OBT Treatment Planning & Dosimetry 4

Question 1

In optical beam therapy, the output factor (Sc,p) is the product of which two components?

Accepted Answer

Collimator scatter factor (Sc) and phantom scatter factor (Sp)

Answer

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:

Accepted Answer

(100/120)^2

Answer

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?

Accepted Answer

Monte Carlo simulation

Answer

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:

Accepted Answer

Overlapping high-dose regions from multiple beams converging in the PTV margin

Answer

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:

Accepted Answer

The ratio of the prescription isodose volume to the PTV volume

Answer

The conformity index (CI) = Volume of prescription isodose / PTV volume; a CI of 1.0 indicates perfect conformity.

(OBT) Optical Beam Therapy Certification Practice Test

(OBT) Optical Beam Therapy Certification Practice Test

OBT Treatment Planning & Dosimetry 4