ABR ABR Imaging Physics and Equipment 3

Question 1

Which quality control test on a linear accelerator uses a Winston-Lutz phantom?

Accepted Answer

Isocenter coincidence between radiation and mechanical isocenters

Answer

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?

Accepted Answer

The region at the beam edge where dose falls rapidly from high to low values

Answer

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?

Accepted Answer

To create complex fluence patterns that conform dose to the tumor while sparing OARs

Answer

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?

Accepted Answer

Patient positioning verification and dosimetric quality assurance

Answer

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?

Accepted Answer

Deflects the electron beam to the correct angle and selects electron energy

Answer

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.

ABR - American Board of Radiology Medical Physics Exam Practice Test

ABR - American Board of Radiology Medical Physics Exam Practice Test

ABR ABR Imaging Physics and Equipment 3