Dielectric Wall Accelerator and Intensity Modulated Proton Therapy

mpss.iop.org

Dielectric Wall Accelerator and Intensity Modulated Proton Therapy

Dielectric Wall Accelerator and

Intensity Modulated Proton

Therapy

Thomas Rockwell Mackie

Professor

University of Wisconsin

Co-Founder and Chairman

of the Board

TomoTherapy Inc.


Financial Disclosure

I am a founder and Chairman of TomoTherapy Inc.

(Madison, WI) which is participating in the

commercial development of helical tomotherapy.


• George Caporaso

• Steve Sampayan

• Yu-Jiuan Chen

• Ryan Flynn

• Evan Sengbusch

Acknowledgements

• Angelica Pérez-Andújar

• David Westerly

• Thomas Bortfeld

• Tony Lomax

• Gustavo Olivera


Conventional Proton Treatment

(Bussiere and Adams, 2003)

• Adenocyctic carcinoma

of the lacrimal gland

• CTV 56 CGE and GTV

72 CGE

• Minimize dose to

temporal lobes


Conventional Proton Treatment

(Bussiere and Adams, 2003)

• Paranasal sinus tumor

• CTV dose 54 CGE and

GTV dose 76 CGE

• Minimize the dose to

the temporal lobes,

brainstem, orbits, optic

nerves and optic

chiasm


Conventional Proton Treatment

(Bussiere and Adams, 2003)

• Hepatoma

• 42 CGE

• Minimize the dose to

the normal liver.


Conventional Proton Treatment

(Bussiere and Adams, 2003)

• Skull base chordoma

• CTV dose 50 CGE and

GTV 80 CGE

• Minimize dose to the

brainstem.


Conventional

Protons Are

Delivered

With Only A

Few Fields

Photon IMRT Vs Conventional Protons

…but less integral dose with protons.

Greater high dose conformity

with IMRT…

Proton

Pinnacle

IMRT

Tomo

(Isodose Lines:

15, 30, and 60 Gy)


Conventional Proton Therapy

• Scattered beam systems with fixed distance range shifting systems

(e.g. “propeller wobblers” or “ridge filters”) create copious neutrons

• Unnecessary dose proximal to the tumor because of fixed distance

range systems.

Scattering

Foils

Fixed Distance

Range Shifting

Distal

Compensator

Collimators


Passive Scattering and Proximal Dose

• Passive scattering

leads to unnecessary

dose to the proximal

side of the target

volume.

• More sophisticated

range shifting and

magnetic scanning

can eliminate this

proximal dose by

preventing Bragg

peaks (beam spots)

from falling outside

the target.


Conventional Protons < IMRT < IMPT

9-Beam

IMRT Conventional Spot Scanning

From Tony Lomax

Passive Scattered

Protons

Protons

IMPT is the Best Solution

IMPT


Example of IMPT

From Thomas Bortfeld


Typical Proton Treatment Paradigm

Shizuoka Proton Center, Japan

3 story

gantry

vault

40 ft

• Large Facility

• Cost ≈ $120M

• Neutrons Flux


Barriers to Entry

• Facility and Equipment

– Large and expensive

• Spot Scanning and Intensity Modulated

Proton Therapy (IMPT) are Desirable

– Retrofitting is Difficult

• Neutron Production

– Unwanted dose to patients

– Massive concrete to protect staff

– Activation of beam components


Dielectric Wall Accelerator* (DWA)

SiC Optical

Switches

Laser

Proton

Source

Monitor

Focusing

Optical

Coupling

Stack of

Blumleins

High

Gradient

Insulator

*Lawrence Livermore National Laboratory, Livermore CA


Dielectric Wall Accelerator (DWA)

Conventional Insulator

High Gradient Insulator

- -

+

Emitted electrons repeatedly

bombard surface

field emitted

electrons

+

floating

conductors

Emitted electrons repelled from

surface

From George Caporaso, Lawrence Livermore


Breakdown Voltage Increases as the

Surface breakdown field stress (MV/m)

