Center for proton therapy - PSI

Pedroni Eros

Paul Scherrer Institute - Villigen PSI, Switzerland

Center for proton therapy - PSI

10.06.2011PSI,

GANTRY DESIGN AND RECENT EXPERIENCE AT PSI

2nd Workshop on Hadron Beam Therapy of Cancer

ERICE

May 23th, 2011

on behalf of the Gantry 2 group

Layout of the presentation

–Gantry 1 experience

–Facility expansion at PSI - new SC cyclotron

–The new Gantry 2 of PSI

– Layout

– Beam optics and beam size

– Energy variations with the beam

– Sweeper magnets calibration

10.06.2011PSI, Page 2

– Sweeper magnets calibration

–Advanced beam scanning techniques

–Possible future clinical use of Gantry 2

Early 90's …

GANTRY 1

USED FOR PATIENT TREATMENTS SINCE 1996

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GOAL in 1989

BASIC FEASIBILITY OF PENCIL BEAM SCANNING

The long term experience of using GANTRY 1

• Designed in 1991 for protons

• On the basis of the scanning experience

with pion therapy 1981-1992

with inverse planning based on CT data

1981

1992

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Gantry 1

1989

• Upstream scanning applied only in the dispersion plane of the beam

• Magnetic scanning started before the last bending magnet

• Eccentric mounting of patient table on the gantry front wheel

(with counter-rotation)

• Gantry radius reduce to 2 m

• Still the smallest proton gantry in the world

System characteristics of Gantry 1

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α rotation

φ rotationβ rotation

If i could do it again …

• Eccentric mounting as with Gantry 1 (R = 2 m)

• But with rotation only on one side (0° 180°) as with the new Gantry 2

• Permanent floor underneath the patient table

• "Treatment cell" moving with the gantry

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• 2 m compact eccentric gantry

A new Gantry 3 ?

Pencil beam scanning

• Small pencil beam: 3 mm σ (7-8 mm FWHM)

• Cartesian scanning (infinite SSD)

• Discrete spot scanning

• “step and shoot” ,method - on a 5 mm grid

• From 1996 until May 2008

the only scanning gantry in the world

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X Sweeper magnet 5 ms / step fast

Y Range shifter 30 ms average

Z Patient table 1 cm/s slow

Dose Monitor+Kicker 100 us reaction time

Elements of scanning:

the only scanning gantry in the world

Patient handling with Gantry 1

• Use of a CT outside of the treatment area

• Position next patient while treating the previous one

• Transfer of positioned patients with automated transporter

• Very useful for treating children with anesthesia

• Patient positioning control with X-ray on Gantry 1 (optional)

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Clinical use of GANTRY 1

• In use since1996

• Full fractionation ~30 fractions

• Treatments 8:00 16:00

• Max 19 patients/day (2.5 per hour)

• 2.8 fields/fractions in average

• 1/3 of patients are children

• Under anesthesia

• 1/3 of treatments are IMPT

Courtesy of B. Timmermann

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• 1/3 of treatments are IMPT

• Weak points of Gantry 1

• Table motion is part of scanning

• Not possible to use collimators

• Not possible to apply repainting

• We treat only

• non moving targets!

Advantages of using scanning (vs. scattering)

• Automated treatments – all by software• Minimal simple equipment

• “The (pencil) beam "does it all"

• Same approach from small to large fields• No individualized hardware (no fabrication of collimators compensators )

• Apply dose fields in sequence without personnel entering the treatment room

• Simplify treatment - reduce treatment time and costs

• True 3-dimensional conformation

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• True 3-dimensional conformation

• Variable modulation of the range • Minimal neutron background (for the patient)

• Lower risk of second cancer than scattering (depends on sophistication)

• Important for pediatric treatments• Less activation of equipment in the nozzle (for the personnel)

• Less activation of the accelerator

• Better lateral fall-off as compared to scattering (?)

