CT for radiotherapy 02/02

Question 1

What did first generation x-ray scanners have?

Answer 1

Single x-ray source, single detector
Slow and inefficient
X-ray emitter and X-ray translator
Rotates around patient

Question 2

What do current systems (3rd generation) now have?

Answer 2

Originally single slice of detectors
Now multiple detector arrays; focussed transversely (axis) and cranio-caudally (head to toe)
Array (group) of detectors
Divergent fan beam

Question 3

Why do we have wide bore CT scanner in Radiotherapy?

Answer 3

• To be able to scan the patient in their treatment position
• Useful for breast patients specifically as breast patients raised to keep breast wall flat
• Tattoo patient, radio-opaque markers, scan
  > One appointment for scan as simulation done in computer space
• Virtual simulation (Vsim software)
  > Full CT capability- 3D and 4D, surface rendering, virtual investigations
  > Eg. Advantage sim (GE), AcQSim (Philips) Prosoma (OSL- independant scanner)
  > ‘Fake treatment’ no additional dose to patient.

Question 4

What does SFOV stand for?

Answer 4

Scan field of view - The parameter that determines how much anatomy is scanned, SFOV larger than anatomy needed.

Question 5

CT simulator couch top

Answer 5

Mimics linac couch top
Flat- most diagnostic couches are shaped /soft/ a bit comfier
Indexed
Minimal flexion
Artefact free
Can use variety of accessories

Question 6

Is the weight capacity different or the same as a linac?

Answer 6

The same

Question 7

CT simulator lasers

Answer 7

Mimics linac lasers both internally and externally

Question 8

What is automatic tube current control

Answer 8

Set up by radiographers in scan protocols
Tube current-mA originally set and constant for length of scan
Modern scanners adjust mA (dose) in accordance with changing attenuation throughout the scan

Question 9

Virtual simulation- surface rendering

Answer 9

Skin surface is reconstructed from CT data set and a virtual light field is projected on to skin surface which aids field placement (extra verification of field placement)

Question 10

What does DRR stand for?

Answer 10

DRR- digitally reconstructed radiograph

Question 11

What is a DRR- digitally reconstructed radiograph?

Answer 11

Digital version of 2D radiograph using CT data set
Equivalent to 2D images taken on simulator (kV)
Comparable to 2D images taken on linac (MV and kV)
Computes radiograph according to the desired energy of x-rays
Slice thickness

Question 12

What does DCR stand for?

Answer 12

DCR- digitally composite radiograph

Question 13

What is a DCR- digitally composite radiograph?

Answer 13

Digital version of 2D radiograph using CT data set
Uses CT data to extract any features required
Maybe different to what is viewed on actual radiograph taken

Question 14

4D CT

Answer 14

Issue associated with CT planning:

• Moveable structures
• Not an issue for direct imaging as a ‘one-off’ image

Question 15

Things to consider for brain…

Answer 15

Cast, contrast, swelling due to steroids, surgery sites, MR Merge

Question 16

Things to consider for head and neck…

Answer 16

Cast, contrast, MR merge, weight loss

Question 17

Things to consider for thorax…

Answer 17

4DCT, immobility, gating, coached breathing, DIBH (breast/lymphoma)

Question 18

Things to consider for abdomen…

Answer 18

Motion Motion Motion! – potentially EEBH, similar tissue densities

Question 19

Things to consider for pelvis…

Answer 19

Bladder, rectum, bowel, genitalia

Question 20

Things to consider for extremities…

Answer 20

Contra lateral limbs, position, vacbags, patient standing up

Question 21

Things to consider for palliative patients…

Answer 21

speed, comfort

Question 22

SRS and SABR considerations

Answer 22

accuracy, head frames, full body vacbags, smaller slice thicknesses?