pedroni eros paul scherrer institute -villigen psi

Center for proton therapy - PSI

15.06.2011PSI,

Gantry 2, the next generation gantry of PSI:

a new system for promoting pencil beam scanning as a universal

beam delivery technique

IOP Half-Day Meeting on Hadron Therapy

Friday 10 June 2011

Manchester

Pedroni Eros

Paul Scherrer Institute - Villigen PSI, Switzerland

15.06.2011PSI,

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

–Advanced beam scanning techniques

–Possible future clinical use of Gantry 2

–Scanning as a universal beam delievery

15.06.2011PSI,

Early 90's …

GANTRY 1

USED FOR PATIENT TREATMENTS SINCE 1996

GOAL in 1989

SHOW THE BASIC FEASIBILITY OF PENCIL BEAM SCANNING

15.06.2011PSI,

The long term experience of PSI of using scanning

Gantry 1

• Gnatry 1 designed in 1991 for protons

• On the basis of the scanning experience

with pion therapy 1981-1992

using inverse planning - based on CT data

1981

1992

1989

15.06.2011PSI,

• Upstream scanning

• Magnetic scanning started before the last bending magnet

• Eccentric mounting of patient table on the gantry front wheel

(with counter-rotation)

• Gantry radius reduced to 2 m

• Still the smallest proton gantry in the world

System characteristics of Gantry 1

α rotation

φ rotationβ rotation

15.06.2011PSI,

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:

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

15.06.2011PSI,

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

• 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!

Courtesy of B. Timmermann

15.06.2011PSI,

Main goal: 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

(biological signal)

• The topic for the

future…

Courtesy of A. Lomax PSI

15.06.2011PSI,

Major disadvantage of scanning : 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?

• Possible remedies :

• Fast Scanning (Repainting)

• Gating

• Tracking?

• or a combination of those

• … the possible points to be developed

with Gantry 2…

The point of scanning to be improved

M.Phillips PSI 1992

15.06.2011PSI,

2000

EXPANSION OF THE PROTON FACILITY AT PSI

SUPERCONDUCTING CYCLOTRON

FAST DEGRADER

LAMINATED BEAM LINES

GOALS:

Stable DC beam

Modulation of the beam intensity at the ion source

Very fast energy changes

15.06.2011PSI,

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

Gantry 2

Gantry 1

OPTIS 2

Exp. Area (PIF)

Disconnected from

PSI ring cyclotron in 2006

Medical cyclotron

15.06.2011PSI,

Facility specifications were derived 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)

• Continuous choice of the beam energy

• Beam line with laminated magnets

• Providing fast changes of beam energy

15.06.2011PSI,

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 with 100% duty factor

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

• 33

nF600 µs

15.06.2011PSI,

2005

A NEW PSI GANTRY - GANTRY 2

MAIN GOAL

DEVELOP FURTHER SCANNING

TO BECOME A UNIVERSAL BEAM DELIVERY TECHNIQUE

15.06.2011PSI,

GANTRY 2

TOPIC 1: INNOVATIVE GANTRY LAYOUT

READINESS FOR

IMAGE GUIDED PROTON THERAPY

VERY EFFICIENT PATIENT HANDLING

15.06.2011PSI,

Beam Line

Support

Bearing axle

From -30°

to +180°

Patient table

Room withfixed floor

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

Design started from the patient table…

ANIMATION

15.06.2011PSI,

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

• 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

Small compact nozzle

Same patient table as

at RPTC in Munich

(Schär Engineering)

15.06.2011PSI,

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

• Patient positioning (tumor in soft tissues region)

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

Beginning of

IGPT ?

15.06.2011PSI,

a) compact gantry b) long throw gantry

Sweepers

X rays tubeProton beam

Bending

magnet

nozzle

Yoke hole

Patient

Imager

Sweeper

or

Scatterer

Collimator

• 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

UPSTREAM SCANNING

Bend and scan

Scan and bend

IGPT

15.06.2011PSI,

BEV - imager

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

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

• Check patient

position at the

isocenter

15.06.2011PSI,

GANTRY 2

TOPIC 2: PENCIL BEAM AND BEAM OPTICS

GOALS:

SMALL PENCIL BEAM FOR PRECISION THERAPY

PARALLELISM OF SCANNING

15.06.2011PSI,

GANTRY 2 beam optics

• 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

Q1

Q3

Q4Q5

Q6

Q7

A1

A2 A3

WU

WT

M1 M2

M3 P

P

S

S

H 3.2 m

Q2

QC

X

15.06.2011PSI,

TRANSPORT beam envelopes through the Gantry 2

A3A2A1

Q2Q1

QC

Q3 Q4 Q5Q6Q7

WTWU

H S Double parallel

scanning

Point to point

focus

Achromatic

Double parallel

scanning

GANTRY 2 beam optics (TRANSPORT and RAYTRACE)

15.06.2011PSI,

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

A big beast - 34 tons

15 cm gap

To accommodate

a scan area of

20 x 12 cm

Lamination leaks …

but in the end

Vacuum problems

well solved !