• No material in the beam path (except air and monitors) - Less MCS effects

Delivery of IMPT and IGPT

• Dose shaping within the target

• IMPT (intensity modulated PT)

• The term of comparison with IMRT (conventional therapy)

• Biological targeting (image guided proton therapy)

• Intentional non-homogeneous dose distributions

• Dose proportional to

the tumor activity

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the tumor activity

(biological signal)

• The topic for the

future…

Courtesy of A. Lomax PSI

Major disadvantage: the organ motion sensitivity

• Interference of organ motion with the scanning sequence

• Disturbance of the dose homogeneity within the target

• With Gantry 1 we treat only immobilized lesions attached to bony structures

• Tumors in the head, spinal chord and low pelvis

• We accept only movements < + -2 mm

• for treatments with full fractionation!

• Can we overcome this drawback?

The point of scanning to be improved

M.Phillips PSI 1992

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• Can we overcome this drawback?

• Possible remedies :

• Fast Scanning (Repainting)

• Gating

• Tracking?

• or a combination of those

• … the possible points to be developed

with Gantry 2…

2000

EXPANSION OF THE PROTON FACILITY AT PSI

SUPERCONDUCTING CYCLOTRON

FAST DEGRADER

LAMINATED BEAM LINES

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LAMINATED BEAM LINES

GOALS:

Stable DC beam

Modulation of the beam intensity at the ion source

Very fast energy changes

Layout of the newly expanded proton facility

• COMET - dedicated superconducting cyclotron [ACCEL - design H. Blosser]

• Beam for Gantry 1 all the year through

• patients treatments restarted in February 2007 no shut-downs since August 07

• Horizontal beam line for OPTIS 2 transfer from OPTIS 1 last year

• Next generation scanning gantry : Gantry 2 1. patient planned for 2012

Exp. Area (PIF)Medical cyclotron

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Gantry 2

Gantry 1

OPTIS 2

Disconnected from

PSI ring cyclotron in 2006

Facility specifications … mainly for the new Gantry 2

• Super-conducting cyclotron

• Very stable beam at the ion source

• Aiming at 2-3% at the 100-200 µs scale

• Deflector plate in the first orbit

• Dynamic control of the beam intensity

• 100-200 µs time scale

• Fast degrader (moving carbon wedges)

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• Fast degrader (moving carbon wedges)

• Continuous choice of the beam energy

• Beam line with laminated magnets

• Providing fast changes of beam energy

Excellent beam current stability … an example

• Change of paradigm of testing the beam monitors

• Use the inherent stability of the beam to check the fine

tuning of the electronics (capacitive matching)

• Result: improved linearity of the dose

• < 0.5% down to a 0.1 Gy dose

• Ideal beam for scanning 100% duty factor• 33

nF600 µs

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Dose measurements for diff. Plugin configurations

0.975

0.98

0.985

0.99

0.995

1

1.005

1.01

0 20 40 60 80 100 120

applied Dose [cGy]

mea

sure

d D

ose

/mea

sure

d D

ose

@ 1

Gy

ISO, 22pf

ISO, 47 pf

ISO, 33 pf

ISO, 39 pf

• 22

nF

nF600 µs

2005

A NEW PSI GANTRY - GANTRY 2

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MAIN GOAL

DEVELOP FURTHER SCANNING

AS A UNIVERSAL BEAM DELIVERY TECHNIQUE

GANTRY 2

TOPIC 1: INNOVATIVE GANTRY LAYOUT

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READINESS FOR

IMAGE GUIDED PROTON THERAPY

EFFICIENT PATIENT HANDLING

Beam Line

Support

Bearing axle

From -30°

to +180°

Design started from the patient table…

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Patient table

Room withfixed floor

0°to +180°would have been a better choice (cheaper) …

Layout of the Gantry 2 room: patient table, compact nozzle

Small compact nozzle

Same patient table as

at RPTC in Munich

(Schär Engineering)

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• Easy access to the patient table on fixed floor

• Fixed walls and ceiling for mounting commercial equipment (Vision RT?)

A system open on

both sides

- lateral and front -

Optimal for using an

in-room sliding CT

In-room sliding CT - within reach with the patient table

The beginning of

IGPT ?

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• Patient positioning (tumor in soft tissues region)

• Setup for treating moving targets - 4D CT (relation external gate - internal motion)

• BEV imaging - equivalent to portal imaging with photons• Very large field-of-view (26 cm x 16 cm)

• not masked by equipment or collimators in the beam path• QA control of gating and tracking

(scanning + pulsed X-rays)• Fluoroscopy mode

• Beam guidance?

