ABR ABR Imaging Physics and Equipment 3 — Questions and Answers
Question 1: Which quality control test on a linear accelerator uses a Winston-Lutz phantom?
- Isocenter coincidence between radiation and mechanical isocenters (Correct answer)
- Output calibration
- Flatness and symmetry
- Leaf position accuracy of the MLC
Correct answer: Isocenter coincidence between radiation and mechanical isocenters
The Winston-Lutz test verifies that the radiation isocenter coincides with the mechanical isocenter by irradiating a small ball bearing from multiple gantry angles and analyzing the resulting fields.
Question 2: What does the term 'penumbra' refer to in radiation therapy beam physics?
- The region at the beam edge where dose falls rapidly from high to low values (Correct answer)
- The central axis dose maximum
- The low-dose region outside the treatment field
- The dose buildup region near the surface
Correct answer: The region at the beam edge where dose falls rapidly from high to low values
The penumbra is the dose gradient region at the lateral edge of the radiation beam where dose transitions from the high-dose treatment field to the low-dose region outside.
Question 3: In IMRT, what is the purpose of multi-leaf collimator (MLC) leaf sequencing?
- To create complex fluence patterns that conform dose to the tumor while sparing OARs (Correct answer)
- To reduce treatment time
- To improve beam flatness
- To calibrate beam output
Correct answer: To create complex fluence patterns that conform dose to the tumor while sparing OARs
MLC leaf sequencing algorithms determine the pattern of leaf positions and movements needed to deliver the complex fluence distributions calculated by the IMRT optimization algorithm.
Question 4: What is the primary purpose of an electronic portal imaging device (EPID) in radiation therapy?
- Patient positioning verification and dosimetric quality assurance (Correct answer)
- Measuring beam energy spectrum
- Calibrating monitor units
- Detecting patient motion during treatment
Correct answer: Patient positioning verification and dosimetric quality assurance
EPIDs are used for portal imaging to verify patient setup against reference DRRs and increasingly for in-vivo dosimetry to confirm delivered dose matches the treatment plan.
Question 5: In a linear accelerator, what is the function of the bending magnet in the beam transport system?
- Deflects the electron beam to the correct angle and selects electron energy (Correct answer)
- Focuses the electron beam to a fine focal spot
- Accelerates electrons to therapeutic energies
- Generates the microwave power for acceleration
Correct answer: Deflects the electron beam to the correct angle and selects electron energy
The bending magnet in a linac deflects the electron beam (typically 270°) to aim it at the x-ray target and acts as an energy-selecting element to improve beam quality.
Question 6: What ACR/AAPM quality control test for CT scanners evaluates the system's ability to reproduce a known CT number for water?
- CT number accuracy and uniformity test (Correct answer)
- Spatial resolution test
- Noise index measurement
- Slice thickness verification
Correct answer: CT number accuracy and uniformity test
The CT number accuracy test uses a water phantom to verify that the scanner reproduces the expected 0 HU value for water within acceptable tolerances, confirming correct calibration.
ABR ABR Imaging Physics and Equipment 3
Which quality control test on a linear accelerator uses a Winston-Lutz phantom?