100

10

1

Pulsewidth Decreases

Conventional

Insulator

y = 104.27x

y = 23.366x

-0.2003

-0.2592

1 10 100 1000 10000

Pulsewidth (ns)

HGI

From George Caporaso, Lawrence Livermore


Proton Source

Matching Phase Velocities

-- - - - --

-

+ + + +

-- - - - --

-

+

+

- -

- - +

- -

- - +

- -

- - +

High Gradient Insulator (100 MV/m)

- -

- - +

- -

- -

+ +

• The phase of the first Blumlein firing has to

match the proton source

• The phase velocity of Blumlein firings have to

match the proton velocity throughout the

accelerator


-

+

-

Operation of a Basic Blumlein

Closing switch

Output

Voltage

V o

0

Time

Matched load

-

+

Output

voltage

Beam


Optical

Energy

Injection

SiC Optical Switches

Wasted

Optical

Energy

With the

absorption of light

the optical switch

rapidly transitions

from electrically

open to closed.

From George Caporaso, LLNL

Conduction Region

SiC


Blumlien Switching Used to Accelerate Protons

Uncharged transmission lines in

this region (not shown)


Focusing to Change the Beam Spot

Size


DWA Can Eliminate Deuteron

Contamination


Beam Capture into the DWA


Proton

injector

F.A.S.T.

Test Assembly

• F.A.S.T. pulsewidth of 3 ns =

crossing time of injected protons

=> 300 keV initial energy

• Acceleration provided by induction

modules

– 5 induction cells simultaneously

triggered

– 300 kV @ 30+ ns

– SF 6 insulation

– Powered by cable Blumleins

• F.A.S.T. specifications

– 7 Kapton Blumleins

– Cree switches

– 532 nm light


Compact Proton Injector @ 300 keV

anode

cathode

gate electrode

source

e-repeller

accelerator grid

grids


anode

cathode

Blumlein stack

Interior Detail

gate electrode

HGI

source

e-repeller

accelerator grid

grids


Hydrated Ti Electrode Proton Source


One section of DWA

Benchtop Test Assembly

2 meters = 200MeV


Vacuum

pump

Test Assembly

5-Induction

cells

Thomson

spectrometer

Spectrometer

& camera


Thomson Ion Spectrometer

Fast phosphor

Principle of the Thomson spectrometer

• Vertical electric field gives rise to vertical

deflection inversely proportional to energy

• Vertical magnetic field gives rise to

horizontal deflection varying inversely

with the square root of the particle energy

spectrometer


Accel.

phase

OFF

Decel.

phase

Documenting Acceleration

Energy

H + C ++

H 2 + or D +

accel

decel


Comparison of Proton Accelerators

The DWA is the only accelerator

technology for which the energy, intensity

and beam spot size can be varied pulse to

pulse.

Cyclotron

Synchrotron

DWA

Energy

X

Intensity

Spot Size

X

X


Distal Edge Tracking (DET)

• Use energy modulation to have the Bragg peak

follow the distal edge of the tumor.

• Provide intensity modulation to control the dose

homogeneity in the target and avoid normal

tissues.

• Needs sufficient treatment directions to enable it,

e.g., arc therapy.

• Oelfke and Bortfeld * have shown that DET

provides the lowest normal tissue dose for a given

target dose.

• Fastest delivery.

*Oelfke U and Bortfeld T. (2000) Intensity modulated radiotherapy

with charged particle beams: Studies of inverse treatment

planning for rotation therapy. Med. Phys, 27:1246-1257.


Tumor

X-Ray IMRT Vs DET

Voxel at the

Tumor Boundary

Sensitive

Structure

X-Ray Proton DET


Spot Scanning and DET

SS Spot Locations (~300)

DET Spot Locations (~20)

Distal Edge of

Target

For DET both intensity modulation and multiple directions or

rotation therapy is required to obtain uniform dose distributions.

From Ryan Flynn


Arc Therapy for Protons

360 deg Intensity Modulated

Arc Therapy (i.e. Tomotherapy)

From Ryan Flynn

135 deg Intensity Modulated

Arc Therapy


Off-Axis Disk Target Volume

360 Deg 180 Deg

90 Deg

From Ryan Flynn

135 Deg

Dose (Gy)


Off-Axis Ring Target Volume

360 Deg 180 Deg

90 Deg

From Ryan Flynn

135 Deg

Dose (Gy)


Head and Neck Structure Sets

L Parotid

PTV 56

PTV 70

PTV 63

Spine

From Gustavo Olivera, TomoTherapy Inc

PTV 56

Four levels of Dose on PTVs

• 70 Gy

• 63 Gy

• 60 Gy

• 56 Gy

Sensitive Structures

• Parotids

• Spine

• Lungs

• Brainstem

• Mandible

• Brain

• Opt chiasm

• Eyes

• Optical Nerves

• Glottis

• Esophagus

• Inner ears


TomoTherapy Photon/IMPT Proton

Lungs Esophagus

Lungs

R Parotid

L Parotid

Cord

R Parotid

L Parotid

Cord

PTV 56

PTV 70

PTV 63

PTV 60

PTV 56

PTV 70

PTV 63

PTV 60


Breast Case

12 directions spread through 180 deg.

From Evan Sengbusch, UW-Madison

No Dose


Craniospinal Case

Tomo Photon

Tomo IMPT No Dose


Craniospinal Case

Tomo Photon Tomo IMPT

Tomo Photon

Tomo Photon

Tomo IMPT

DVH: Full line: Photons Dash line: IMPT


Dose Contrast

Resolution

Boost (+) and

Avoidance (-) Dose

Base tumour

region

Boost region

circle

Boost

region

r 2

r 3

R P

r 1

r 4

y

R B

r 5

R T

r 6

From Ryan Flynn

θ

Tomo

DET

180 deg

Local

evaluation

region

x

Normal

tissue

Spot Scan

180 deg


Design for a Conventional Vault

Click To Access

Concept Drawings

We feel it is important to build a CT scanner into our unit


What Energy is Required for Arc Delivery

From Evan Sengbusch, UW


200 MeV is Sufficient for 90 to 95% of

100

90

80

70

60

50

40

30

20

10

From Evan Sengbusch, UW

0

Method 1 (Split Arc)

Method 2 (Continuous Arc)

Patients

0 50 100 150 200 250

Maximum Proton Energy (MeV)

100

consecutive

patients at

UW


Neutrons

Neutron Energy Fluence (MeV cm -2 p -1 )

4.0E-03

3.5E-03

3.0E-03

2.5E-03

2.0E-03

1.5E-03

1.0E-03

5.0E-04

y = 4E-12x 3.76

0.0E+00

140 160 180 200 220 240 260

Proton Energy (MeV)

• Fluence Increases Rapidly with

Proton Energy

• Neutrons Go Mainly in the Forward

Direction

From Angelica Pérez-Andújar and Paul Deluca, UW


Neutrons

• Conventional proton radiotherapy produces

copious quantities of neutrons due to slowing

down the protons to produce the desired range.

• Our machine will produce the desired proton

energy in the linac – no slowing down required.

• Neutrons are still produced in the patient.

• Minimizing the proton energy needed will

reduce neutron production.

• Forward directed neutrons can be attenuated in

the machine like tomotherapy does with the

beam stop and this also serves as a

counterweight.


Timeline

• First Article DWA

• Acceleration of Protons from

Source

• Subscale DWA Q1 2009.

• Clinically useful energy Q1 2010

• Pilot release late 2011.

• Regulatory approval and

commercial release 2012.

• Continue to work on heavier ions


Conclusions

• Not all proton radiotherapy is the same.

• Need for intensity modulated proton

therapy.

• Rotation therapy is best.

• CT-guidance is necessary for proton

radiotherapy, especially DET.

• Compact proton linac being developed with

Lawrence Livermore National Laboratory.

• Fit within a conventional vault footprint.

• Low neutron dose and lower facility cost.

• Target commercialization in 2012.


Questions

More magazines by this user
Similar magazines