The difficult piece: the last 90° bending magnet

15.06.2011PSI,

• 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

• Telescopic motion of the nozzle

• To reduce air gap (keep patient at isocenter)

• Option to add collimators and compensators

• 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)

Compact optimized nozzle

15.06.2011PSI,

Results: pencil beam size (on axis)

• Minimize material in the nozzle for having a sharp lateral fall-off

beam size between 2.5 - 5 mm sigma at all energies (70-230 MeV)

0

0.1

0.2

0.3

0.4

0.5

0.6

0 50 100

Adding piece by piecethe materials in the nozzle

Region 70 - 100 MeV

15.06.2011PSI,

GANTRY 2

TOPIC 3 :

ENERGY VARIATIONS WITH THE BEAM LINE

COMPENSATED INTENSITY-ENERGY LOSSES

VERY FAST ENERGY CHANGES

15.06.2011PSI,

SC cyclotron

Degrader

Beam analysis

Collimators forIntensity suppression

Coupling point

Gantry 2

Compensation of beam losses in the range 100-200 MeV

Rate Monitor 1

0

100

200

300

400

500

600

100 120 140 160 180 200

Energy [MeV]

kHz

200-400 nA

0.2-0.4 nA

Constant ratio

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

15.06.2011PSI,

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

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

Uncorrected

Corrected

1 mm

15.06.2011PSI,

Scintillator block

the beam of Gantry 2 seen with a

TV camera

• But - we observe

• Slow change of lateral beam position after big

energy changes in the range 230-70 MeV

• 1-3 mm shift with exponential decay

with decay time of ~2 s

• < 0.3 mm position change after small energy

changes within the SOBP

• 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

With very fast energy changes …. 80 ms

VIDEO

15.06.2011PSI,

GANTRY 2

TOPIC 4 : SWEEPER MAGNETS

COMMISSIONING ISSUES

NON LINEARITIES OF THE SWEEPERS

15.06.2011PSI,

−15 −10 −5 0 5 10 15−8

−6

−4

−2

0

2

4

6

8210 MeV

U (cm)

T (

cm)

−15 −10 −5 0 5 10 15−8

−6

−4

−2

0

2

4

6

8150 MeV

U (cm)

T (

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)

210 MEV

80 MEV

150 MEV

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

15.06.2011PSI,

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

70 MEV 120 MEV

170 MEV 220 MEV

After a proper mapping of the sweepers

15.06.2011PSI,

GANTRY 2

Topic 5 : the main goal of Gantry 2

NEW ADVANCED BEAM DELIVERY TECHNIQUES

TO PROMOTE SCANNING AS A UNIVERSAL BEAM DELIVERY

TECHNIQUE

15.06.2011PSI,

• For reducing organ motion errors

• Goal - Fast painting with volumetric repainting

• Painting lines

• < 5-10 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

Aiming for highest repainting From spots

To lines

To contours

15.06.2011PSI,

Use of FPGAs for dose painting

• Vertical deflector plate for intensity modulation

• Installed inside accelerator after the first turn close to the ion source

• Fast intensity control at the time scale of 100 µs

• Requires flexible control system

• Synchronous control of fast actuators (sweepers, deflector plate) with 100 kHz

• Tabulated dose delivery based on state-of-the-art electronics (FPGA)

• Example: Painting shaped energy iso-layer

15.06.2011PSI,

Time driven beam delivery Beam path downloaded as tables of the sweepers U - T and of the beam intensity IDose control with a feed-back loop:Monitor 1 required dose -> vertical deflector plate

A) If sweeper speed is the limitwe use variable intensity (IM)

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

C) or we use both !

Dose delivery as a function of the real time

Full range of dynamic dose control

from zero IM

to any dose SSM

23 times

Max T speed

Variable intensity

0.2 0.7 10

500

1000

1500

2000

Required Dose

Delivered MUs

Dose linearity of simple T-lines

5ms - 10 cm lines - painted with IM

15.06.2011PSI,

Video showing fast conformal line scanning

• feed-forward only

VIDEO

15.06.2011PSI,

GANTRY 2

POSSIBLE CLINICAL USE OF GANTRY 2

GOAL:

PROMOTE SCANNING

AS A UNIVERSAL BEAM DELIVERY TECHNIQUE

make scattering obsolete

15.06.2011PSI,

• Often required

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

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

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

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

Factor 1.4 gain

Difference Gauss to

error-function (1.7)

Scanning with

collimation betterScanning alone better

Idealized scattering

(zero phase space)

Realistic scanning

FWHMmm

Rangemm

Prostate

Brain stem

Improve lateral fall-off for treating static targets

15.06.2011PSI,

Example: a paediatric case treated in 2004 with Gantry 1

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

VIDEO

15.06.2011PSI,

• 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

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 TARGET REPEATING…

Moderately moving targets (~5 mm)

15.06.2011PSI,

Largely moving targets

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

• Using Conformal line painting

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

• If not working, Gating

7s

Lung liver

15.06.2011PSI,

Retinoblastoma ?

Simulated scattering

• Dose shaping using compensators

• But applying uniform scanning

• Energy layer with homogeneous fluence

• Magnetic line scanning at max. speed

• Very high repainting number

• Collimator is optional

• As a sub-mode of conformal scanning

• To simulate scattering on a scanning-gantry

• To render scattering based gantries obsolete

15.06.2011PSI,

• Distal layer: repainted 48 times in 30s

• 8 cm diameter sphere

Feasibility demonstration of "simulated scattering"

• Field shape = target projection

• minimal neutron background

• Variable modulation of the range

• Layer shrinking

• Parallel beam

• no compensator dose errors

• Highly repainted

• From S. Zenklusen PhD - Medical Physics 2011

15.06.2011PSI,

Gantry 2 coming soon …

• On behalf of the Gantry 2 team

• D. Meer, C. Bula, S.Safai, S. König, M.Rejzek, S.

Zenklusen ….

THANK YOU

pedroni eros paul scherrer institute -villigen psi

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