Modelling Treatment Planning Algorithms

Question 1

What parameters can be altered to tweak the TPS model?

Answer 1

Energy spectrum
Primary beam profile
Relative primary/secondary contributions
Off-axis corrections
Gaussian sigmas for penumbra fitting
electron contamination

Question 2

What data needs to be provided for modelling?

Answer 2

Geometry of treatment head
Energy spectrum, electron contamination, fluence distribution, source size, inter-leaf leakage, absolute dose for each beam

Question 3

Who models the beam?

Answer 3

Depends on manufacturer - some it’s them, some it’s Physicist, some it’s both

Question 4

How is thee linac head geometry found?

Answer 4

From the manufacturer

Question 5

Why is the linac head geometry important?

Answer 5

Provides graphical interface, defines mechanical behaviour and prevents undeliverable plans

Question 6

What measured data needs to be provided for the linac head?

Answer 6

Interleaf leakage - scan across T shaped fields
Leaf tip widths - scan across a series of in and out leaves
Absolute dose - calculate under reference conditions

Question 7

What are the requirements of the measured data?

Answer 7

Orientation/co-ordinate system
Position/calibration
Set-up consistency
Geometric accuracy of plotting equipment
Sufficient spatial resolution of data
Appropriate measurement devices

Question 8

Why are the PDDs measured?

Answer 8

To determine the spectrum of the beam and therefore how the beam interacts with the linac head
MC simulations sum the contributions of mono-energetic beams

Question 9

Whey are profiles measured?

Answer 9

Model off axis softening, source/target size, penumbras from primary and secondary sources
Determine best fit
Need to correct for small and large fields

Question 10

Why is the field size dependence important?

Answer 10

Output factor corrections

Flattening filter source corrections

Question 11

What are measured for the collimators?

Answer 11

Offset
Gain
Curvature

Question 12

What are measured for the MLCs?

Answer 12

Leaf tip
Tongue and groove
Root mean square of MLC transmission

Question 13

Why is electron contamination measured?

Answer 13

It adds surface dose

Question 14

How is direct fluence measured?

Answer 14

Star profiles

2D dose distribution in water

Question 15

How is head scatter fluence modelled?

Answer 15

Using output factors measured in a mini-phantom

Question 16

What are the possible limitations of a TPS?

Answer 16

Modelling of depth hardening
Modelling of off-axis softening
Lack of independent calculation checks

Question 17

What are the inaccuracies in treatment and what are the major sources?

Answer 17

~3.8%

Beam and patient set-up error

Question 18

What validation data is needed for the TPS model?

Answer 18

Independent data

Covers the range of clinical use, push the limitations of the TPS model ie situations not explicitly modelled