OBT Treatment Planning & Dosimetry 2

Question 1

When calculating the tissue-phantom ratio (TPR) for an optical beam, which variable remains constant as depth increases in a homogeneous medium?

Accepted Answer

Field size at the isocenter

Answer

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?

Accepted Answer

210 MU

Answer

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?

Accepted Answer

Tissue-maximum ratio (TMR)

Answer

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:

Accepted Answer

Evaluate beam-target geometric alignment and critical structure avoidance

Answer

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:

Accepted Answer

The dose received by 95% of the PTV volume

Answer

D95 indicates the dose that at least 95% of the PTV volume receives, used to confirm adequate target coverage.

(OBT) Optical Beam Therapy Certification Practice Test

(OBT) Optical Beam Therapy Certification Practice Test

OBT Treatment Planning & Dosimetry 2