approx. 60 days?
  • A) Palladium-103
  • B) Iodine-125
  • C) Gold-198
  • D) Cesium-131
Reveal Answer
Correct: B) Iodine-125 I-125 (59.4 days) is a gold standard for LDR prostate.
4. Palladium-103 (Pd-103) emits photons with an average energy of approximately:
  • A) 21 keV
  • B) 28 keV
  • C) 662 keV
  • D) 1.25 MeV
Reveal Answer
Correct: A) 21 keV This low energy limits the range, making it ideal for aggressive tumors.
5. The average energy of Iridium-192 gamma rays is:
  • A) 28 keV
  • B) 380 keV
  • C) 662 keV
  • D) 1.25 MeV
Reveal Answer
Correct: B) ~380 keV 0.38 MeV is the commonly cited average energy.
6. Which brachytherapy source has the longest half-life?
  • A) Cobalt-60
  • B) Cesium-137
  • C) Iridium-192
  • D) Iodine-125
Reveal Answer
Correct: B) Cesium-137 30 years. (Cobalt-60 is 5.26 years).
7. In the TG-43 formalism, source strength is specified by which quantity?
  • A) Apparent Activity (mCi)
  • B) Air Kerma Strength (Sk)
  • C) Exposure Rate Constant
  • D) Absorbed Dose to Water
Reveal Answer
Correct: B) Air Kerma Strength (Sk) Measured in units of 'U'.
8. One "mgRaEq" (milligram Radium equivalent) corresponds to a filtration of:
  • A) 0.5 mm Pt
  • B) 1.0 mm Al
  • C) 0.5 mm Cu
  • D) 1.0 mm Pb
Reveal Answer
Correct: A) 0.5 mm Pt This is the historical standard for Radium needles.
9. The exposure rate constant for a point source of Radium-226 filtered by 0.5 mm Pt is approximately:
  • A) 2.3 R-cm²/mCi-hr
  • B) 4.6 R-cm²/mCi-hr
  • C) 8.25 R-cm²/mg-hr
  • D) 12.8 R-cm²/mg-hr
Reveal Answer
Correct: C) 8.25 R-cm²/mg-hr This is a classic value to memorize for boards/exams.
10. Which isotope decays via Electron Capture?
  • A) Cobalt-60
  • B) Iodine-125
  • C) Cesium-137
  • D) Strontium-90
Reveal Answer
Correct: B) Iodine-125 It decays to Tellurium-125.
Dosimetry Systems
11. The Manchester (Paterson-Parker) system is designed to achieve:
  • A) Uniform distribution of sources
  • B) Uniform dose distribution (± 10%)
  • C) Maximum dose at the center
  • D) A conical dose distribution
Reveal Answer
Correct: B) Uniform dose distribution Achieved by using non-uniform source placement.
12. In the Manchester system (area < 25 cm²), how is radium distributed?
  • A) 100% in the center
  • B) 67% (2/3) on periphery, 33% (1/3) in center
  • C) 50% periphery, 50% center
  • D) Uniformly spaced
Reveal Answer
Correct: B) 2/3 periphery, 1/3 center This compensates for the lack of scatter from outside the implant.
13. Which system uses uniform source distribution resulting in a non-uniform (hot center) dose?
  • A) Manchester System
  • B) Paris System
  • C) Quimby System
  • D) ICRU System
Reveal Answer
Correct: C) Quimby System The "Quimby" system is characterized by physical uniformity of sources.
14. The Paris System was developed specifically for which type of sources?
  • A) Radium-226 needles
  • B) Iridium-192 wires/hairpins
  • C) Iodine-125 seeds
  • D) Cobalt-60 spheres
Reveal Answer
Correct: B) Iridium-192 wires Used historically for breast and head & neck implants.
15. In the Manchester cervix system, Point A is defined as:
  • A) 2 cm superior to external os, 2 cm lateral to central canal
  • B) 5 cm lateral to the midline
  • C) On the surface of the ovoids
  • D) At the pelvic sidewall
Reveal Answer
Correct: A) 2 cm up, 2 cm lateral It represents the crossing of the uterine artery and ureter.
16. What does Point B represent in the Manchester cervix system?
  • A) The dose to the bladder
  • B) The dose to the rectum
  • C) The dose to the pelvic lymph nodes
  • D) The surface dose of the cervix
Reveal Answer
Correct: C) Pelvic lymph nodes Specifically the obturator nodes (3 cm lateral to Point A).
17. In the Paris system, the "Basal Dose Rate" is calculated as:
  • A) Average of the source centers
  • B) Average of the minimum dose rates between sources
  • C) Dose rate at 1 cm from center
  • D) Maximum surface dose
Reveal Answer
Correct: B) Average of local minimums It uses the minima between radioactive lines.
18. Which rule is a requirement for the Paris System?
  • A) Sources must be perpendicular
  • B) Sources must be parallel and equidistant
  • C) Sources must be in a circle
  • D) Peripheral loading is required
Reveal Answer
Correct: B) Parallel and equidistant This is essential for the volume definitions in the Paris system.
