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1 PROTON Physics and Technology Alfred Smith, Ph.D. M. D. Anderson Cancer Center Houston, TX Topics Covered Rationale for Proton Therapy Physics of Proton Beams Treatment Delivery Techniques Proton Therapy Technology Rationale for Proton Therapy Need for Improved Local Control in Cancer Treatment (selected sites) (all numbers are estimates) Head/Neck 22,000 13,200 (60%) Gastrointestinal 135,000 54,000 (40%) Gynecologic 28,000 14,000 (50%) Genitourinary 55,000 27,500 (50%) Lung 160,000 40,000 (25%) Breast 41,000 4,920 (12%) Lymphoma 20,000 2,400 (12%) Skin, Bone, Soft Tissue 15,000 5,000 (33%) Brain 12,000 10,800 (90%) Total 488,000 171,820 (35%) Over 1,350,000 new cancer patients per year in the US Tumor Site Deaths/ Deaths due to year Local Failure

Topics Covered PROTON Physics and Technology · Topics Covered • Rationale for ... electronic collisions in traversing the distance dx in a material of density

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Page 1: Topics Covered PROTON Physics and Technology · Topics Covered • Rationale for ... electronic collisions in traversing the distance dx in a material of density

1

PROTON Physics and

Technology

Alfred Smith, Ph.D.M. D. Anderson Cancer Center

Houston, TX

Topics Covered

• Rationale for Proton Therapy

• Physics of Proton Beams

• Treatment Delivery Techniques

• Proton Therapy Technology

Rationale for Proton Therapy

Need for Improved Local Control in Cancer Treatment (selected sites)

(all numbers are estimates)

Head/Neck 22,000 13,200 (60%)Gastrointestinal 135,000 54,000 (40%)Gynecologic 28,000 14,000 (50%)Genitourinary 55,000 27,500 (50%)Lung 160,000 40,000 (25%)Breast 41,000 4,920 (12%)Lymphoma 20,000 2,400 (12%)Skin, Bone, Soft Tissue 15,000 5,000 (33%)Brain 12,000 10,800 (90%)

Total 488,000 171,820 (35%)

Over 1,350,000 new cancer patients per year in the US

Tumor Site Deaths/ Deaths due toyear Local Failure

Page 2: Topics Covered PROTON Physics and Technology · Topics Covered • Rationale for ... electronic collisions in traversing the distance dx in a material of density

2

Dosimetric advantages of proton beams

• Protons Stop!• Photons don’t stop.• Proton dose at depth

(target) is greater than dose at surface.

• Photon dose at depth (target) is less than dose at dmax.

Protons

Ideal dose distribution

Photons

Photons

Protons

PHOTONS

PROTONS

MEDULLOBLASTOMA

100

60

10

PHOTONS

PROTONS

Advantages of Proton Therapy

Highly localized dose distributions→– Increased local control of tumors– Decreased treatment-related side effects– Improved Quality of Life

Proton Physics

Protons lose energy by:

• ionizations

• multiple Coulomb scattering

• non-elastic nuclear reactions.

Page 3: Topics Covered PROTON Physics and Technology · Topics Covered • Rationale for ... electronic collisions in traversing the distance dx in a material of density

3

Electromagnetic energy loss of protons

e

pp

Mass Electronic Stopping Power is the mean energy lost by protons in electronic collisions in traversing the distance dx in a material of density ρ.

S/ρ = 1/ρ[dE/dx] ∝ 1/v2

Where v = proton velocity

This is the main interaction that causes formation of Bragg peak.

0%

20%

40%

60%

80%

100%

120%

0 5 10 15 20 25 30

Depth in Water[cm]

Rel

ativ

e D

ose

[%]

1. The incident beam has a narrow energy spread (ΔE/E ≈ 0.2%)

2. Bragg peak is “broadened” by range straggling (statistical differences in energy losses in individual proton paths).

• A certain fraction of protons undergo nuclear interactions, mainly on 16O (∼ 1%/cm)

• Nuclear interactions lead to secondary particles and thus to local and non-local dose deposition, including neutrons.

Nuclear interactions of protons

γ

n

p’

pp

p’

Primary fluence

Effect on the lateral dose distribution

Secondaries from nuclear interactions

Pedroni et al PMB, 50, 541-561, 2005

Depth in Water [cm]

0 5 10 15 20 25 30 35

Rel

ativ

e D

ose

[%]

0102030405060708090

100110

Total Absorbed Dose

'Primary' Dose 'Secondary' Dose

230 MeV protons

Effect on Depth Dose

Normalized (at peak) Bragg Curves for Various Proton Incident EnergiesRange Straggling will cause the Bragg peak to widen with depth of penetration

0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30 35 40

Depth in Water (cm)

Rel

ativ

e D

ose

100 MeV130 MeV160 MeV180 MeV200 MeV225 MeV250 MeV

-10

0

10

20

30

40

50

0 10 20 30 40 50

Depth in Water (cm)

100 MeV

130 MeV

160 MeV

180 MeV

200 MeV

225 MeV

250 MeV

Normalized (at entrance) Bragg Curves for Various Proton Incident Energies

Rel

ativ

e D

ose

Page 4: Topics Covered PROTON Physics and Technology · Topics Covered • Rationale for ... electronic collisions in traversing the distance dx in a material of density

4

Dose depositions in water from 160 MeV protons. Beam slit delimiters with width W cm.

