Lomax Treatment Planning - American Association of ... compound The relationship between chemical composition and proton stopping power [KN] coh coh ph µ= ρTissueNTissue ph 3.62Z

Treatment planning for Pencil Beam Scanning

Tony Lomax, Alessandra Bolsi, Francesca Albertini,

Damien Weber

Paul Scherrer Institute, Switzerland

1.  Field shaping and op0misa0on 2.  Plan design

3.  Lateral penumbra and PBS

4.  Range and posi0oning uncertain0es 5. Mo0on

Overview of presentation





Single field, uniform dose (SFUD) planning

The combination of individually optimised fields, each of which

deliver a (more or less) homogenous dose across the target volume

SFUD is the spot scanning equivalent of treating with ‘open’ fields.

Field shaping and optimisation

SFUD planning

Spot definition

Incident field Incident field

Spot selection

Selected spots

Initial dose distribution

Dose calculation

Spot weight optimisation

Optimised dose

Dose Calculation


An example SFUD plan.

Note, each individual field is homogenous across the target volume

F1 F2

F3 F4

Combined distribution


1st series (0-40CGE)

3 field SFUD plan to PTV

2nd series (40-74CGE)

3 field SFUD plan to

‘TechPTV’

Full treatment + =

An example SFUD plan


The TechPTV or ‘Virtual 3d block’

In order to carve-out dose to neighbouring critical

structures, need to be able to ‘block’ out dose

Modified target volume used to define ‘Virtual 3d blocks’

Alternatively, dose can be spared to critical structure by

Single Field Optimisation (SFO)


Intensity Modulated Proton Therapy (IMPT)

The simultaneous optimisation of all Bragg peaks from all fields (with or

without additional dose constraints to neighbouring critical structures)

IMPT is the spot scanning equivalent of IMRT (and field patching for passive

scattering proton therapy).


An example IMPT plan

F1 F2

F3 F4

Combined distribution

Note, each individual field is highly in-homogenous (in dose) across the target volume (c.f. SFUD plans)


Example clinical IMPT plans delivered at PSI

Skull-base chordoma

4 fields

3 field IMPT plan to an 8 year old boy

3 fields


Two, 5 field IMPT dose distributions

A B

Corresponding spot weight distributions from field 2

3D IMPT DET

There’s more than one way to optimise an IMPT plan…


In-fi

eld

inte

nsity

mod

ulat

ion

Technique

DET

Degeneracy: A continuum of modulation*

* Inspired by Stephen Dowdell, MGH, Boston

SFUD IMPT


2500 spots per field 150 spots per field 120 spots per field

SFUD/IMPT DET Beyond DET…

Degeneracy: Beyond DET... Field shaping and optimisation

Multiple Criteria Optimisation (MCO)

Pareto surface (David Craft,

MGH)

Modulation

Chen et al 2010, Med. Phys. 37:4938-4945






Geometric avoidance of organs at risk.

The selection of beam incidences which avoid critical structures leads…

…‘automatically’ to reduced doses to the critical structures

Plan design

…and 19%

For same mean dose to target, 15MV photons deliver an integral dose of….

16%…

The corresponding values for two proton fields are..

7%… …and 13%

Field selection and integral dose – protons vs photons

Plan design

Avoidance of coarse density heterogeneities.

•  Accuracy of dose calculations

•  Effects on dose homogeneity and conformity

•  Sensitivity of a plan to spatial delivery uncertainties.