19. If source active length doubles (constant activity per length), the dose at a distant point on the perpendicular bisector:
  • A) Remains the same
  • B) Decreases by half
  • C) Increases by approx. factor of 2
  • D) Increases by factor of 4
Reveal Answer
Correct: C) Increases by approx. factor of 2 Because the total activity has doubled.
20. The "elongation factor" is used in which system to correct for non-square shapes?
  • A) Paris
  • B) Manchester (Paterson-Parker)
  • C) Quimby
  • D) TG-43
Reveal Answer
Correct: B) Manchester system Used when the area to be treated is rectangular rather than square.
TG-43 Formalism
21. In the TG-43 equation, what does Λ (Lambda) represent?
  • A) Radial Dose Function
  • B) Anisotropy Function
  • C) Dose Rate Constant
  • D) Geometry Factor
Reveal Answer
Correct: C) Dose Rate Constant It converts Air Kerma Strength to Dose Rate in water at the reference point.
22. The Geometry Function G(r, θ) in TG-43 accounts for:
  • A) Scattering in the tissue
  • B) Attenuation in the capsule
  • C) Spatial distribution of activity (Geometric fall-off)
  • D) Isotope decay
Reveal Answer
Correct: C) Spatial distribution / Geometric fall-off Essentially the inverse square law correction for the specific source shape.
23. The Anisotropy Function F(r, θ) corrects for:
  • A) Attenuation by source encapsulation
  • B) Radial fall-off in water
  • C) Decay of the source
  • D) Patient inhomogeneity
Reveal Answer
Correct: A) Attenuation by source encapsulation Dose varies with angle due to self-absorption in the metal seed/wire.
24. What is the unit of Air Kerma Strength (Sk)?
  • A) Gy/hr
  • B) Ci
  • C) µGy · m² · h⁻¹ (or U)
  • D) Bq · s⁻¹
Reveal Answer
Correct: C) µGy · m² · h⁻¹ This unit is also known as 'U'.
25. The Radial Dose Function g(r) accounts for:
  • A) Inverse square law only
  • B) Absorption and scattering in the medium
  • C) Anisotropy of the source
  • D) Source geometry
Reveal Answer
Correct: B) Absorption and scattering in the medium It describes how dose falls off in tissue/water along the transverse axis.
26. In TG-43, the reference point is usually defined at:
  • A) 1 cm, 0 degrees
  • B) 1 cm, 90 degrees (transverse axis)
  • C) 5 cm, 90 degrees
  • D) Surface of the source
Reveal Answer
Correct: B) 1 cm, 90 degrees This is the standard calibration point relative to the source center.
27. TG-43 calculations assume the surrounding medium is:
  • A) Air
  • B) Infinite homogeneous water phantom
  • C) Heterogeneous tissue
  • D) Lead shielding
Reveal Answer
Correct: B) Infinite homogeneous water phantom It does not account for tissue heterogeneity (like bone or air cavities).
28. Which parameter is NOT required for a point-source approximation in TG-43?
  • A) Anisotropy Function F(r, θ)
  • B) Geometry Function G(r, θ)
  • C) Air Kerma Strength
  • D) Active length
Reveal Answer
Correct: A) Anisotropy Function In a simple point check, anisotropy is often assumed to be 1.0 or averaged.
29. Which radioisotope has the steepest dose fall-off (lowest energy) used in permanent implants?
  • A) Ir-192
  • B) Cs-137
  • C) Pd-103
  • D) I-125
Reveal Answer
Correct: C) Pd-103 21 keV average energy provides a very rapid dose fall-off.
30. Why is water used as the reference medium in TG-43?
  • A) Easy to obtain
  • B) Similar absorption to soft tissue
  • C) Does not boil easily
  • D) Regulatory requirement
Reveal Answer
Correct: B) Similar absorption to soft tissue It is the universal standard for radiation dosimetry.
Level 4: Clinical & Delivery
31. What is the primary advantage of HDR over LDR brachytherapy?
  • A) Better biology
  • B) Reduced staff exposure / outpatient capability
  • C) Cheaper equipment
  • D) Longer lived isotopes
Reveal Answer
Correct: B) Reduced staff exposure / outpatient Remote afterloading means staff are not in the room during treatment.
32. In LDR prostate brachytherapy, "Post-implant dosimetry" is typically performed:
  • A) Immediately in OR
  • B) ~30 days (4 weeks) after implant
  • C) 1 year after
  • D) Never
Reveal Answer
Correct: B) ~30 days after implant This allows implant edema (swelling) to resolve for accurate CT calculation.
33. Which applicator is commonly used for vaginal vault cuff recurrence?
  • A) Fletcher-Suit
  • B) Vaginal Cylinder
  • C) Tandem and Ring
  • D) Syed-Neblett
Reveal Answer
Correct: B) Vaginal Cylinder Provides a uniform surface dose to the vaginal mucosa.
