IMRT Deliveries

Question 1

What does IMRT stand for

Answer 1

Intensity modulated radiation therapy

Question 2

What is IMRT

Answer 2

The use of non-uniform beam intensities to produce dose distributions which conform to irregular target shapes

Question 3

What is the rationale behind IMRT

Answer 3

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)

Question 4

Types of IMRT

Answer 4

Fixed gantry:
Step and shoot
Dynamic MLC IMRT

Rotational IMRT:
VMAT
Tomotherapy

Question 5

Step and shoot

Answer 5

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

Question 6

Accuracy and efficiency in IMRT

Answer 6

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?

Question 7

What is a control point

Answer 7

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

Question 8

Dynamic MLC IMRT

Answer 8

Leaves move during delivery
Want leaves moving in one direction to maximise open field and prevent collisions

Question 9

Accuracy of dynamic MLC

Answer 9

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

Question 10

Linac issues with IMRT

Answer 10

Small segments have larger variation of output factor
Low dose segments
MLC accuracy
Tongue and groove effect

Question 11

What is the tongue and groove effect

Answer 11

MLC leaf edges are designed to minimise interleaf leakage, causes shading in overlap region and can lead to underdosing

Question 12

VMAT

Answer 12

Volumetric modulated arc therapy
Gantry rotates while beam is on, MLC position, dose rate, gantry speed usually all vary

Question 13

Disadvantages of VMAT

Answer 13

More intense QA needed
Low dose wash - dose is everywhere

Question 14

Control points in VMAT

Answer 14

More complicated - now where is the linac too
Field shape and cumulative MU defined at discrete gantry angles

Question 15

What is the interplay effect?

Answer 15

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.

Question 16

Planning options in VMAT

Answer 16

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

Question 17

Why have slightly off collimator angle?

Answer 17

Leakage will not always be in the same plane, spreads out dose, minimises tongue and groove.

Question 18

What is tomotherapy

Answer 18

Purpose built IMRT/IGRT machine, hybrid between a linac and a CT scanner. Helical deliver, highly conformal.

Question 19

What varies in tomotherapy plans?

Answer 19

Constant speed in one plan
Constant couch velocity in one plan
Field width/pitch selected for individual patient

Question 20

What kind of beam is tomotherapy?

Answer 20

FFF, forward peaked.
Much higher dose rate than traditional linac as a result.

Question 21

What is complexity analysis?

Answer 21

A measure of the complexity of a plan
Agreement between delivered and planned dose is function of TPS calculation and linac delivery accuracy

Question 22

Tomoedge

Answer 22

Dynamic movement of jaws, close when close to OAR, steeper dose distribution

Question 23

How is imaging done on tomotherapy unit?

Answer 23

6MV beam detuned to improve image quality.

Question 24

Tomodirect

Answer 24

The use of tomotherapy unit in non-rotational manor. Gantry fixed while couch moves, can treat at 12 discrete angles.

Question 25

VMAT commissioning considerations

Answer 25

Need to commission and do routine QA on more variables, dose rate, gantry speed, leaf speed, synchromisation.

Question 26

Patient specific QA

Answer 26

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.

Question 27

How would you check MLC leaf calibration?

Answer 27

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

Question 28

Why are small segments a problem?

Answer 28

Output varies with field size for small fields

Question 29

What is issue with low dose segments?

Answer 29

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

Question 30

How can we reduce problems with low dose segments and small segments?

Answer 30

Define minimum field opening
Define minimum MU/segment

Question 31

How can we minimise T&G effect?

Answer 31

Optimise for larger apertures, set minimum opening or synchronise motion of adjacent leaves

Question 32

What is pitch in tomotherapy?

Answer 32

Couch movement per gantry rotation

Question 33

What can affect plan complexity?

Answer 33

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?