Treatment Simulation & Immobilization Flashcards
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Read the first 7 Treatment Simulation & Immobilization flashcards as text
What immobilization system is most appropriate for stereotactic radiosurgery (SRS) of an intracranial lesion?
Answer: Relocatable or rigid stereotactic frame providing sub-millimeter reproducibility
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.
What does 'indexed immobilization' mean in the context of radiation therapy simulation?
Answer: Using a numbered couch rail system to reproducibly position immobilization devices at the identical couch location each session
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.
What Hounsfield Unit (HU) value corresponds to water in CT imaging?
Answer: 0 HU
The Hounsfield scale defines water as 0 HU by convention, with air at -1000 HU and cortical bone at approximately +1000 HU.
Why are a patient's arms positioned above the head during CT simulation for breast cancer treatment?
Answer: To move the arms and shoulders out of the planned tangential beam paths
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.
What is the function of the orthogonal room lasers (sagittal, coronal, and transverse) in the CT simulation suite?
Answer: To define a fixed 3D coordinate reference system aligned with the CT isocenter for precise skin mark and tattoo placement
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.
Which of the following best describes the purpose of acquiring a scout (topogram) image at the start of CT simulation?
Answer: To survey anatomy and define the superior and inferior extent of the diagnostic CT acquisition volume
The scout image is a low-dose planar projection radiograph used to survey anatomy and set the scan range, ensuring the full target volume and relevant anatomy are captured in the CT dataset.
When performing image-guided radiation therapy (IGRT) setup verification, the daily CBCT is registered to which reference image set?
Answer: The planning CT acquired at simulation, using the same coordinate system as the treatment plan
Daily CBCT is registered to the planning CT simulation dataset, which shares the coordinate system of the approved treatment plan, enabling detection and correction of positional deviations.