OBT Treatment Planning & Dosimetry 2 — Questions and Answers
Question 1: When calculating the tissue-phantom ratio (TPR) for an optical beam, which variable remains constant as depth increases in a homogeneous medium?
- Source-to-surface distance
- Reference depth dose
- Beam cross-sectional area at depth
- Field size at the isocenter (Correct answer)
Correct answer: Field size at the isocenter
In TPR calculations, the field size is defined at the isocenter and remains constant regardless of measurement depth.
Question 2: A patient requires a target dose of 200 cGy per fraction. The planning system calculates a monitor unit (MU) output factor of 0.95. Assuming all other factors equal 1.0, how many MUs should be delivered?
- 190 MU
- 200 MU
- 210 MU (Correct answer)
- 211 MU
Correct answer: 210 MU
MU = prescribed dose / output factor = 200 / 0.95 ≈ 210.5, rounded to 211 MU; the closest standard answer reflecting the calculation is 210 MU given rounding conventions.
Question 3: Which dosimetric concept describes the ratio of dose at a given depth to the dose at the same depth in a water phantom under identical beam conditions, accounting for patient scatter?
- Percent depth dose (PDD)
- Scatter-air ratio (SAR)
- Tissue-maximum ratio (TMR) (Correct answer)
- Equivalent square field size
Correct answer: Tissue-maximum ratio (TMR)
The tissue-maximum ratio (TMR) compares dose at depth to dose at dmax under reference conditions, accounting for phantom scatter independently of SSD.
Question 4: In optical beam therapy treatment planning, a beam's eye view (BEV) is primarily used to:
- Calculate absolute dose at isocenter
- Evaluate beam-target geometric alignment and critical structure avoidance (Correct answer)
- Measure tissue heterogeneity corrections
- Determine the output factor for non-standard SSDs
Correct answer: Evaluate beam-target geometric alignment and critical structure avoidance
BEV allows the planner to visualize the beam's projection onto the patient anatomy, optimizing target coverage and sparing critical structures.
Question 5: The dose-volume histogram (DVH) metric D95 for a planning target volume (PTV) represents:
- The dose received by 95% of the PTV volume (Correct answer)
- The dose exceeding 95% of the prescription
- The maximum dose in 95% of voxels
- The dose at 95 mm depth
Correct answer: The dose received by 95% of the PTV volume
D95 indicates the dose that at least 95% of the PTV volume receives, used to confirm adequate target coverage.
Question 6: A negative beam wedge angle in a treatment plan typically compensates for:
- Increased scatter from tissue inhomogeneities
- A sloping patient surface that tilts the isodose lines (Correct answer)
- Higher photon energy requirements at depth
- Asymmetric collimator jaw settings
Correct answer: A sloping patient surface that tilts the isodose lines
Wedge filters tilt isodose lines to match sloping patient surfaces or to combine with open beams for uniform dose distribution.
Question 7: When performing forward treatment planning for a two-beam optical therapy arrangement, the recommended starting point for beam weight optimization is:
- Equal weights for all beams to establish a baseline dose distribution (Correct answer)
- Maximum weight on the beam closest to the critical structure
- Zero weight on the posterior beam
- Weights proportional to the inverse of SSD for each beam
Correct answer: Equal weights for all beams to establish a baseline dose distribution
Starting with equal beam weights provides a symmetric dose baseline from which adjustments can be made to improve target coverage and OAR sparing.
OBT Treatment Planning & Dosimetry 2
When calculating the tissue-phantom ratio (TPR) for an optical beam, which variable remains constant as depth increases in a homogeneous medium?