Uniform particle distributions.

0

5

10

15

20

25

30

0 5 10 15

Depth (cm)

Dos

e (M

eV/c

m)

W = 0.1 cmW = 0.16 cmW = 0.24 cmW = 0.5 cmW = 1 cmW = 2 cm

Depth dose for 160 MeV Protons for various field sizes

For small fields, loss of in-scattering (charged particle equilibrium) results in deterioration of Bragg peak and non uniformity of SOBP.

• Protons undergo multiple deflections through elastic coulomb interactions with atomic nuclei

• Beam broadening can be approximated by a Gaussian distribution

p’p θ

Multiple Coulomb Scattering (MCS) leads to broadening of lateral penumbra as beam penetrates in depth.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-150 -100 -50 0 50 100 150Off-axis Distance (mm)

Rel

ativ

e D

ose

Depth=8 cmDepth=15.9 cmDepth = 12 cm

Lateral penumbra:• Dominated by Multiple Coulomb Scattering• d80-20 ≈ 1.68 σ ≈ 3.3% of range ⇒ ∼ 5 mm at 15 cm depth• Wide angle scattering and nuclear interaction products add• Total penumbra ∼ 6-7 mm at 15 cm depth

8 cm depth

16 cm Depth

12 cm Depth

80/20 Penumbra Comparison

0.00

0.25

0.50

0.75

1.00

15 cm 20 cm 25 cm

Norm alization Depth

80 -

20 %

Dis

tanc

e (c

m)

Lateral dose fall-off: Protons vs. Photons

15 MV Photons

Protons

∼ 17 cm

Paganetti

Page 5: Topics Covered PROTON Physics and Technology · Topics Covered • Rationale for ... electronic collisions in traversing the distance dx in a material of density

5

Urie

Large air gaps will degrade the lateral penumbra

Proton Therapy Beam Delivery Technology

Physics of the Passive Scattering Mode of Proton Beam Delivery

Goitein, Lomax, Pedroni - Med. Phys.

Passive Scattering Nozzle with Range Modulation Wheel

Hitachi Passive Scattering Nozzle

Page 6: Topics Covered PROTON Physics and Technology · Topics Covered • Rationale for ... electronic collisions in traversing the distance dx in a material of density

6

How a Spread Out Bragg Peak (SOBP) is formed.• Modulation wheel rotates in the

beam.• Pull-back (energy shift)

determined by height of step.• Weight determined by width of

step.• Multiple SOBPs can be obtained

by gating beam.

Deficiencies of Proton Passive Scattering Techniques

• Excess normal tissue dose.

• Increases effective source size

which increases lateral

penumbra.

• Requires custom aperture and

compensator

• Inefficient - high proton loss

produces activation and

neutron production.

Chu, Ludewigt, Renner - Rev. Sci. Instrum.

PTCOG 46 Educational Workshop

Prostate Patient Treatment Plan

CTVRectumBladderFemoral heads

Al Smith PTCOG 46 Educational Workshop

QA of Prostate Treatment using patient treatment parameters/appliances and EBT film in water

phantom.Treatment plan on CT anatomy converted to dose distribution in water phantom.

PTCOG 46 Educational Workshop

Measurements in water phantom using EBT film, patient aperture, and range compensator

Cross Field Profile A to P through Isocenter

0

0.2

0.4

0.6

0.8

1

-65 -45 -25 -5 15 35 55

Lateral Distance (mm)

Dose

(arb

uni

ts)

EBT film measurement

Eclipse

Cross Field Profile F to H through isocenter

0

0.2

0.4

0.6

0.8

1

-70 -50 -30 -10 10 30 50 70

Lateral Distance (mm)

Dos

e (a

rb u

nits

)

EBT film measurementEclipse

Al Smith PTCOG 46 Educational Workshop

Patient Treatment QA – Measurements compared with treatment planning calculations converted to water phantom.

Eclipse vs. Measured - Crossplane

0

20

40

60

80

100

0 2 4 6 8 10 12

Eclipse Measured

Data measured in water phantom using Pin-Point ion chamber. Treatment aperture and range compensator were both inserted.

Al Smith

The Pencil Beam Scanning Mode of Proton Beam Delivery

Goitein, Lomax, Pedroni - Med. Phys.