Plan design

Vol = 86%

Vol = 99%

A ‘homogenous’ field direction

Analytical Dose difference (MC-RC %)

0 5 10 -10 -5 10 20 30 40 50

Volu

me

(%)

Difference histogram

MC

Dose difference (MC-RC %) 0 5 10 -10 -5

10 20 30 40 50

Volu

me

(%)

Analytical

Difference histogram Vol = 64%

Vol = 89%

MC

An ‘inhomogenous’ field direction

Plan design

Effects on (single field) dose conformity

Example field through relatively homogenous

anatomy

Example field through very inhomogenous

anatomy

Plan design

Plan design

The importance of multiple fields

•  Improve dose homogeneity

•  Reduce dose to normal tissues (but increase irradiated volume)

•  Improve plan robustness

Plan design The importance of multiple fields

Improved plan homogeneity and reduced dose to normal tissues

Plan design

Albertini et al 2011 PMB 56:4399-4413, Lowe et al 2015 manuscript in preparation

The importance of multiple fields

Improved plan robustness

Dose error bars for frac0onated posi0oning errors

σ85% = 3.3mm

Frac0ons = 23





~6MV photons

The problem of lateral fall-off for PBS

Safai et al 2008 PMB 21;1729-1750

Passive scaHering (collimated)

PBS (uncollimated)

Lateral penumbra

The problem of superficial spots… •  Superficial spots (ranges < ~5 cm) require low energies (< ~80MeV) ~5cm

Typical solution is to use a preabsorber. E.g…

5cm (wer)

Nozzle Patient

•  Low energy protons are difficult to transport through the beamline/gantry

•  Low energy Bragg peaks are very sharp

Lateral penumbra

The problem of superficial spots… •  However, scattering in the absorber broadens the beam…

+

Air gap

Air gap

•  …and the beam broadens as a function of the gap between absorber and patient

When using a preabsorber then, the nozzle-patient

distance must be minimised to reduce beam size.

This is not always possible…

Lateral penumbra

Beam widths (σ in air) for PSI Gantry 2 at iso-centre

The problem of the preabsorber…

Without preabsorber 80 MeV, without

preabsorber ~ 4.7mm

80 MeV, with preabsorber ~ 10.5mm

Lateral penumbra

Lateral profiles

-20

0

20

40

60

80

100

120

-10 -8 -6 -4 -2 0 2 4 6 8 10

X coordinate (cm)

Dos

(%)

80-20% Fall offs: No absorber: 8mm

Absorber (9cm gap): 12mm

Absorber (27cm gap): 16mm

No absorber

Absorber (9cm) Absorber (27cm)

The problem of the preabsorber… Effect on lateral fall-off

No absorber

Absorber (9cm gap)

Absorber (27cm gap)

Geometric box calculations

Lateral penumbra

Are superficial spots important?

An orbital rhabdomyosarcoma (pediatric case)

•  Critical structure - lacrimal gland

•  Maximum range per field: ~5cm

Lateral penumbra

Spot-range histogram

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250

Range (mm)

Per

cent

age

of B

ragg

pea

ks

Spot-range histogram

Cumulative BP-range frequency plot

calculated from 3829 fields applied

at PSI

Are superficial spots important?

More than 30% of delivered spots have ranges of <=5cm

Lateral penumbra

Lateral penumbra

Collimation and PBS

Hyer et al 2014 Med Phys 41:091701

Time for a break...





The advantage of protons is that they stop.

The disadvantage of protons is that we don’t always know where…

10% range error

The influence of range uncertainties

Range and positioning uncertainties

Potential magnitude

Beam energy [σ]

Patient positioning [σ]

Inherent CT uncertainties (beam hardening, calibration etc) [Σ]

Distal end RBE enhancements [Σ]

CT artifacts [Σ]

Variations in patient anatomy [Σ,σ]

The ‘Bermuda Triangle’ of range uncertainties


Potential magnitude

Beam energy [σ]




CT artifacts [Σ]




WaterWater

TissueTissueTissuee

Tissue

NNSP

ρρ

ρ =≈

∑=i

iiA

Tissue

AZNN ω

where

NA is Avagadro’s number Zi, Ai and ωi are the atomic number, atomic weight and the proportion by weight of the ith element of

the compound

The relationship between chemical composition and

proton stopping power

[ ]KNcoh

cohph

phTissueTissue KZKZKN ++= 86.162.3ρµ

where

and again Zi and ωi are the atomic number and the proportion by weight of the ith

element of the compound

The relationship between chemical composition and

Hounsfield units

[ ] 62.31

62.3∑= iiph ZZ ω

[ ] 86.11

86.1∑= iicoh ZZ ω

Calibrating CT to stopping power (1)

