OBT Treatment Planning & Dosimetry 5

Question 1

In a 3D conformal optical beam therapy plan, coplanar beams are defined as beams that:

Accepted Answer

All lie within the same transverse plane through the isocenter

Answer

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:

Accepted Answer

267 MU

Answer

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:

Accepted Answer

Reduce hot spots within the PTV by adding subfields that selectively block high-dose regions

Answer

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:

Accepted Answer

Median dose (D50) of the PTV

Answer

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:

Accepted Answer

Using steep dose gradients and sharp beam penumbra to rapidly drop dose outside the PTV

Answer

Achieving a steep dose gradient adjacent to serial OARs requires sharp penumbra and optimized beam geometry to protect the structure.

(OBT) Optical Beam Therapy Certification Practice Test

(OBT) Optical Beam Therapy Certification Practice Test

OBT Treatment Planning & Dosimetry 5