IMRT - Conventional Techniques

Question 1

What is IMRT?

Answer 1

A technique where the beam is altered from its flat or wedged profile. This produces dose distributions which conform to irregular target shapes

Question 2

Why is IMRT used?

Answer 2

Greater dose conformality
Greater sparing of normal tissue (lower toxicity/complications)
Potential for dose escalation
Easy to create non-uniform distributions
Potential efficiency savings
Can block OAR
Can shape to concavities

Question 3

How can varying the MLCs deliver the desired dose?

Answer 3

Each leaf pair gives a 1D profile which is stacked to give a 2D fluence
For step and shoot, split the profile into a series of static fields to be delivered

Question 4

Why is the leaf sweep technique used?

Answer 4

Saves time

Less chance of leaves colliding

Question 5

Why can we not increase the number of segments to infinity to deliver a perfect distribution?

Answer 5

This is less efficient as the beam is suspended more

For very small segments the beam will not be stable for ~2s and the dose/MU can very up to 2% initially

Question 6

How many control points are associated with each segment?

Answer 6

2 - 1 sets the shape, 1 increments the MU

Question 7

How are dynamic MLC moves used?

Answer 7

Can create field gradients by sweeping a jaw across the beam to change the beam profile.
Intensity profiles are converted into leaf moves in the TPS - need to consider constraints on leaf velocity, adjacent profiles, treatment time

Question 8

What is thee dosimetric accuracy most dependent on?

Answer 8

Positional accuracy of collimators

Question 9

What are the problems that IMRT presents to the linac?

Answer 9

Small segments
Low dose segments - start up characteristics affect low MU segments, MU linearity, flatness, and energy
MLC accuracy

Question 10

Why is inverse planning used for IMRT?

Answer 10

It is too complex for a person

Question 11

How is inverse planning performed?

Answer 11

Planner specifies dose distribution through dose limits and volume constraint to PTV and OARs
TPS calculates beam profiles, iterates to reduce cost/objective function
Planner assesses plan and alters dose limits/constraints if needed

Question 12

What is a cost function?

Answer 12

A physical way of summarising the merit of a plan in a single figure to drive the plan optimisation

Question 13

What is the simple cost function based on the weighted least squares minimisation algorithm?

Answer 13

C = sum(penalty function.(calculated dose - prescribed dose)^2
This = 0 if all the plan constraints are met

Question 14

What are the common stopping criterion for the TPS?

Answer 14

Stop after x iterations

Stop when the difference in the cost function between successive plans is a specified percentage

Question 15

What is the difference between an objective and a constraint?

Answer 15

An objective is something you would like, weighted against other objectives for importance
A constraint is something which must be achieved for the plan to be acceptable

Question 16

What parameters are used for the optimisation of target volumes and OARs?

Answer 16

Target - min dose, max dose, dose gradient/uniformity parameter
OAR - max dose, dose volume controls

Question 17

What are the optimisation algorithms on offer?

Answer 17

Analytic techniques - inverse CT
Deterministic iterative techniques - gradient methods - most common in comercial TPS - fine if not optimising gantry too
Stochastic iterative techniques - simulated annealing, genetic algorithms, neural networks - prevents optimisation getting stuck in local minima

Question 18

How does the gradient descent method work?

Answer 18

Walks with a predefined step size to find the global minima. Needs good first guess and a simple objective function

Question 19

How does simulated annealing work?

Answer 19

Step size starts large and reduces over time. Always accept a step reducing the cost function. But also accept steps which increase the cost function with probability p(i) = e^(-delta.OF/kb.T) to get out of local minima

Question 20

How is beamlet optimisation achieved?

Answer 20

Field split into beamlets of discrete fluence, with the corresponding dose distribution computed
Beamlet weights are optimised to produce an optimal fluence map for each beam direction
Optimal fluence map translated into deliverable segments or leaf trajectory - leaf segmentation
The final deliverable dose distribution is slightly degraded

Question 21

What is aperture based optimisation?

Answer 21

Segment shapes and weights optimised together

Takes limitations of MLCs into account

Question 22

What is sequencing in IMRT planning?

Answer 22

Converting the theoretical/ideal fluence into deliverable segments

Question 23

What is the compromise between the number of levels in the optimisation?

Answer 23

Fewer levels - greater efficiency, easier to verify, poorer plan
More levels - closer to optimisation dose distribution, less efficient

Question 24

What are current optimisations based on an what may they move to in the future?

Answer 24

Currently - absorbed dose

Future - outcomes ie biological effectiveness

Question 25

What considerations need to be made in inverse planning?

Answer 25

Use odd number of equidistant beams - no opposing beams
Direction less important as number of beams increases
Can come through OAR
Choice of energy doesn’t matter >10MV not used

Question 26

What is the purpose of tuning volumes?

Answer 26

Drive optimisation to acceptable solution by having more control over dose distribution

Question 27

How can overlapping ROIs be dealt with?

Answer 27

Set overlap priority

Create rinds to reduce the dose in that area

Question 28

How is normalisation performed for IMRT?

Answer 28

Normalise to a volume
Can have large absorbed dose gradients in individual beams
Use level 2 reporting

Question 29

What is a class solution?

Answer 29

A set of inverse planning parameters which can be used for patients with the same disease and staging to speed up planning. Can be a stand alone or a good starting point

Question 30

What is included in a class solution?

Answer 30

Volumes needed
Beam numbers and orientation
Optimisation parameters
Sequencing parameters

Question 31

What is the common method for making an IMRT plan?

Answer 31

Make a plan with good dose to the PTV then push on OARs