Page 7: Topics Covered PROTON Physics and Technology · Topics Covered • Rationale for ... electronic collisions in traversing the distance dx in a material of density

7

Beam3.2m

ScanningMagnets

Profile Monitor

CeramicChamber

Spot Position Monitor

Dose Monitor 1, 2

Energy AbsorberIsoCenter

Energy Filter

PerformanceRange 4 – 36 g/cm2

Adjustability 0.1 g/cm2

Max. field size 30 x 30 cm

Beam size in air 5 – 10 mm σ

SAD > 2.5 m

Dose compliance +/- 3% (2 σ)

Irradiation time < 1.5 min to deliver 2 Gy to 1 liter at any depth.

Pencil Beam Scanning Nozzle

HeliumChamber

Hitachi Spot Scanning Nozzle

A major problem with spot scanning:The target can move during treatment leading to dose errors!

Remedies:Rescanning (spot, layer, volume)

ΔD/D ∝ 1/√n, where n = number of scansBeam GatingReal time tracking with markers

Orthogonal IC array measurements performed at different water depths using a computer controlled water column and

compared with calculations.

‘Beam’s-eye-view’ of dosein water

U axis profile

T axis profile

Pedroni, PSI, Switzerland

PTCOG 46 Educational Workshop

Ionization Chamber ArrayWater column with 26 small ionization chambers of 0.1 cm3

Dose box

Pedroni, PSI, Switzerland

Beam

Mirror

CCD Camera

Scintillating Plate

Scintillating Plate, Mirror and CCD Camera used for pencil beam scanning QA.

Spot Pattern Test Uniform Field Scanning Test

M D Anderson Cancer Center

PTCOG 46 Educational Workshop

Scintillating screen viewed with a CCD through a 45° mirror

– ideal for non homogeneous dose distributions

WER6.65CM

WER7.82CM

W= 6.65 cm

W= 7.82 cm

Calculation Measurement vs.

Pedroni, PSI, Switzerland

Martin Bues

1. Proton Accelerators2. Isocentric Gantries

3. Typical Facility

Page 8: Topics Covered PROTON Physics and Technology · Topics Covered • Rationale for ... electronic collisions in traversing the distance dx in a material of density

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Accelerators used in proton therapy facilitiesIBA 230 MeV CyclotronHitachi 250 MeV synchrotron

Varian/ACCEL Superconducting Cyclotron

250 MeV; 90 tons; 3.2 m dia.

Still River Superconducting Synchrocyclotron

250 MeV; 20 tons; 1.7 m dia.

220 tons

63 tons8 m dia.

ProTom International Inc.A Scanning-Optimized Synchrotron

• Total weight = 15 tons; 4.9 m diameter• 330 MeV → Proton tomography• 0.1 to 10 sec extraction• Variable intensity•

Hitachi

M. D. Anderson Gantry

Siemens

Heidelberg

600 tons

190 tons

120 tons

Proton

Carbon

Gantries

Page 9: Topics Covered PROTON Physics and Technology · Topics Covered • Rationale for ... electronic collisions in traversing the distance dx in a material of density

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• Accelerator mounted on Gantry

• Entire system contained in one room

• Multiple independent rooms can be installed

Still River

Proton Therapy System

Typical Proton Therapy Facility

12

5

43

6

1. Accelerator 4. Gantry room

2. Beam transport line 5. Fixed beam room

3. Gantry room 6. Patient support area

PHASE llAdd Light ions

Phase I Protons only

A Proton + Light Ion Facility built in two phases

Robotic Applications

Page 10: Topics Covered PROTON Physics and Technology · Topics Covered • Rationale for ... electronic collisions in traversing the distance dx in a material of density

10

New Technologies

Laser Accelerated Proton BeamsProton acceleration is achieved by focusing a high-power laser on a thin target. The short (10-16 sec) laser pulse width produces a high peak power intensity that causes massive ionization in the target, expelling a large number of relativistic electrons. The sudden loss of electrons gives the target a high positive charge and this transient positive field accelerates protons to high energies.

∼ 5 m laser system

Proton beam source

Laser accelerated proton therapy has a time frame of 5 – 10 years.

Page 11: Topics Covered PROTON Physics and Technology · Topics Covered • Rationale for ... electronic collisions in traversing the distance dx in a material of density

11

Dielectric Wall Proton Accelerator (DWA)

Conventional accelerator cavities have an accelerating field only in the gaps which occupy only a small fraction of their length. In a DWA, the beam pipe is replaced by an insulating wall so that protons can be accelerated uniformly over the entire length of the accelerator yielding a much higher accelerating gradient.

Artist Concepts for DWA clinical installation

The goal is to have a full scale prototype in ~ 4-5 years, which will be installed at UC Davis CC.

Thomotherapy is the private sector partner

Thank You!