OBT Treatment Planning & Dosimetry 4 — Questions and Answers
Question 1: In optical beam therapy, the output factor (Sc,p) is the product of which two components?
- Collimator scatter factor (Sc) and phantom scatter factor (Sp) (Correct answer)
- Tissue-maximum ratio and percent depth dose
- Wedge transmission factor and tray factor
- Monitor unit conversion factor and inverse-square correction
Correct answer: Collimator scatter factor (Sc) and phantom scatter factor (Sp)
The total output factor Sc,p combines the collimator scatter factor (head scatter) and the phantom scatter factor (in-patient scatter).
Question 2: A dosimetrist is planning treatment at an extended SSD of 120 cm instead of the reference 100 cm. The appropriate correction factor to apply to the dose calculation is:
- (100/120)^2 (Correct answer)
- (120/100)^2
- (100/120)
- (120/100)
Correct answer: (100/120)^2
The inverse-square law requires multiplying by (reference SSD / treatment SSD)^2 to account for increased distance.
Question 3: Which treatment planning system (TPS) dose calculation algorithm is considered the gold standard for accuracy in heterogeneous tissues?
- Pencil beam convolution
- Ray-tracing algorithm
- Monte Carlo simulation (Correct answer)
- Clarkson sector integration
Correct answer: Monte Carlo simulation
Monte Carlo simulation tracks individual particle histories and is considered the most accurate method for dose in heterogeneous media.
Question 4: During plan review, a hot spot of 115% of the prescription dose is found within the PTV but outside the clinical target volume (CTV). This is most likely caused by:
- Overlapping high-dose regions from multiple beams converging in the PTV margin (Correct answer)
- Incorrect tissue density assignment in the TPS
- Failure to apply the wedge transmission factor
- An error in the inverse-square correction at isocenter
Correct answer: Overlapping high-dose regions from multiple beams converging in the PTV margin
Hot spots within PTV margins commonly result from constructive dose addition where multiple beam paths intersect.
Question 5: The conformity index (CI) of a treatment plan is best described as:
- The ratio of the prescription isodose volume to the PTV volume (Correct answer)
- The percentage of the PTV receiving 95% of the prescription
- The maximum dose divided by the mean PTV dose
- The volume of tissue receiving more than 110% of the prescription
Correct answer: The ratio of the prescription isodose volume to the PTV volume
The conformity index (CI) = Volume of prescription isodose / PTV volume; a CI of 1.0 indicates perfect conformity.
Question 6: In treatment planning, 'gantry angle optimization' is performed primarily to:
- Reduce the number of monitor units required per fraction
- Avoid beam paths through high-density implants or critical structures (Correct answer)
- Increase the field size at the isocenter
- Decrease the source-to-skin distance for all fields
Correct answer: Avoid beam paths through high-density implants or critical structures
Selecting gantry angles that avoid critical structures and high-density implants minimizes dose perturbations and OAR exposure.
Question 7: The gamma analysis criterion of 3%/3mm in patient-specific QA means that a measured dose point passes if:
- Its dose difference is within 3% AND its distance-to-agreement is within 3 mm simultaneously (Correct answer)
- Either its dose difference is within 3% OR its distance-to-agreement is within 3 mm
- The dose at 3 mm depth matches within 3% of the plan
- The total dose uncertainty is less than 3% at 3 mm from isocenter
Correct answer: Its dose difference is within 3% AND its distance-to-agreement is within 3 mm simultaneously
The gamma criterion requires that both the dose difference and spatial distance-to-agreement criteria are satisfied simultaneously for a point to pass.
OBT Treatment Planning & Dosimetry 4
In optical beam therapy, the output factor (Sc,p) is the product of which two components?