Protons - 2

Question 1

What are the field arangements for proton planning?

Answer 1

1-5 fields - mean of 2.5 - beams not necessarily coplanar

Question 2

What are the parameters that are configured during planning?

Answer 2

Planner defined: technique, objective, beam angles, beam modifiers
Computer optimised: spot positions, spot weights

Question 3

What is the planning technique used dependent on?

Answer 3

Anatomy of targets and OARs
Type of cancer
Delivery technology
On-treatment imaging technology
Treatment planning system features
Department protocol

Question 4

What are the three planning techniques?

Answer 4

Single field optimisation
Multi field optimisation
Distal edge tracking

Question 5

What are the advantages of MFO?

Answer 5

Provides more control of the combined dose distribution
Only combined dose distribution needs to be uniform - not each field’s dose distribution
Gives optimiser more freedom to produce combined dose in any way it likes

Question 6

What are the issue with MFO?

Answer 6

How do we know optimiser picked a safe solution?

How sensitive is it to uncertainties?

Question 7

What is distal edge tracking?

Answer 7

In SFO - distal edge spots have highest weighting
In distal edge tracking - only the spots on the distal edge are used - use multiple fields to get dose distribution
Quicker to deliver, less robust
Not clinical

Question 8

What is the range uncertainty for protons?

Answer 8

2.7-4.6% + 1.2mm

Question 9

What are the sources of range uncertainty?

Answer 9

CT calibration and artefacts - systemic, unrelated to patient position - HU to stopping power has uncertainty
Beam paths going through inhomogeneities - patient set-up/movement, gas/liquid in patient cavities
Patient anatomy changes from planning scan - weight loss/gain, tumour regression

Question 10

What is the effect of range uncertainty?

Answer 10

Spot can move deeper or shallower, resulting in a different dose distribution, creating an over/underdose situation

Question 11

Why is PBS more sensitive to inhomogeneities?

Answer 11

Spots move due to cavities, dense targets in low surroundings, moving targets

Question 12

What will patients losing weight result in for the dose distribution?

Answer 12

Less tissue therefore the dose will overshoot the target

Question 13

How can PBS plans cope with uncertainties?

Answer 13

Fields contain 1000 spots
Need to weight spots for high degree of degeneracy
Need to make sure plan remains in tolerance under a range of error scenarios

Question 14

How can the beam directions be used to cope with uncertainties?

Answer 14

Avoid beam directions where OAR is directly behind target
Using the lateral edge avoids range uncertainty problems
Additional fields/patched fields may help

Question 15

Why do uniform CTV to PTV expansions not necessarily work for protons?

Answer 15

The static dose cloud approximation does not approximate the proton dose distribution well due to the energy dependent range of the protons

Question 16

How can target definitions be used to cope with uncertainties?

Answer 16

Use beam specific PTVs, grow the PTV based on direction (along or lateral to beam) and heterogeneities in the beam path

Question 17

How can robust optimisation be used to reduce uncertainties?

Answer 17

Optimise to CTV - ignore PTV
Information describing the uncertainties are supplied to the optimiser
Optimiser looks for plans that meet the objectives for both the nominal case and a number of error scenarios
Available on commercial TPS, slower optimisation

Question 18

What is the issue with creating a robust plan?

Answer 18

Plan quality without the error reduces
Irradiate a larger volume of healthy tissue
Compromise target coverage to ensure OAR doses are acceptable
The larger the uncertainty, the larger the compromise

Question 19

Is it possible to have a globally robust plan?

Answer 19

No - plans are robust to specified objectives under certain conditions

Question 20

How can treatment plan verification be done?

Answer 20

Physical measurement - takes 1-3 hours per plan for SPB, for 750 plans a year do 3 plan checks an evening
Independent dose calculation

Question 21

What is the procedure for a physical plan check?

Answer 21

Deliver each field at gantry 0 to a 2D array in water
Measure proton fluence to compare to TPS
Measure 2-3 depths for validation of range
Compare absolute dose vs TPS
May adjust MU to scale the dose required due to difficulty modelling halo and range shifter in TPS
Validates range and dose to water not patient

Question 22

Why use MC as a tool for software plan verification?

Answer 22

Independent validation of spot weights
Independent validation of Bragg peak range in CT
Improved dose calculation of inhomogeneities
Full modelling of nuclear interactions

Question 23

How are MC systems compared?

Answer 23

Do gamma analysis of doses produced for same plan on each system

Question 24

What is the equation for the RBE?

Answer 24

RBE = Dphoton/Dion that gives an isoeffect

Question 25

What does RBE depend on and what value is usually clinically used?

Answer 25

Use 1.1 -conservative

Depends on tissue type, tissue oxygenation, endpoint, proton energy/linear energy transfer

Question 26

Why is the LET used?

Answer 26

RBE depends on physical factors that are hard to model, LET only depends on the physical characteristics of the beam

Question 27

What is the LET?

Answer 27

A measure of the energy deposited over distance - highest at distal edge where the majority of protons deposit most of the energy

Question 28

How is the LET of a beam calculated?

Answer 28

All protons cotribute to LET
Can combine using track averaged LET or dose averaged LET
Can;t measure LET directly - use MC simulations

Question 29

What future developments are being worked on?

Answer 29

Adaptive treatment
Moving targets - gating, repainting, scan path optimisation
Reducing stopping power uncertainty - dual energy CT, proton imaging
Extended indications list - clinical trials evidence