ABR ABR Radiation Dosimetry 2 — Questions and Answers
Question 1: What phantom material is required for reference dosimetry measurements per TG-51?
- Liquid water (Correct answer)
- Solid water
- PMMA
- RW3 plastic
Correct answer: Liquid water
TG-51 requires measurements in a liquid water phantom as it is the reference medium for absorbed dose-to-water calibrations.
Question 2: Which of the following best describes the percent depth dose (PDD) curve for a megavoltage photon beam?
- Builds up to a maximum then exponentially decreases (Correct answer)
- Decreases monotonically from the surface
- Remains constant throughout the phantom
- Peaks at the surface then rapidly drops
Correct answer: Builds up to a maximum then exponentially decreases
Megavoltage photon beams exhibit a skin-sparing buildup region before reaching dose maximum (dmax), after which dose decreases with depth due to attenuation.
Question 3: What is the tissue-phantom ratio (TPR) used to characterize in radiation dosimetry?
- Beam quality and off-axis output factors (Correct answer)
- Absolute dose calibration
- Electron contamination
- Brachytherapy source strength
Correct answer: Beam quality and off-axis output factors
The tissue-phantom ratio (TPR) characterizes beam quality and is used to calculate output factors independent of the inverse square law.
Question 4: In electron beam dosimetry, what is the recommended reference depth for calibration per TG-51?
- dref = 0.6·R50 – 0.1 cm (Correct answer)
- dmax
- 5 cm
- 10 cm
Correct answer: dref = 0.6·R50 – 0.1 cm
For electron beams, TG-51 specifies calibration at dref = 0.6·R50 − 0.1 cm, where R50 is the depth of 50% ionization.
Question 5: What is the primary purpose of a farmer chamber's graphite wall in dosimetry?
- To provide charged particle equilibrium (Correct answer)
- To reduce energy dependence
- To shield against neutrons
- To improve spatial resolution
Correct answer: To provide charged particle equilibrium
The graphite wall of a Farmer chamber helps establish charged particle equilibrium and has a water-equivalent mass to minimize perturbation.
Question 6: Which AAPM protocol provides dosimetry recommendations for low-energy photon brachytherapy sources such as 125I seeds?
- TG-43 (Correct answer)
- TG-51
- TG-61
- TG-142
Correct answer: TG-43
AAPM TG-43 and its update TG-43U1 provide the dosimetry formalism for calculating dose distributions around brachytherapy sources including 125I seeds.
ABR ABR Radiation Dosimetry 2
What phantom material is required for reference dosimetry measurements per TG-51?