34. The "Tandem and Ovoid" applicator is primarily used for cancer of the:
  • A) Prostate
  • B) Breast
  • C) Cervix
  • D) Esophagus
Reveal Answer
Correct: C) Cervix The tandem treats the uterus, ovoids treat the fornices/cervix face.
35. An "Interstitial" implant implies:
  • A) Sources inside a cavity
  • B) Sources placed directly into tissue
  • C) Sources on skin surface
  • D) Sources in blood vessels
Reveal Answer
Correct: B) Sources placed directly into tissue Example: Prostate seeds or breast catheters.
36. Which modality has a dose rate effect that spares normal tissue the most (repair of sublethal damage)?
  • A) HDR
  • B) LDR
  • C) External Beam
  • D) Electronic Brachy
Reveal Answer
Correct: B) LDR Continuous low dose irradiation allows healthy cells to repair damage during treatment.
37. High Dose Rate (HDR) is generally defined as a dose rate greater than:
  • A) 2 Gy/hr
  • B) 5 Gy/hr
  • C) 12 Gy/hr
  • D) 100 Gy/hr
Reveal Answer
Correct: C) 12 Gy/hr Or 0.2 Gy/min (ICRU definition).
38. A common prescription dose for monotherapy LDR prostate brachytherapy using I-125 is:
  • A) 45 Gy
  • B) 85 Gy
  • C) 145 Gy
  • D) 200 Gy
Reveal Answer
Correct: C) 145 Gy TG-43 standard prescription.
39. Electronic Brachytherapy (eBx) typically uses:
  • A) Ir-192 source
  • B) Miniature X-ray source (50 kVp)
  • C) Co-60 source
  • D) Proton beam
Reveal Answer
Correct: B) Miniature X-ray source It can be turned on and off, unlike isotopes.
40. Which organ is the primary "Organ at Risk" (OAR) immediately anterior to the cervix?
  • A) Rectum
  • B) Bladder
  • C) Sigmoid
  • D) Urethra
Reveal Answer
Correct: B) Bladder The rectum is posterior.
Safety & QA
41. A patient with a permanent implant can be released if the effective dose to a public member is less than:
  • A) 1 mSv
  • B) 5 mSv (0.5 rem)
  • C) 50 mSv
  • D) 0 mSv
Reveal Answer
Correct: B) 5 mSv (0.5 rem) NRC regulation for patient release.
42. The leak test for a sealed source must be capable of detecting:
  • A) 0.005 µCi
  • B) 0.05 µCi
  • C) 0.1 µCi
  • D) 1.0 µCi
Reveal Answer
Correct: A) 0.005 µCi (185 Bq) Standard threshold for leakage.
43. How often must HDR unit timer accuracy and linearity be checked?
  • A) Daily
  • B) Monthly
  • C) Annually
  • D) Only at commissioning
Reveal Answer
Correct: A) Daily Part of the morning QA routine.
44. What is the most critical safety device in an HDR treatment room?
  • A) Video Monitor
  • B) Independent Radiation Monitor (Zone Monitor)
  • C) Intercom
  • D) Patient restraints
Reveal Answer
Correct: B) Independent Radiation Monitor It tells you if the source is exposed, even if the console says it is safe.
45. If an HDR source fails to retract (stuck source), what is the FIRST action?
  • A) Run into the room
  • B) Press the Emergency Stop button
  • C) Call manufacturer
  • D) Manually crank
Reveal Answer
Correct: B) Press the Emergency Stop button This cuts power to the motor (often allowing spring return).
46. A "Well Chamber" (Re-entrant Ion Chamber) is used for:
  • A) Measuring patient dose
  • B) Calibrating source strength (Sk)
  • C) Room background
  • D) Detecting leaks
Reveal Answer
Correct: B) Calibrating source strength Standard tool for brachytherapy physics.
47. According to AAPM, HDR source output should be calibrated by a physicist:
  • A) Every treatment
  • B) Upon source exchange
  • C) Once a year
  • D) Only when timer error is high
Reveal Answer
Correct: B) Upon source exchange Usually every 3-4 months for Ir-192.
48. Which instrument is best for locating a lost radioactive seed in the OR?
  • A) Ion chamber
  • B) Geiger-Muller (GM) counter
  • C) TLD
  • D) Well chamber
Reveal Answer
Correct: B) Geiger-Muller (GM) counter Due to its high sensitivity to find small sources.
49. The transportation index (TI) on a radioactive package indicates:
  • A) Activity in Curies
  • B) Dose rate in mrem/hr at 1 meter
  • C) Surface dose rate
  • D) Half-life
Reveal Answer
Correct: B) Dose rate in mrem/hr at 1 meter Used for shipping and handling safety.
50. Instructions must be given to a permanent implant patient if the dose rate at 1 meter exceeds:
  • A) 1 mrem/hr
  • B) 2 mrem/hr
  • C) 5 mrem/hr
  • D) 10 mrem/hr
Reveal Answer
Correct: A) 1 mrem/hr (0.01 mSv/hr).

End of Quiz. Great job practicing!

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