Using these relationships, we can calculate a calibration curve between SP and HU for biological samples


Stoichiometric calibration

1. ‘Parametrize’ CT scanner using tissue substitutes (Kph, Kcoh, KKN)

Parametrization (measured

versus theoretical)

Fitted calibration curve (SP

versus HU)

Accuracy c.f. experiments 1-2% (Schaffner et al, PMB, 1998)

2. From parametrization, calculate HU and SP of biological tissues using their chemical composition

3. Fit multi-linear calibration curve through calculated points (Schneider et al, PMB 1996)

Calibrating CT to stopping power (2)


SFUD

Lomax AJ (2007) in ‘Proton and charged particle Radiotherapy’, Lippincott, Williams and Wilkins

IMPT

Degeneracy and range uncertainty


SFUD

IMPT




CTV (IMPT)

0

20

40

60

80

100

120

80 85 90 95 100 105 110 115

Dose (%)

Vol

ume

(%)

NominalCT +5%CT -5%

CTV (SFUD)

0

20

40

60

80

100

120

80 85 90 95 100 105 110 115

Dose (%)

Vol

ume

(%)

NominalCT +5%CT - 5%

+5% CT -5% CT




Modeling the effect of delivery uncertainties

SFUD IMPT

Dose distributions

Dose error-bar distributions (+/-3% range)

Albertini et al 2011 PMB 56;4399-4413


Error bars (% of prescription dose)

Volu

me

(%)

5 10 15 20 25 0 0

20

40

60

80

100

Modeling the effect of delivery uncertainties – Error-bar distributions

Error-bar volume histograms (EVH’s)

SFUD (PTV)

IMPT (PTV)

SFUD (CTV)

IMPT (CTV)


Albertini et al 2011 PMB 56;4399-4413

Potential magnitude

Beam energy [σ]




CT artifacts [Σ]




CT artifacts [Σ]


The problem of metal artefacts

Spinal stabilisation Hip prosthesis


The problem of metal artefacts kV-CT

MV-CT

0

0.5

1

1.5

2

2.5

3

3.5

0 20 40 60 80 100 120 140 160 180 200 220 240

X (voxels)

Sto

pp

ing

po

wer

KV SPMV SP

Prosthesis kV-CT artifacts

Stopping power profiles

PTV

Albertini et al, PTCOG 2006


Correcting metal artefacts

The manual approach

Uncorrected CT Artefact corrected CT


Dietlicher et al 2014 PMB 59:7181-‐7194


Experimental validation

Plans calculated to uncorrected and manually corrected versions of the CT Each plan delivered to phantom and dose measured with film in three planes (as indicated)


Gamma agreement Manually corrected CT

97% with γ<1 (3mm/3%)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Dietlicher et al 2014 PMB 59:7181-‐7194


Experimental validation

89% with γ<1 (3mm/3%)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Gamma agreement Uncorrected CT

Potential magnitude

Beam energy [σ]




CT artifacts [Σ]





Planning CT Planning CT

Repeat CT

Repeat CT, 1st fraction

E.g. Variations in bowel filling

Francesca Albertini and Alessandra Bolsi (PSI)


Recalculated dose Francesca Albertini and Alessandra Bolsi (PSI)


Robust planning in the bowel

Planning (modified) CT

Original CT Planned dose

IMPT for robust planning

Lomax et al 2001, Med. Phys. 28:317-324


IMPT for robust planning


Liu et al 2012, Med Phys 39:1079-1091

Unkelbach et al 2009, Med Phys 36:149-163





Motion

The three challenges to PBS proton therapy

Dose blurring Interplay Range varia0ons

1.8σ 1.3σ

Assume σ = 0.5cm

For this example, dose errors of ~20% can result from motion

(positioning) errors of 2.5mm

Phillips et al., PMB, 37:223-234,1992

Interplay

Motion

Max dose 111.2%

Max dose 112.8%

Static Max dose 120.2%

With motion (~6mm S-I)

Single field

Max dose 104.5%

Three fields

Interplay and multiple fields Motion

Knopf et al, Phys. Med. Biol. 56 (2011) 7257-7271

Max dose 111.2%

Static 4x rescanning

Single field

Max dose 104.5%

Three fields

Interplay and rescanning Motion

Max dose 108.6%

Max dose 104.9%


Range variations

Motion


Zhang et al 2015, PTCOG 54, San Diego.

