OBT Treatment Planning & Dosimetry 5 — Questions and Answers
Question 1: In a 3D conformal optical beam therapy plan, coplanar beams are defined as beams that:
- All lie within the same transverse plane through the isocenter (Correct answer)
- Share the same collimator angle
- Have identical gantry angles but different couch rotations
- Use the same wedge orientation
Correct answer: All lie within the same transverse plane through the isocenter
Coplanar beams all rotate within the same plane (typically the axial plane through isocenter) with couch rotation at 0°.
Question 2: A treatment plan uses a 30° physical wedge with a wedge transmission factor of 0.75. If the open-field MU calculation yields 200 MU, the MU with the wedge in the beam is:
- 150 MU
- 200 MU
- 267 MU (Correct answer)
- 230 MU
Correct answer: 267 MU
Wedged MU = open-field MU / wedge transmission factor = 200 / 0.75 = 266.7 ≈ 267 MU.
Question 3: The concept of 'field-in-field' (FIF) technique in treatment planning is used to:
- Reduce hot spots within the PTV by adding subfields that selectively block high-dose regions (Correct answer)
- Increase the dose to the PTV margin by adding overlapping segments
- Replace the need for physical wedges in oblique beam arrangements
- Calculate equivalent square fields for irregular apertures
Correct answer: Reduce hot spots within the PTV by adding subfields that selectively block high-dose regions
FIF uses additional smaller segments delivered within the main field to block dose from hot spots, flattening the dose distribution.
Question 4: According to ICRU Report 83 recommendations for IMRT, the prescribed dose should ideally be assigned to the:
- Mean dose of the PTV
- Maximum dose point within the GTV
- Minimum dose to 98% of the PTV (D98)
- Median dose (D50) of the PTV (Correct answer)
Correct answer: Median dose (D50) of the PTV
ICRU Report 83 recommends prescribing to the median dose (D50) of the PTV as the reference dose for IMRT planning.
Question 5: When a treatment plan shows a large gradient between the PTV and a nearby serial OAR such as the optic nerve, the planner should prioritize:
- Maximizing dose homogeneity within the PTV
- Using steep dose gradients and sharp beam penumbra to rapidly drop dose outside the PTV (Correct answer)
- Increasing the number of beams to spread low dose over more normal tissue
- Raising the prescription dose to improve tumor control probability
Correct answer: Using steep dose gradients and sharp beam penumbra to rapidly drop dose outside the PTV
Achieving a steep dose gradient adjacent to serial OARs requires sharp penumbra and optimized beam geometry to protect the structure.
Question 6: The radiobiological concept used in treatment planning to compare different fractionation schedules is:
- Linear-quadratic (LQ) model and biologically effective dose (BED) (Correct answer)
- Tissue-maximum ratio (TMR) and scatter-air ratio (SAR)
- Percent depth dose (PDD) and output factor
- Equivalent square field and inverse-square law
Correct answer: Linear-quadratic (LQ) model and biologically effective dose (BED)
The LQ model calculates BED, enabling comparison of the biological effect of different dose-per-fraction and fractionation schedules.
Question 7: A systematic error in patient setup during treatment planning simulation will most likely result in:
- Random variation in daily isocenter position
- A consistent offset of the delivered dose distribution from the planned distribution across all fractions (Correct answer)
- Increased dose to the PTV due to overshoot
- Unpredictable hot spots in the dose distribution
Correct answer: A consistent offset of the delivered dose distribution from the planned distribution across all fractions
Systematic errors shift the entire dose distribution by a fixed amount in the same direction every fraction, causing consistent geographic miss.
OBT Treatment Planning & Dosimetry 5
In a 3D conformal optical beam therapy plan, coplanar beams are defined as beams that: