First Year Exam: CT Sim and Motion Management Flashcards
At what point in the pre-treatment workflow is isocenter set?
Treatment planning by dosimetry
How do therapists, on the day of treatment, know where isocenter is? Give the steps required to make this happen.
- Sim therapist marks user origin on patient using BBs
- Dosimetrist sets user origin on TPS
- Dosimetrist marks isocenter on TPS
- Dosimetrist provides therapists with couch shifts to go from user origin to new isocenter
- Therapists, on VSim day or first treatment, will make location of new isocenter on patient by moving patient using couch shifts and using lasers to mark new iso
Give two scenarios in which it’s beneficial for dosimetry to set the isocenter, and not any therapists.
- Half beam blocking, only dosimetry will know where a half beam block has to occur. Of course HBB depends directly on location of isocenter, so dosi needs to set
- Wanting to use small MLCs to shape target
What slice thicknesses do we use for…
HDR:
Stereotactics:
Everything else:
HDR: 0.625 mm
Stereotactics: 1 mm
Everything else: 2.5 mm
What is the main downside to using a smaller slice thickness?
You worsen low contrast resolution (since you’re reconstructing the same image but with less data per voxel)
What is the main benefit to using a smaller slice thickness?
Helps avoid volume averaging effects and gives more accurate spatial dose distribution
What measures higher spatial resolution, small or large FOV?
Small FOV
In one short sentence, what is pitch?
Amount of table movement per gantry rotation
What does a small pitch yield in terms of data and dose? What about a large pitch?
Small pitch: more data per slice, but also more dose
Large pitch: less data per slice, but also less dose
What is the mA range used in CT?
10-800 mA
What image quality metric does mA affect?
contrast
What are 4 typical kVp’s used for CT sims? Which do we use in our clinic for all sims?
80, 100, 120, 140
We use 140 kVp in our clinic
What is the main difference in simming considerations for palliative vs curative treatments?
Palliative - sim with patient comfort as the main concern, we wan them to get through treatment reproducibly
Curative - sim with dosimetry in mind, because it’s higher dose and we need to focus more on getting the best delivery
What are the 5 most common CT artifacts?
Ring
Beam hardening
Streaking (or starvation)
Motion artifacts
Partial volume averaging
What causes ring artifacts?
Bad detector that goes out of calibration and doesn’t properly record incoming data
What does a beam hardening artifact look like? Why does it happen?
The effective energy of a beam changes with attenuation, leading to errors in attenuation coefficients and numbers
Appears as a region of low HU near bone or other large density. The beam in that region is hardened, so the soft tissue attenuates relatively less than it would if it weren’t hardened. The reconstruction algorithm then calculates a lower HU value
What causes streaking artifacts?
Photon starvation due to complete beam attenuation from very high density material
What causes partial volume averaging artifacts?
More than one tissue type in a CT slice becomes averaged into a single voxel, yielding an incorrect CT number
How do you handle contrast in a CT?
Contour the contrast and override HU to set it to water
In general, what is the function of contrast used in CT?
To either distend portions of the stomach, bowel or bladder, or to outline structures such as loops of bowell, vessels, and lymphatic system
What is the typical threshold suspected motion error used in our clinic that would warrant the use of motion management?
~1 cm
How is an ITV drawn?
Using the maximum projection from the 4DCT
Which CT do you play on for lungs?
average intensity projection, or free breathing
What is a slow CT and how long does it usually take?
It’s a CT that’s used to scan slowly so there are multiple breathing cycles and it results in an average intensity projection
~60 seconds
Describe the use of Inhale/Exhale CTs
You take a CT during inhale and exhale and you assume these are the sup/inf max and min to use for the MIP contour
How is a breathing cycle established for 4DCT?
By placing an RPM block on the patient’s chest and tracking the vertical motion of the block
How many bins (phases of breathing cycle) does a 4DCT take?
10
If motion of target is < 1 cm, what does the physician use to contour lung target?
Physician contours on the 30% bin because it’s between inspiration and expiration (less movement)
Which scan is used for treatment planning?
Average MIP
When do you use minimum intensity projection?
Contouring liver target, where the tumor is the lowest HU value than the surrounding normal liver tissue
(unlike lung where it’s the opposite, so you use maximum)
Why do some sites not do IMRT for lung treatments?
Tumor may modulate in and out of open and closed MLC areas. This can result in unintentionally hot or cold spots if the of the MLCs are missing or not
Give a general description on how Calypso works
Used for target localization and/or tracking during treatment
3 tiny EM transponder beacons implanted into prostate
Emit signals when excited by non-ionizing EM field generated by array
Signal is triangulated like GPS
What is the slant board weight limit?
300 lb
What is the slant board used for?
Breast patients
What is typical setup considerations/techniques for a prostate patient?
Full bladder
Empty rectum
Vacloc immobilization
(Possible use of calypso)
When is a belly board used?
For prone rectal patients
What are some parameters defined by a protocol in CT Sim? (I’ve got 8 listed)
Slice thickness
kVp
mA
mAs
FOV
Patient positioning
Types of contrast to use
Immobilization devices
Should prostate patients have a full or empty bladder? Why?
Full bladder
What is approximate motion of a lung tumor in the axial plane?
Within 2 mm
The vast majority of tumor motion in lung is superior/inferior, so it won’t show much in axial
What is the upper end of dosimetric single fraction errors from IMRT or wedged plans?
What about throughout the course of a 30 fx treatment?
Single fraction - can be upwards of 20% error
30 fx - Errors average out and reduce to about 1-2%
What is an “Interplay Effect”
An effect that occurs when two distinct timelines of motion are in effect simultaneously
An example of this is movement of MLCs in sliding window IMRT and movement of tumor
Interplay effects usually result in inhomogenous dose distributions
What is the main benefit to using respiratory motion management?
You cand ecrease your treatment volume
How are breast patients usually set up? (not prone breast)
Arm up on side being treated
Placed on an incline with slant board (300 lb limit)
Can be DIBH, 4D, or free breathing
When a 4D scan is performed, what images/scans are actually usually taken?
Scout image
A free breathing CT without contrast
A free breathing CT with contrast
The 4DCT
When the physicians come into CT sim, what are they usually reviewing?
Sometimes they review the scan itself, but not always
Most of the time they come in the review setup, immobilization, isocenter lcoation, and sign any notes that need to be signed (this is all mostly for billing purposes)
In general, what sites of the body MIGHT you see motion management done for?
Thorax and Abdomen
They’re the most likely locations for respiratory motion
At what expected tumor motion would you want to use respiratory motion management?
>= 5 mm suspected movement
What is the general design of an AlignRT system?
Dual camera optical stereoscopic approach
Tracks a 3D contour
Consists of two pods
The pods have two cameras each, and one projector each
What are some different types of non-radiographic systems? (4 major types, give the most commonly known example of each one) (think what we use in our clinic)
Radiofrequency (like Calypso)
Optical dual camera (like AlignRT)
Single camera (like RPM)
Infrared dual camera (like ExacTrac)
Quickly: how does calypso work?
Small transponders (or beacons) are implanted in a body site
The beacons are alternating current circuits
An array of source coiuls produces an oscillating electromagnetic field which induces a resonance inside of the transponders
The decay of the resonant signal is detected by a second array of receiver coils
Triangulation is used to determine transponder location
What systems are shown below?
Does AlignRT utilize deformable or rigid registration?
Rigid