Motion Range adapted PTV’s

3 field plan to gITV

3 field plan to raITV

4D-‐CT

Motion

Zhang et al 2015, PTCOG 54, San Diego.

Range adapted PTV’s and rescanning

gITV

raITV

no rescanning

HI: 32%

Rescanning

HI: 20%

HI: 25% HI: 12%

•  Treatment planning for PBS is a flexible and automated process

•  Lateral penumbra is a limi0ng factor for PBS. Collimated PBS therapies will help to improve this in the future.

•  The planning process is highly degenerate. This can be good and bad…

•  Uncertainty in range is unavoidable, but is manageable if understood.

•  Organ mo0on can severely degrade the delivered dose distribu0on, but re-‐scanning and ‘smart’ target design can help mi0gate the

problem

•  The easiest form of robust planning is to use mul0ple beams. They can mi0gate (and hide) a mul0tude of evils…

Take home messages

The lateral dose fall-‐off of PBS proton therapy is…

20%

20%

20%

20%

20% 1.  Below 5cm range is worse than photons but beHer than passive scaHering

2.  Above 15cm range is beHer than for passive scaHering but worse than photon therapy

3.  Is always beHer than photons or passive scaHered protons

4.  Is always worse than passive scaHered protons 5. Is only dependent on the beam width in air

10

ANSWER

2. Above 15cm range is beHer than for passive scaHering but worse than photon therapy

The air gap in PBS therapies …

20%

20%

20%

20%

20% 1.  Is important due to large amount of scaHer in air that substan0ally broadens the beam

2.  Can be neglected

3.  Should be minimized only for the treatment of deep seated tumours

4.  Should be minimized, in par0cular for the treatment of superficial tumors

5.  Is only determined by the shape of the pa0ent.

10

ANSWER

4. Should be minimized, in par0cular for superficial tumors

20%

20%

20%

20%

20% 1.  Is mainly due to uncontrollable varia0ons in proton beam energy

2.  Is independent on the quality of CT data used for planning

3.  Is typically in the sub-‐millimeter range

4.  Can easily be reduced to sub-‐millimeter levels with careful calibra0on of the CT scanner

5.  Will typically be of a few percent of proton range in the best case

10

Range uncertainty for proton therapy…

ANSWER

5. Will typically be of a few percent of proton range in the best case

20%

20%

20%

20%

20% 1.  Is not a problem

2.  Is less problema0c than for passive scaHering

3.  Is less of an issue than for IMRT

4.  Is subject to interplay effects that effect target dose homogeneity

5.  Is only a problem if mo0on is more than 1cm

10

The treatment of moving targets with

PBS…

ANSWER

4. Are subject to interplay effects that effect target dose homogeneity

20%

20%

20%

20%

20% 1.  Use as few beam orienta0ons as possible

2.  Use the distal fall-‐off to spare cri0cal organs such as the spinal cord and brain stem

3.  Maximise the path length to the target to minimize pencil beam size

4.  Use mul0ple beam orienta0ons to improve plan robustness

5.  Use mul0ple fields to minimize integral dose

10

When designing plans and selecSng beam orientaSons for PBS proton therapy, it is good

pracSce to…

ANSWER

4. Use mul0ple field direc0ons to improve plan robustness

Documents

Lomax Treatment Planning - American Association of ... compound The relationship between chemical composition and proton stopping power [KN] coh coh ph µ= ρTissueNTissue ph 3.62Z