ABR Quality Assurance and Quality Control 2 — Questions and Answers
Question 1: The gamma (γ) index used in IMRT patient-specific QA combines which two criteria?
- Dose difference and distance-to-agreement (Correct answer)
- Dose rate and monitor unit accuracy
- Beam energy and field size
- Flatness and symmetry
Correct answer: Dose difference and distance-to-agreement
The gamma index simultaneously evaluates dose difference (DD) and distance-to-agreement (DTA) to provide a single pass/fail metric for IMRT QA comparisons.
Question 2: AAPM TG-119 was developed primarily to establish:
- Commissioning benchmarks and test cases for IMRT planning and delivery (Correct answer)
- QA protocols for CT simulators
- Acceptance testing procedures for new linear accelerators
- Dosimetry standards for brachytherapy sources
Correct answer: Commissioning benchmarks and test cases for IMRT planning and delivery
TG-119 provides IMRT commissioning test cases with known dose distributions to help institutions benchmark their IMRT planning and delivery systems.
Question 3: Which AAPM report provides QA guidelines specifically for CT simulators used in radiation therapy?
- TG-66 (Correct answer)
- TG-142
- TG-40
- TG-51
Correct answer: TG-66
AAPM TG-66 addresses the quality assurance of CT simulators and establishes acceptance testing and periodic QA procedures for these devices.
Question 4: An EPID (Electronic Portal Imaging Device) is most commonly used in linac QA for:
- Absolute dose calibration at isocenter
- Patient position verification and transit dosimetry (Correct answer)
- Measuring electron beam percent depth dose
- Commissioning photon beam energy
Correct answer: Patient position verification and transit dosimetry
EPIDs are primarily used for patient setup verification via portal imaging and for transit dosimetry to detect delivery errors during or after treatment.
Question 5: What is the recommended passing criteria (gamma pass rate) for patient-specific IMRT QA according to AAPM TG-218?
- 90% of points with γ(3%/3mm) ≤1
- 95% of points with γ(3%/3mm) ≤1 (Correct answer)
- 95% of points with γ(2%/2mm) ≤1
- 99% of points with γ(5%/5mm) ≤1
Correct answer: 95% of points with γ(3%/3mm) ≤1
TG-218 recommends a universal tolerance of ≥95% passing rate using 3%/3 mm criteria with a 10% dose threshold for IMRT patient-specific QA.
Question 6: Machine-Specific Reference Fields (MSRF) are used in small-field dosimetry primarily because:
- They allow direct comparison between institutions without corrections
- Standard 10×10 cm² fields cannot be produced by some small-field devices like CyberKnife or MRI-linacs (Correct answer)
- They eliminate the need for ionization chamber measurements
- They define the calibration field for all electron beams
Correct answer: Standard 10×10 cm² fields cannot be produced by some small-field devices like CyberKnife or MRI-linacs
Some specialized machines (CyberKnife, Gamma Knife, MRI-linacs) cannot produce a 10×10 cm² reference field, so an MSRF of the largest available field is used as the reference instead.
Question 7: Which detector type is generally preferred for small-field dosimetry (fields < 3×3 cm²) due to its small sensitive volume and minimal perturbation?
- Farmer-type ionization chamber
- Diode or diamond detector (Correct answer)
- Large parallel-plate chamber
- Thermoluminescent dosimeter (TLD) powder
Correct answer: Diode or diamond detector
Diode and diamond detectors have very small sensitive volumes (~1 mm³), making them well-suited for small-field dosimetry where volume-averaging effects with large chambers cause errors.
ABR Quality Assurance and Quality Control 2
The gamma (γ) index used in IMRT patient-specific QA combines which two criteria?