Rotational Techniques Flashcards
How are VMAT treatments delivered?
Gantry rotate continuously while MLCs and dose rate vary
Partial, single or multiple arcs can be used
What is the difference between VMAT and IMRT techniques?
VMAT is more efficient - lower MU, quicker
Increased complexity - needs extra QA checks
How are VMAT field shapes defined?
Defined at discrete gantry angle (control points)
How is the dose calculated for a VMAT delivery?
TPS compute dose by sampling delivery with a certain number of control points - the more control points the slower the optimisation
What is the leaf motion between control points restricted by?
Speed of MLC leaf motion
Gantry speed
Why do lung VMAT deliveries limit the MLC motion?
To minimise interplay effects by setting limits on the leaf speed with distance and/or gantry angle, selecting parameters to encourage chunky segments, change max delivery time
What parameters can be controlled in VMAT deliveries?
Gantry speed Dose rate MLC leaf motion Single or double arc Angle of arc Collimator angle Control point spacing Max leaf speed Max delivery time
Why is the collimator usually fixed at a non-zero angle?
Spreads out leakage
Minimise tongue and groove
More than 1 leaf pair irradiates each slice
Easier to shield OAR
Gives another degree of freedom for multiple arcs
What is the compromise between single and double arc deliveries?
Single is quicker
Double gives better dose distributions
What is a Tomotherapy machine?
Integrated IMRT/IGRT machine
Hybrid of a linac and CT
Delivers a helical delivery - highly conformal
Integrated imaging - precise patient positioning
How is the Tomotherapy helical delivery achieved?
Gantry rotates with a constant speed, period 12-60s
Couch moves with a constant velocity
Field width and pitch selected for the individual patient
Computer planning optimisation to 51 gantry angles
How are Tomotherapy beams collimated?
1 set of independent jaws - field width of 1, 2.5, 5cm
64 pneumatically driven binary MLC, transit time ~10ms
Modulate the fan beam to enable IMRT
The curve of the MLCs sharpens the penumbra between leaves. not focused exactly on the target to minimise interleaf leakage
Why do conventional QA test need to be adapted for Tomotherapy?
FFF beam - high dose rate
No transverse bore laser
No light field
SSD = 85cm so get quicker fall off on PDD
What does Tomo Edge enable?
Dynamic jaws
What is the Tomotherapy sinogram?
Equivalent to the energy fluence
Each row shows leaf open time - black = open, white = shut
2D sinogram represents leaf pattern to deliver the modulated dose
Off centre tumours has a sinusoidal shape
Each structure has its own sinusoid
How is treatment planning performed for Tomotherapy?
Choose pitch, modulation factor, field size, dose grid
TPS works out dose to voxels hit by beamlet of sinogram.
Plan like normal IMRT
Makes very conformal dose due to many degrees of freedom
How is IGRT used in Tomotherapy?
Use inbuilt IGRT and register the patient daily
If the patient loses weight - overlay original dose distribution on verification scan, look at the difference - may need a replan
What is Tomodirect?
Non-rotational treatment using Tomotherapy Fixed gantry Move the couch through the bore 12 discrete angles Can define modulation Full inverse planning Tissue compensated conformal delivery Can treat up to 160cm length
What is a Pareto optimal solution?
A solution to a problem where there is no feasable solution to minimise all the objective functions simultaneously
How can Pareto solutions be used in inverse planning?
Produce a series of pareto solutions, minimising each objective function in turn
Plot on a Pareto surface graph and explore to find the ideal solution
How does Raysearch make a variety of plans?
It makes an ‘anchor plan’ which meets the acceptable criteria
It then makes a balance plan and 10-12 minimising certain objectives
Based on beam optimisation, merge fluence for the difference solutions until you allow it to create deliverable plan
What extra commissioning needs to be performed for IMRT and VMAT capable machines?
Collect validation data for highly modulated fields and off-axis segments
Check variables associated with arc deliveries: dose rate (0-600MU), gantry speed (0-6 degrees/s), leaf speed (0-2.5cm/s)
Ensure all changes are synchronised together
Check interruption/resumption procedures
What are the 3 ling tests?
1: arc with a static picket fence
2: 7 segmented field, 6 dose rates, 3 gantry speeds
3: 4 segmented fields, 4 leaf speeds
What patient specific QA is used for VMAT deliveries?
Independent MU check
Check by measurement - recommnded for first 10 patients of new technique/hardware or software upgrade
Can use film and ionisation chamber or QA devices
IMRT deliveries should be included in machine QA
What proportion of patients are expected to be treated with IMRT?
24% - breasts don’t count towards IMRT figure
What was the error in the national RT IMRT audit?
+/-2%
How much money was available for IMRT upgrades and what was it spent on?
£23mil - spent on upgrading linac, TPS, IGRT, training staff