BEV X-ray - synchronized with proton beam delivery

Scan and bend

IGPT

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a) compact gantry b) long throw gantry

Sweepers

X rays tubeProton beam

Bending

magnet

nozzle

Yoke hole

Patient

Imager

Sweeper

or

Scatterer

Collimator

UPSTREAM SCANNING

Bend and scan

BEV - imager

• Check patient

position at the

isocenter

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• Photograph of the retractable arm for holding the X-ray panel

• behind the patient on the side opposite to the nozzle (BEV X-ray).

GANTRY 2

TOPIC 2: PENCIL BEAM AND BEAM OPTICS

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GOALS:

SMALL PENCIL BEAM FOR PRECISION THERAPY

PARALLELISM OF SCANNING

GANTRY 2 beam optics

Q1

Q3

Q4Q5

Q6

Q7

A1

A2 A3

WU

WT

M1 M2

M3 P

P

S

S

3.2 m

Q2

QC

X

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• Rotational symmetric (large) phase space +- 3mm +- 10 mrad +- 0.5% dp/p

• Complete achromatism

• Scanning-invariant beam focus

• Orthogonal (x-,y-) focal planes in T and U

• Focus to focus with 1:1 imaging from the coupling point of the gantry to the iso-center

• 2D- parallelism of scanning (in T and U) (on Gantry 1 only U)

SPECIFICATIONS

A3A2A1QC

WTWU

H S Double parallel

scanning

Point to point

GANTRY 2 beam optics (TRANSPORT and RAYTRACE)

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TRANSPORT beam envelopes through the Gantry 2

Q2Q1 Q3 Q4 Q5Q6Q7

focus

Achromatic

Double parallel

scanning

Advantages of coupling the gantry at a small beam focus

• At a small beam focus the spot image is insensitive to scattering

• Decouple vacuum (gantry from facility)

• Collimate the beam at the coupling point

• Forced beam centering on axis

• Measure the beam characteristics on-line before the gantry

• On-line independently of Gantry rotation and scanning

• Manipulate the beam shape imaged on the scanning beam

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• Manipulate the beam shape imaged on the scanning beam

• Large Gaussian beam shape ?

• Virtual dynamic collimation at coupling point ?

• Disadvantage

• A focus coupling needs a longer beam line coupling with a parallel beam

Advantage of using parallel scanning

• Less skin dose

• Simplify treatment planning

• No inverse squared distance effects

• Simplify dosimetry and QA control

• Dose value and dose distribution in the patient and in the phantoms are “the same”

• Easier field patching (expansion of the scan range for treating long targets)

• Can be done without optimization within treatment planning

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• Just exchange magnetic scan position with patient table offset

• Compensators (simulated scattering)

• No dose errors due to inverse R-square effects

• Collimators

• No tapered faces necessary

A big beast - 34 tons

15 cm gap

To accommodate

a scan area of

20 x 12 cm

Lamination leaks …

The difficult piece: the last 90° bending magnet

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Photograph of the mounting of the 90° bending magnet.

Integrated vacuum chamber embracing the poles of the magnet

180 tons … mechanical isocenter within + - 0.4 mm

Lamination leaks …

but in the end

Vacuum problems

well solved !

• Vacuum “up to the patient”

• Sharp pencil beam - 3 mm sigma

• Two monitors and a strip monitor

• 2 mm strips (TERA collaboration)

• Removable pre-absorber

• IN and OUT of beam

• For ranges below 4 cm

Compact optimized nozzle

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• For ranges below 4 cm

• Telescopic motion of the nozzle

• To reduce air gap (keep patient at isocenter)

• Option to add collimator (compensator)

• To shield OAR on top of scanning

• To simulate passive scattering with a scanning beam

• Collision protection to treat patients remotely (multiple

fields in one go)

Results: pencil beam size (on axis)

• Minimize material in the nozzle for keeping To have a sharp lateral fall-off

the beam size between 3 and 4 mm sigma at all energies (100-200 MeV)

0.5

0.6

Adding piece by piecethe materials in the nozzle

Region 70 - 100 MeV

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0

0.1

0.2

0.3

0.4

0 50 100

GANTRY 2

TOPIC 3 :

