CMD Treatment Planning & Dosimetry 3 — Questions and Answers
Question 1: The tissue-maximum ratio (TMR) is preferred over the tissue-air ratio (TAR) for high-energy photon beams because:
- TMR is independent of field size
- TMR eliminates the need for inverse square corrections
- TAR requires impractically large phantom measurements at high energies (Correct answer)
- TMR accounts for electron contamination
Correct answer: TAR requires impractically large phantom measurements at high energies
At high photon energies, measuring TAR requires enormous phantoms to achieve electronic equilibrium at depth, making TMR with scatter factors more practical.
Question 2: Which contouring structure type is used in treatment planning to expand a target volume to account for subclinical disease extension?
- Internal Target Volume (ITV)
- Clinical Target Volume (CTV) (Correct answer)
- Planning Target Volume (PTV)
- Gross Tumor Volume (GTV)
Correct answer: Clinical Target Volume (CTV)
The CTV expands the GTV to include areas of suspected microscopic disease extension, representing the true anatomical extent of disease requiring treatment.
Question 3: In 4D CT-based treatment planning for lung cancer, the Internal Target Volume (ITV) is derived by:
- Contouring the tumor on the average intensity projection image only
- Taking the union of GTVs contoured across all respiratory phases (Correct answer)
- Applying a fixed 1.5 cm isotropic expansion to the GTV
- Contouring on peak exhale phase only
Correct answer: Taking the union of GTVs contoured across all respiratory phases
The ITV is the union of all phase-specific GTVs across the breathing cycle, encompassing the full range of tumor motion.
Question 4: A symmetric 4-field box technique for prostate treatment typically uses which beam arrangement?
- AP, PA, right lateral, left lateral (Correct answer)
- AP, right anterior oblique, left anterior oblique, PA
- Two opposed lateral beams only
- AP and PA beams with 30° wedges
Correct answer: AP, PA, right lateral, left lateral
The classic 4-field box for prostate uses anterior-posterior, posterior-anterior, and bilateral lateral beams to distribute dose and spare anterior normal tissue.
Question 5: Which effect describes the increase in relative biological effectiveness (RBE) of radiation as linear energy transfer (LET) increases?
- Oxygen enhancement ratio increases with LET
- RBE peaks around 100 keV/μm LET then decreases
- RBE is constant regardless of LET for photons
- High-LET radiation reduces the oxygen enhancement ratio and increases RBE simultaneously (Correct answer)
Correct answer: High-LET radiation reduces the oxygen enhancement ratio and increases RBE simultaneously
High-LET radiation both increases RBE (more lethal per unit dose) and reduces the oxygen enhancement ratio (less dependent on oxygen for cell kill).
Question 6: For a wedged pair technique, the hinge angle between two beams is related to the wedge angle by which formula?
- Wedge angle = hinge angle
- Wedge angle = 90° - (hinge angle / 2) (Correct answer)
- Wedge angle = hinge angle / 2
- Wedge angle = 180° - hinge angle
Correct answer: Wedge angle = 90° - (hinge angle / 2)
For a wedged pair, the optimal wedge angle equals 90° minus half the hinge angle between the two beams to produce a uniform dose distribution.
Question 7: In brachytherapy, the term 'dose rate constant' (Λ) for a source is defined as the dose rate at:
- 1 cm in water per unit air kerma strength at 1 cm in air (Correct answer)
- The surface of the source per unit activity
- The prescription point per unit reference air kerma
- 1 m in air per unit activity
Correct answer: 1 cm in water per unit air kerma strength at 1 cm in air
The dose rate constant Λ is the dose rate to water at 1 cm from the source per unit air kerma strength (1 U = 1 cGy·cm²/h), linking source strength to dose rate.
CMD Treatment Planning & Dosimetry 3
The tissue-maximum ratio (TMR) is preferred over the tissue-air ratio (TAR) for high-energy photon beams because: