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E. Pedroni CPT - Paul Scherrer Institute - Erice 20 -04-2009

Workshop on HadronBeam Therapy of Cancer

EriceApril 2009

Gantry design and experience at PSIGantry design and experience at PSIGantry design and experience at PSIGantry design and experience at PSI

Eros Pedroni

for the R&D Technology Team

Center for Proton Therapy

Paul Scherrer InstituteVilligen-PSI

SWITZERLAND


1. Experience of using GANTRY 1

Gantry 1

• A system designed in 1991 for protons

• Based on the scanning experience with

pion therapy between 1981-1992


– Magnetic scanning started before the last bending magnet

• “Upstream scanning” (applied only in the dispersion plane of the beam)

– “Excentric” mounting of patient table on the gantry front wheel (with counter-rotation)

• Reduce gantry radius to 2 m

– Patient couch rotation in the horizontal plane by +,-120°

– 360° axial rotation (a mistake?)

System characteristics of Gantry 1

α rotation

φ rotationβ rotation


α+β rotationα rotationBODY HEAD

A very compact system

• Flexibility to apply the beam from any direction

– Patients safety concerns when

treating with beam from below

• patient rescue in case of

motors failures


If I could do it again …

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

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

• Floor underneath the patient table

– Room moving with the gantry

– With a counter-rotation

• 2 m compact eccentric gantry Gantry 3 ?


X Sweeper magnet 5 ms / step fast

Y Range shifter 30 ms average

– Gaussian pencil beam: 3 mm σ in air

– Cartesian scanning (infinite SSD)

“step and shoot” on a 5 mm grid

– From 1996 until May 2008 the only

scanning gantry in the world

Dynamic pencil beam scanning

Z Patient table 1 cm/s slow

Dose Monitor+Kicker 100 us reaction time

Elements of scanning:


SKULL

PELVIS

Clinical use of GANTRY 1

• > 400 patients treated since 1996

• Mainly skull, spinal chord and low pelvis

– Only non moving targets!

– First human patient treated in 1996

– 30 fractions (treatment lasting ~ 6 weeks)

• Capability

– Max 19 patients/day (2-3 per hour)

– 2.8 fields/fractions in average

– Multiple fields delivered without personnel

entering the treatment room

• Weak points of Gantry 1

– Table motion is part of scanning (3. axis)

– Not possible to use collimators

– Not possible to apply repainting


Advantages of using scanning

• Automated treatments – all by software

• Avoid the use of individualized hardware

– Save money and personnel to fabricate and mount field-specific equipment

• Apply dose fields in sequences in one go without personnel entering the treatment room

• To reduce treatment time

• 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)

• Use of a lower extracted beam intensity

– Less activation of the accelerator

• Minimal simple equipment

– “The (pencil) beam does it for you”


Variable modulation of the range

• Avoid unnecessary 100% dose on the healthy tissues

– Especially relevant for large tumors

– Reduce skin dose


The possibility to deliver IMPT and IGPT

• IMPT (intensity modulated PT)

– The term of comparison with IMRT (conventional therapy)

– PSI: the only facility delivering routinely proton-IMRT (30% of pts)

• 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


Major disadvantage: organ motion sensitivity

• Scanning is very sensitive to organ motion errors during beam delivery -> disturbance of the dose homogeneity

– At the moment we can treat at PSI only well 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:

• Repainting

• Gating

• Tracking?

– or a combination of those

– …possible points to be developed

with Gantry 2…

The points to learn about


2. Next step in 2000 at PSI: the PROSCAN project

• Facility expansion in the NA-Hall

• Problems before 2007

– Parasitic use of the beam in a physics

research environment

• Split and degrade beam from the

(2 mA) 590 MeV main beam

– A multi-user complex facility

• Long shut-down each year

• Beam available only

from May until December

GANTRY 1

OPTIS 1

NEW PROSCAN AREA


Layout of the PROSCAN facility

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

– Beam for Gantry 1 all the year through

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

– Next generation scanning gantry : Gantry 2 (1. patient planned for 2010))

– Horizontal beam line for OPTIS 2 (1. patients in 2009 – higher priority)

Gantry 2

Gantry 1

OPTIS 2

Exp. Area (PIF)

Disconnect from

Ring cyclotron

Medical cyclotron


Dedicated accelerator COMET

• Super-conducting cyclotron

• Requirements for Gantry 2

– Constant energy

– Constant intensity

– Stable beam at the ion source

• Aiming at 2-3% in a few 100 us scale

– Dynamic control of the beam intensity

• With a deflector plate in the first turn

• Beam intensity modulation

– Aiming at 100 µs time scale

– For fast direct control of the dose

delivery while painting dose lines

• For delivering repainted scanning

– See talk of D. Meer tomorrow


Energy variations with a degrader

• Fast energy changes with degrader + beam line (GANTRY 2)

• Aiming at 100 ms for a 5 mm proton range step

Carbon wedges

moving against each other

in the beam


3. Next generation scanning gantry: Gantry 2

• A tool for developing advanced beam scanning techniques

– Iso-centric layout (~ 4 m radius)

– Double magnetic scanning (parallel) – started upstream of the last 90° bend

– Dynamic beam energy variations with the beam

(gantry beam line with laminated magnets)

• New characteristic

– The new PSI gantry rotates

only on one side

by -30°to 185°

– Flexibility of beam delivery

achieved by rotating the

patient table in the

horizontal plane

– Analogy with longitude and

latitude in world-geography


Beam Line

Support

Bearing axle

From -30°

to +180°

Patient table

Room with

fixed floor

ServicesX-ray console

Patient anddoctor

Design started from the patient table…

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


…without loosing any functionality

Sliding CT

(Siemens)