ENERGY VARIATIONS WITH THE BEAM LINE

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COMPENSATED INTENSITY-ENERGY LOSSES

VERY FAST ENERGY CHANGES

SC cyclotron

Degrader

Beam analysis

Gantry 2

Compensation of beam losses in the range 100-200 MeV

200-400 nA Constant ratio

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Collimators forIntensity suppression

Coupling point

Rate Monitor 1

0

100

200

300

400

500

600

100 120 140 160 180 200

Energy [MeV]

kHz

0.2-0.4 nA

Goal : reserve the use of the deflector plate for intensity modulated dose painting

Optimization for maximum transmission at 100 MeV

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Beam envelopes at 100 MeV.

Horizontal (red) and vertical (blue) envelopes

Green line = transmitted beam intensity Dots beam sizes calculated with TURTLE

3 4

Intentional beam defocusing at higher energies

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1 2

Beam envelopes at 200 MeV.

yellow we four beam line sections.

Beam defocusing at collimators (black bars)

Same intensity losses as at 100 MeV

• Energy loop Red: up-down Blue: down-up

Taking into account hysteresis effects …(70-230 MeV)

Uncorrected

Corrected

1 mm

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Corrected

• But - we observe

• Slow change of lateral beam position after a

big energy change in the range 230-70 MeV

• 1-3 mm with exponential decay

with decay time of ~2 s

• < 0.3 mm position change after small energy

changes within the SOBP

With very fast energy changes …. 80 ms

10.06.2011PSI, Page 37

Scintillator block

the beam of Gantry 2 seen with a

TV camera

• We plan to correct these shift with sweeper

offsets as a function of the time after the last

energy change

• Strategy:

• Fixed targets - range precision of < 0.5 mm

while working with full ramp 70-230 MeV

• Moving targets - repainting - precision ~ 1mm

• repaint only the SOBP up and down

GANTRY 2

TOPIC 4 : SWEEPER MAGNETS

10.06.2011PSI, Page 38

COMMISSIONING ISSUES

NON LINEARITIES OF THE SWEEPERS

−8

−6

−4

−2

0

2

4

6

8210 MeV

T (

cm)

−6

−4

−2

0

2

4

6

8150 MeV

T (

cm)

210 MEV 150 MEV

Need of a very precise mapping of the sweeper's action

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−15 −10 −5 0 5 10 15−8

U (cm) −15 −10 −5 0 5 10 15−8

U (cm)

−15 −10 −5 0 5 10 15−8

−6

−4

−2

0

2

4

6

880 MeV

U (cm)

T (

cm)

Measured (red) and calculated (blue) spot maps of

the (linear input) action of the sweeper

magnets on the scanned beam position.

The non linearities are due to a curvature

of the effective boundary of the magnetic field

of the 90° bending magnet which is

changing with energy (changing sign)

80 MEV

100 MeV

U

T

QMF1: 0% QMF1: -10% QMF1: -20%

Invariant beam focusing … needs some "tricks"

First tests in 2008

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• Anticipated by Raytrace calculations

• Tapering of the poles surface of the U

sweeper

• Quadrupole corrector QMFC switched

in series with the U-Sweeper

• Results:

acceptable scanning-invariant focus

QMFC in series

70 MEV 120 MEV

After a proper mapping of the sweepers

10.06.2011PSI, Page 41

Beam spots (2 cm steps) at the isocenter covering the scan region of 12 cm x 20 cm

170 MEV 220 MEV

GANTRY 2

Topic 5 : the main goal of Gantry 2

NEW ADVANCED BEAM DELIVERY TECHNIQUES

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TO PROMOTE SCANNING AS A UNIVERSAL BEAM DELIVERY

TECHNIQUE

• For reducing organ motion errors

• Goal - Fast painting with volumetric repainting

• Painting lines

• < 10 ms per line (10cm + line change)

Aiming for highest scanning speed From spots

To lines

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• < 10 ms per line (10cm + line change)

• Painting energy layers

• 200 ms per plane (20 lines x 5 mm)

• Change of energy (100 ms - 5mm range)

• Painting of volume

• 6 s per liter (20 energies at 5mm steps)

