Rotational Techniques

Question 1

How are VMAT treatments delivered?

Answer 1

Gantry rotate continuously while MLCs and dose rate vary

Partial, single or multiple arcs can be used

Question 2

What is the difference between VMAT and IMRT techniques?

Answer 2

VMAT is more efficient - lower MU, quicker

Increased complexity - needs extra QA checks

Question 3

How are VMAT field shapes defined?

Answer 3

Defined at discrete gantry angle (control points)

Question 4

How is the dose calculated for a VMAT delivery?

Answer 4

TPS compute dose by sampling delivery with a certain number of control points - the more control points the slower the optimisation

Question 5

What is the leaf motion between control points restricted by?

Answer 5

Speed of MLC leaf motion

Gantry speed

Question 6

Why do lung VMAT deliveries limit the MLC motion?

Answer 6

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

Question 7

What parameters can be controlled in VMAT deliveries?

Answer 7

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

Question 8

Why is the collimator usually fixed at a non-zero angle?

Answer 8

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

Question 9

What is the compromise between single and double arc deliveries?

Answer 9

Single is quicker

Double gives better dose distributions

Question 10

What is a Tomotherapy machine?

Answer 10

Integrated IMRT/IGRT machine
Hybrid of a linac and CT
Delivers a helical delivery - highly conformal
Integrated imaging - precise patient positioning

Question 11

How is the Tomotherapy helical delivery achieved?

Answer 11

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

Question 12

How are Tomotherapy beams collimated?

Answer 12

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

Question 13

Why do conventional QA test need to be adapted for Tomotherapy?

Answer 13

FFF beam - high dose rate
No transverse bore laser
No light field
SSD = 85cm so get quicker fall off on PDD

Question 14

What does Tomo Edge enable?

Answer 14

Dynamic jaws

Question 15

What is the Tomotherapy sinogram?

Answer 15

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

Question 16

How is treatment planning performed for Tomotherapy?

Answer 16

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

Question 17

How is IGRT used in Tomotherapy?

Answer 17

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

Question 18

What is Tomodirect?

Answer 18

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

Question 19

What is a Pareto optimal solution?

Answer 19

A solution to a problem where there is no feasable solution to minimise all the objective functions simultaneously

Question 20

How can Pareto solutions be used in inverse planning?

Answer 20

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

Question 21

How does Raysearch make a variety of plans?

Answer 21

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

Question 22

What extra commissioning needs to be performed for IMRT and VMAT capable machines?

Answer 22

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

Question 23

What are the 3 ling tests?

Answer 23

1: arc with a static picket fence
2: 7 segmented field, 6 dose rates, 3 gantry speeds
3: 4 segmented fields, 4 leaf speeds

Question 24

What patient specific QA is used for VMAT deliveries?

Answer 24

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

Question 25

What proportion of patients are expected to be treated with IMRT?

Answer 25

24% - breasts don’t count towards IMRT figure

Question 26

What was the error in the national RT IMRT audit?

Answer 26

+/-2%

Question 27

How much money was available for IMRT upgrades and what was it spent on?

Answer 27

£23mil - spent on upgrading linac, TPS, IGRT, training staff