2nd Year: SRS/SRT Flashcards
For small fields, why do you NOT measure PDD? Only TMR?
It’s very difficult to measure PDD due to chamber movement from CAX as the chamber changes depth. Your alignment is critical and has to be centered perfectly the entire range of depths
For TMR, since the chamber only stays in one spot, it only needs to be aligned once
For cone, why do we always use the same jaw setting for every cone attachment and every cone plan you do?
Changing jaw size affects output factor due to differences in scattering surfaces. You need to stay consistent since your output factors are defined purely for one jaw setting
What does isocentricity measure? What tests do we do that check it?
Size of isocenter
Spoke shot and WL both check it
What does isocenter coincidence check? What test do we do that checks it
Coincidence of radiation and mechanical isocenter
WL checks it
You can argue, if lining WL up using imaging, that imaging isocenter coincidence with the others is also checked, since you define imaging isocenter as the center pixels, so if center sphere aligns to that, then you say imaging isocenter is checked against radiation and mechanical
What test done daily for TB3 checks imaging and radiation isocentricity?
Isocal
(WL also does check mechanical and radiation isocentricity)
For WL, do we collimate with MLC or Jaws?
Always MLC
This is because…
A) We always define the last layer of field collimation using the MLCs
B) MLC positioning is more accurate than Jaw
C) You always want to test the mechanical aspect which is defining a field. For all of IMRT, that is MLCs
What is the isocenter of an image, by definition?
The literal location of the center pixel in your imaging array, rotated about all angles
That is, if something is aligned to imaging isocenter, Ex. a WL cube, then it should be at the center pixel across all projections
What is the benefit of frame delivery vs frameless? What is the drawback
Frame delivery is more accurate and does not require IGRT or any external tracking. It forces the skull to a certain orientation
However, it’s often much more uncomfortable and painful for the patient and requires more setup/time commitment to attach and to do everything same day
How are frames attached? How is patient aligned for treatment?
Screwed to the skull with local anesthesia
There’s a special device that shows torque on skull so you don’t crack it
The frame attaches to the couch top in a known and indexed manner. This makes alignment as close to exact as possible, and does not allow for wiggle
Frame has a known coordinate system for alignments
What are the benefits to using noncoplanar beams over coplanar beams?
Noncoplanar beams can achieve higher conformality and steeper dose gradients, in addition to lower surface dose
In general, what is mechanical tolerances for mecahnical devices?
< 1 mm
What are the main defining characteristics of SRS?
Steep dose gradients
High targeting accuracy (<1 mm)
High dose per fraction ( >= 5 Gy)
High conformity
1 to 5 Fx
Target diameter < 4 cm
How do cones achieve steeper dose fall-off?
Multiple noncoplanar arcs
Typically use lower energies
Smaller collimator-to-tumor distances
How does GammaKnife manage to get steeper dose gradients than Linac based SRS?
Large number of noncoplanar isocentric beams
Small collimator-to-target distances
Lower energy photons, that means lower energy and lower range secondary electrons
How often will a GammaKnife unit require a source exchange?
Every 5 to 6 years
(about 1 half-life)
What is the typical initial dose rate at isocenter of a GammaKnife unit when the sources are first received?
3 Gy/min
What is typical prescription (1 Fx) for the following metastases sizes?
<= 2 cm
2.1 - 3 cm
3.1 - 4 cm
<= 2 cm - 20-24 Gy
2.1 - 3 cm - 18 Gy
3.1 - 4 cm - 15 Gy
What is typical prescription dose for a trigeminal neuralgia?
80 - 90 Gy x 1 Fx to the 100% isodose line
What are the advantages to using GammaKnife over Linac based SRS?
Higher accuracy and pecision
Less moving parts (easier to get everything within QA tolrances)
Quicker QA
Used longer in the field (more research/experience)
Very rare downtimes
What are the disadvantages to using GammaKnife over Linac based SRS?
Can only treat brain
Usually only treats single fractions
Longer treatment times
Not many units come with IGRT capabilities
Requires source exchange after 5 - 6 years
Radiation safety precautions
When would multi-fraction SRT be preferred over single fraction SRS?
Retreatments
Tumors > 4 cm max dimension
Lesion located near critical structures
For a 1.5 T MRI scanner, what the is approximate geometric distortion due to fluctuations of gradient field near the center of the stereotactic space?
What about near the edge (near headframe base for example)
< 0.5 mm near center
about 2 mm near headframe base
How does the strength of magnetic field influece the geometric distortion thatis seen in MRI?
Distortion magnitudes are approximately proportional to field strength
Ex. For a 1.5 T field, the distortion at center is < 0.5 mm. For a 3T field it is < 1 mm
Ex. For a 1.5 T field, distortion near edge is 2 mm. For a 3T field it is 4 mm
What MRI protocol is the go-to for drawing GTV for SRS/SRT?
T1 with contrast