IMRT Deliveries Flashcards
What does IMRT stand for
Intensity modulated radiation therapy
What is IMRT
The use of non-uniform beam intensities to produce dose distributions which conform to irregular target shapes
What is the rationale behind IMRT
Highly conformal dose distributions
Greater sparing of normal tissue
Potential for dose escalation
Easy to create deliberately non-uniform dose distribution (boost)
Potential efficacy savings (less gantry angles/couch twists)
Types of IMRT
Fixed gantry:
Step and shoot
Dynamic MLC IMRT
Rotational IMRT:
VMAT
Tomotherapy
Step and shoot
Beam is suspended between delivery of subfields that add up to create field
Intensity profile created using a sequence of static subfields each delivering a small dose increment
Accuracy and efficiency in IMRT
Less steps is clunkier and less accurate but will be quicker to deliver.
More clunks will take longer, how much can beam be trusted for small MU segments?
What is a control point
Instructions to linac, what it should be doing throughout delivery. For step and shoot there are 2 control points for each section, shape and then number of MU
Dynamic MLC IMRT
Leaves move during delivery
Want leaves moving in one direction to maximise open field and prevent collisions
Accuracy of dynamic MLC
Dosimetric accuracy is dependent on positional accuracy of collimators as delivering throughout, not the case for step and shoot. Modulation achieved by moving colimators while beam is on.
Less dependent on positional accuracy in S&S except in penumbral regions.
Need to consider ability of linac
Linac issues with IMRT
Small segments have larger variation of output factor
Low dose segments
MLC accuracy
Tongue and groove effect
What is the tongue and groove effect
MLC leaf edges are designed to minimise interleaf leakage, causes shading in overlap region and can lead to underdosing
VMAT
Volumetric modulated arc therapy
Gantry rotates while beam is on, MLC position, dose rate, gantry speed usually all vary
Disadvantages of VMAT
More intense QA needed
Low dose wash - dose is everywhere
Control points in VMAT
More complicated - now where is the linac too
Field shape and cumulative MU defined at discrete gantry angles
What is the interplay effect?
The relationship between dynamic delivery and motion of the target. Dynamic techniques are sensitive to motion, target could be over or underdosed if leaves are not well synched, concerning in SABR.
Planning options in VMAT
Similar to IMRT, inverse planning, might use additional dummy structures
Gantry speed, dose rate, MLC leaf motion options, single or double arc, length of arc, collimator angle
Why have slightly off collimator angle?
Leakage will not always be in the same plane, spreads out dose, minimises tongue and groove.
What is tomotherapy
Purpose built IMRT/IGRT machine, hybrid between a linac and a CT scanner. Helical deliver, highly conformal.
What varies in tomotherapy plans?
Constant speed in one plan
Constant couch velocity in one plan
Field width/pitch selected for individual patient
What kind of beam is tomotherapy?
FFF, forward peaked.
Much higher dose rate than traditional linac as a result.
What is complexity analysis?
A measure of the complexity of a plan
Agreement between delivered and planned dose is function of TPS calculation and linac delivery accuracy
Tomoedge
Dynamic movement of jaws, close when close to OAR, steeper dose distribution
How is imaging done on tomotherapy unit?
6MV beam detuned to improve image quality.
Tomodirect
The use of tomotherapy unit in non-rotational manor. Gantry fixed while couch moves, can treat at 12 discrete angles.
VMAT commissioning considerations
Need to commission and do routine QA on more variables, dose rate, gantry speed, leaf speed, synchromisation.
Patient specific QA
Independent MU check required for all RT plans. Can use independent MU check programme such as Racalc or measurement, which is recommended for first patients of new technique, Delta4.
How would you check MLC leaf calibration?
Picket fence test
Used to be done with film, now would use EPID
Create plan with lines of uniform intensity, these lines would be in the wrong place or would be uneven if the MLC movement was not acceptable
Why are small segments a problem?
Output varies with field size for small fields
What is issue with low dose segments?
Start up characteristics affect low MU segments
-MU linearity
-flatness and energy
Problem for S&S
100 segments of 1MU might not give same output as 1 segment of 100MU
How can we reduce problems with low dose segments and small segments?
Define minimum field opening
Define minimum MU/segment
How can we minimise T&G effect?
Optimise for larger apertures, set minimum opening or synchronise motion of adjacent leaves
What is pitch in tomotherapy?
Couch movement per gantry rotation
What can affect plan complexity?
Number of segments
Size of segments
Complexity of aperture shape
Amount of leaf motion
Number of MU
How much does shape change? how much does gantry speed change?