Setup for

treating

moving targets

(relation of the

external

motion

surrogate

with the

internal true

organ motion)


– The new idea to use BEV X-rays simultaneously to the proton beam

• …to be investigated

imager

X-ray

BEV X-ray: for QA of moving targets

Hole in the return

yoke of the 90°

bending magnet


a) compact gantry b) long throw gantry

SweepersX rays tube

Proton 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)

• Single image in the beam direction

BEV X-ray images simultaneous to proton beam

Bend and scan

Scan and bend


Example:

paediatric cases treated in 2004

with Gantry 1

Recurrent Medulloblastoma,

5 year old boy (with

Anaesthesia)

Remote control of patient table motion

• Expand range of magnetic scanning

– Beyond 20 cm x12 cm

• Taking advantage of the parallelism of the beam

– Field patching independent

of treatment planning


GANTRY 2 beam optics

– Rotational symmetric (large) phase space +- 3mm +- 10 mrad

– Complete achromatism

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

– Scanning-invariant beam focus

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

– 1:1 imaging from the coupling point of the gantry to the iso-center

11.7m

3.2m

0

7.9m

Q1 Q2

QC

Q3

Q4

Q5 Q6 Q7

A1

A2A3

Sy

S1y

H1

S2y

S2x

H2

K

WT WU

M1

M2

M3

P1

REQUIREMENTS:


Beam optics confirmed by RAY-TRACING

Dispersive plane

Action of the U-sweeper

Transverse plane

Action of the T-sweeper

Harald Enge raytrace code


Advantage of using parallel scanning

• Simplify treatment planning

– Dose homogeneity achieved with standard SOBP rules

• Simplify dosimetry and QA control

– No sensitivity to the distance from the apparent source (small on a gantry)

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

• Dose homogeneity preserved

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

– Can be done without optimization within treatment planning

• 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


• Gantry 2 improvements With respect to Gantry 1

– Double parallel magnetic scanning One magnetic axis + patient table

– Energy variations with the beam Range shifter before patient

– Modulation of the beam intensity Discrete spot scanning

Single pencil beam

Dynamic beam energy

Repainting

Lateral scanning Beam intensitymodulation

GatingMulti-gating

Tracking

4. Gantry 2: aiming at new faster scanning modes


• Painting of lines (contours)

– At max. velocity (~1-2 cm/ms)

– Dose shaping with Beam Intensity Modulation (I.M.)

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

• Painting of energy iso-layers

– 200 ms per plane (20 lines x 5 mm)

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

• Repainting of iso-layers

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

• Volumetric repainting capability (aim)

– 10-20 repaintings / liter in 2 minutes

– See talk of David Meer tomorrow

Aiming at the highest scanning speed


• 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 10cm

• 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)

Compact optimized nozzle


5. Initial commissioning of Gantry 2

• First beam through the new PSI Gantry 2 on May 2008

Only beam line completed


Results: pencil beam size (on axis)

• Minimize material in the nozzle for keeping the beam size between 3 and 4 mm sigma at all energies (100-200 MeV)

Adding piece by piece

the materials in the nozzle

To have a sharp lateral fall-off


100 MeV

120 MeV

140 MeV

160 MeV

180 MeV

200 MeV

For all energies…

• Parallelism

– Max deviation ~4 mrad


Intensity vs. beam energy

• Equalization of the intensity losses of the degrader

– By detuning quadrupoles in the beam line

(gain a factor of 50:1)

– Constant beam intensity at patient location

[0.3 nA]

– With constant extracted

intensity [120 nA]

• Independently of the

energy setting

• Goal

– Reserve the use of the

modulation of the beam

intensity at the ion source

for dynamic dose painting !

Rate Monitor 1

0

100

200

300

400

500

600

100 120 140 160 180 200 220

Energy [MeV]

kH

z down

up


• Integral depth dose curves measured with a wide integrating (8-cm diameter) ionizations chamber in a water phantom( + measurements, curves dose model calculation)

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

Taking into account hysteresis effects …


• Verification with a stack of 128 10cm-broad ionization chambers

– We can measure the whole range curve on a spot by spot basis (Torino TERA chip)

• Energy of a tunes defined within an hysteresis loop of 100-230 MeV

• Blue curves – up/down down/up – correct order

• Red curves - up/down down/up - wrong order

• We should be able to scanwith energy stepsup-down anddown-up

Range control with an ionization chambers stack


With very fast energy changes …. 80 ms

• Example : Time pattern of scanning devices of a scan for a 65 mm dose box (2940 Spots)

Degrader

90°Bending magnet

Beam monitor

80 ms

An order of magnitude faster than any other system


Planned clinical use of Gantry 2

• The new instrument for treating moving targets with IMPT

– With Repainting

– Gating

– Tracking (?) – multiples gates(?)

• New indications

– Moving targets: Liver, Lung, upper GI

– Large Targets:

Craniospinal axis (medulloblastoma),

Central Thorax (Mesothelioma),

Abdominal and Pelvic Lymphnodes

– Small Targets: Integrated boost,

Retinoblastoma, Proton Radiosurgery

(AVM etc)

– Breast cancer


Conclusions:

– Beam optics commissioning of Gantry 2 successfully completed

– The beam spot size is small as planned

– The double parallel scanning is satisfactory

– We can apply very fast dynamic beam energy changes with the beam line

– We hope to show the feasibility of delivering highly repainted scanning for treating moving target (with and without gating)

– Still missing: control of the patient

table, the patient handling equipment

and the finishing of the area

• And the detailed precision work!

Scintillator block

the beam of Gantry 2 seen with a TV camera

THANK YOU

VIDEO OF THE PROTON BEAM

OF GANTRY 2

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