CMD Treatment Simulation & Immobilization 3

Question 1

What immobilization system is most appropriate for stereotactic radiosurgery (SRS) of an intracranial lesion?

Accepted Answer

Relocatable or rigid stereotactic frame providing sub-millimeter reproducibility

Answer

SRS delivers ablative doses in one to five fractions and demands sub-millimeter accuracy, making stereotactic frames or high-precision relocatable systems the required immobilization standard.

Question 2

What does 'indexed immobilization' mean in the context of radiation therapy simulation?

Accepted Answer

Using a numbered couch rail system to reproducibly position immobilization devices at the identical couch location each session

Answer

Indexed immobilization uses a standardized couch indexing rail so immobilization devices can be returned to the same precise position on the couch for every treatment fraction.

Question 3

What Hounsfield Unit (HU) value corresponds to water in CT imaging?

Accepted Answer

0 HU

Answer

The Hounsfield scale defines water as 0 HU by convention, with air at -1000 HU and cortical bone at approximately +1000 HU.

Question 4

Why are a patient's arms positioned above the head during CT simulation for breast cancer treatment?

Accepted Answer

To move the arms and shoulders out of the planned tangential beam paths

Answer

Elevating the arms clears the shoulder and arm anatomy from the lateral tangential beam paths used in breast treatment, preventing unnecessary dose to those structures.

Question 5

What is the function of the orthogonal room lasers (sagittal, coronal, and transverse) in the CT simulation suite?

Accepted Answer

To define a fixed 3D coordinate reference system aligned with the CT isocenter for precise skin mark and tattoo placement

Answer

Room lasers define a coordinate system fixed to the CT isocenter, enabling therapists to place reference skin marks and tattoos at known 3D positions relative to the planned treatment isocenter.

(CMD) Certified Medical Dosimetrist Practice Test

(CMD) Certified Medical Dosimetrist Practice Test

CMD Treatment Simulation & Immobilization 3