• Volumetric repainting capability (aim)

• 10-20 repaintings / liter in 2 minutes

To contours

Conformation requires non-homogenous proton fluences

3d-Dosehomogeneous

U

T

depth

10.06.2011PSI, Page 44

Proton fluenceof energy layerisnon homogeneous

Repainted for coping with organ motion

Time driven beam delivery Beam path downloaded as tables of the sweepers U - T and beam intensity IDose control with feed-back loop:

Monitor 1 required dose -> vertical deflector plate

A) If sweeper speed is the limituse variable intensity (IM)

FPGA based delivery of tabulated energy layers

0.2 0.7 10

500

1000

1500

2000

Required Dose

Del

iver

ed M

Us

Dose linearity of simple T-lines

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B) If beam dose rate is the limituse variable sweeper speed (SSM)

C) or use both !

Full range of dynamic dose control

from zero IM

to any dose SSM

23 times

Max T speed

Variable intensity

Required Dose

5ms - 10 cm lines - painted with IM

GANTRY 2

POSSIBLE CLINICAL USE OF GANTRY 2

10.06.2011PSI, Page 46

POSSIBLE CLINICAL USE OF GANTRY 2

GOAL:

PROMOTE SCANNING AS A UNIVERSAL BEAM DELIVERY

TECHNIQUE

to completely replace scattering

• Often required

• sharp lateral fall-off at the boundary between tumor and sensitive structure

• Scanning alone better at high energy (edge enhancement with pencil beam)

• Scanning with added collimation better at low energy (beam size limitations)

• Scanning with varying beam energy superior to scattering ?… less material in the beam path

Idealized scattering

(zero phase space)

FWHMmm

Prostate

Brain stem

Improve lateral fall-off for treating static targets

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Factor 1.4 gain

Difference Gauss to

error-function (1.7)

Scanning with

collimation betterScanning alone better

(zero phase space)

Realistic scanning

Rangemm

Dorsal irradiation

Very big tumors

• Combine the use of fast scanning with patient table displacements

• Needs remote control of the patient table (collision detection)

• Take advantage of the parallelism of the beam

• trivial patching - shift table - and apply intensity filter to spot pattern

Medulloblastoma

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Example: a paediatric case

treated in 2004 with Gantry 1

COMPARE

PancreasRectumBreast

• Discrete spot scanning applied with iso-layered repainting (max dose per spot visit)

• Treatments in the trunk (pancreas, cervix, colon), breast, lymph nodes, etc.

• Advantage of having fast energy changes -> Volumetric repainting

• FAST SCANNING IS MORE THAN JUST REPEATING…

Moderately moving targets (0-5 mm)

10.06.2011PSI, Page 49

• From

• Simulation of repainting strategies …

• S. Zenklusen's thesis work

• PMB 55 (2010) 5103-5121

COMPARE

G1 spots, scaled

G2 spots, scaled

G2 spots, iso-layer

G2 lines, scaled

Largely moving targets

• Volume painting within a single breath hold” ? (repeated)

• Using Conformal line painting

• Speed of painting 0.2 Gy of a sphere of (17 layers) – ½ liter in 7 secs

• If not working, Gating

Lung liver

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7s

• Dose shaping with compensators

• Uniform scanning

• Energy layer with homogeneous fluence

• Magnetic line scanning at max. speed

• Very high repainting number

• Collimator optional

Simulated scattering

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• As a sub-mode of conformal scanning

• To simulate scattering on a scanning-gantry

• To replace completely scattering ??

Retinoblastoma ?

• 8 cm diameter sphere

Feasibility demonstration of "simulated scattering"

• Field shape = target projection

• minimal neutron background

• Variable modulation of the range

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• Distal layer: repainted 48 times in 30s

• Variable modulation of the range

• Layer shrinking

• Parallel beam

• no compensator dose errors

• Highly repainted

• From S. Zenklusen PhD - Medical Physics 2011

Status of the Gantry 2 project

• Significant project delays due to the

• Priority given to OPTIS 2 in 2008

• Difficulties with the vacuum of the 90° bending

magnet

• PSI internal development of the software for the

control of the patient table … insufficient resources

10.06.2011PSI, Page 53

THANK YOU