PROTON Physics
and
Technology
Topics Covered
• Rationale for Proton Therapy
• Physics of Proton Beams
Alfred Smith, Ph.D.
M. D. Anderson Cancer Center
Houston, TX
• Treatment Delivery Techniques
• Proton Therapy Technology
Need for Improved Local Control
in Cancer Treatment (selected sites)
(all numbers are estimates)
Tumor Site
Rationale for Proton Therapy
Head/Neck
Gastrointestinal
Gynecologic
Genitourinary
Lung
Breast
Lymphoma
Skin, Bone, Soft Tissue
Brain
Total
Deaths/
year
Deaths due to
Local Failure
22,000
135,000
28,000
55,000
160,000
41,000
20,000
15,000
12,000
488,000
13,200 (60%)
54,000 (40%)
14,000 (50%)
27,500 (50%)
40,000 (25%)
4,920 (12%)
2,400 (12%)
5,000 (33%)
10,800 (90%)
171,820 (35%)
Over 1,350,000 new cancer patients per year in the US
1
Photons
Dosimetric
advantages of
proton beams
Protons
MEDULLOBLASTOMA
PHOTONS
PHOTONS
100
Photons
Ideal dose
distribution
Protons
•
Protons Stop!
•
Photons don’t stop.
•
Proton dose at depth
(target) is greater than
dose at surface.
•
60
PROTONS
PROTONS
10
Photon dose at depth
(target) is less than
dose at dmax.
Advantages of Proton Therapy
Proton Physics
Highly localized dose distributions→
– Increased local control of tumors
– Decreased treatment-related side effects
– Improved Quality of Life
Protons lose energy by:
• ionizations
• multiple Coulomb scattering
• non-elastic nuclear reactions.
2
Electromagnetic energy loss
of protons
p
Nuclear interactions of protons
1. The incident beam has a narrow
energy spread (ΔE/E ≈ 0.2%)
p
e
p’
p’
2. Bragg peak is “broadened” by range
p
straggling (statistical differences in
γ
p
energy losses in individual proton
n
paths).
• A certain fraction of protons undergo nuclear
120%
100%
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.
60%
S/ρ = 1/ρ[dE/dx] ∝ 1/v2
40%
20%
Where v = proton velocity
0%
5
10
15
20
25
30
Depth in Water[cm]
interactions, mainly on 16O (∼ 1%/cm)
• Nuclear interactions lead to secondary particles
and thus to local and non-local dose deposition,
including neutrons.
This is the main interaction that
causes formation of Bragg peak.
Normalized (at peak) Bragg Curves for Various Proton Incident Energies
Range Straggling will cause the Bragg peak to widen with depth of penetration
Pedroni et al PMB, 50, 541-561, 2005
Effect on the lateral dose
distribution
1.2
Primary
fluence
Relative Dose
1
0.8
100 MeV
130 MeV
160 MeV
180 MeV
200 MeV
225 MeV
250 MeV
0.6
0.4
0.2
Secondaries from
nuclear interactions
0
0
110
100
90
80
70
60
50
40
30
20
10
0
5
10
15
20
25
30
35
Normalized (at entrance) Bragg Curves for Various Proton Incident Energies
230 MeV protons
Total Absorbed Dose
50
40
'Primary' Dose
0
5
10
'Secondary' Dose
15
20
Depth in Water [cm]
25
40
Depth in Water (cm)
Effect on Depth Dose
Relative Dose
0
Relative Dose [%]
Relative Dose [%]
80%
30
35
100 MeV
130 MeV
30
160 MeV
180 MeV
20
200 MeV
10
225 MeV
250 MeV
0
-10
0
10
20
30
40
50
Depth in Water (cm)
3
Multiple Coulomb Scattering (MCS) leads to
broadening of lateral penumbra as beam
penetrates in depth.
Dose depositions in water from 160 MeV protons.
Beam slit
delimiters
widthMeV
W cm.
Depth
dose
forwith160
Uniform particle distributions.
Protons for various field sizes
30
D o se (M eV /cm )
25
•
W = 0.1 cm
W = 0.16 cm
W = 0.24 cm
W = 0.5 cm
W = 1 cm
W = 2 cm
20
•
15
Protons undergo multiple deflections
through elastic coulomb interactions
with atomic nuclei
Beam broadening can be
approximated by a Gaussian
distribution
p’
θ
p
10
5
0
0
5
10
15
Depth (cm)
For small fields, loss of in-scattering (charged particle equilibrium)
results in deterioration of Bragg peak and non uniformity of SOBP.
Lateral penumbra:
• Dominated by Multiple Coulomb Scattering
• d80-20 ≈ 1.68 σ ≈ 3.3% of range ⇒ ∼ 5 mm at 15 cm depth
Lateral dose fall-off: Protons vs. Photons
• Wide angle scattering and nuclear interaction products add
80/20 Penumbra Comparison
• Total penumbra ∼ 6-7 mm at 15 cm depth
80 - 20 % D is ta n ce (c m )
1.2
1.0
R elative D o se
0.8
Depth=8 cm
Depth=15.9 cm
0.6
Depth = 12 cm
12 cm
Depth
0.75
15 MV Photons
0.50
∼ 17 cm
0.25
0.00
15 cm
0.4
16 cm
Depth
0.2
Protons
1.00
20 cm
25 cm
Norm alization De pth
8 cm
depth
0.0
-150
-100
-50
0
Off-axis Distance (mm)
50
100
150
Paganetti
4
Large air gaps will
degrade the lateral
penumbra
Proton Therapy
Beam Delivery Technology
Urie
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
5
How a Spread Out Bragg Peak (SOBP) is formed.
Deficiencies of Proton Passive
Scattering Techniques
•
Modulation wheel rotates in the
beam.
•
Pull-back (energy shift)
determined by height of step.
•
Excess normal tissue dose.
•
Weight determined by width of
step.
•
Increases effective source size
•
Multiple SOBPs can be obtained
by gating beam.
which increases lateral
penumbra.
•
Requires custom aperture and
compensator
•
Inefficient - high proton loss
produces activation and
neutron production.
Chu, Ludewigt, Renner - Rev. Sci. Instrum.
Prostate Patient Treatment Plan
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.
The Pencil Beam Scanning Mode
of Proton Beam Delivery
CTV
Rectum
Bladder
Femoral heads
PTCOG 46
Educational Workshop
Al Smith
Measurements in water
phantom using EBT film,
patient aperture, and
range compensator
Cross Field Profile A to P through Isocenter
Educational Workshop
PTCOG 46
Patient Treatment QA – Measurements compared with treatment
planning calculations converted to water phantom.
Data measured in water phantom using Pin-Point ion chamber.
Treatment aperture and range compensator were both inserted.
EBT film measurement
1
Eclips e
Eclipse vs. Measured - Crossplane
Dose (arb units)
0.8
0.6
100
0.4
0.2
80
0
-65
-45
-25
-5
15
35
55
Lateral Distance (mm)
60
Cross Field Profile F to H through isocenter
40
1
Dose (arb units)
0.8
20
0.6
EBT film measurement
Eclipse
0.4
0
0
0.2
0
-70
-50
-30
-10
10
30
50
2
4
6
Eclipse
70
8
10
12
Goitein, Lomax, Pedroni - Med. Phys.
Meas ured
Lateral Distance (mm)
PTCOG 46
Educational Workshop
Al Smith
PTCOG 46
Educational Workshop
Al Smith
6
Pencil Beam Scanning Nozzle
Beam
3.2m
Profile
Monitor
Scanning
Magnets
Ceramic
Helium
Chamber
Chamber
Spot Position
Monitor
Dose Monitor 1, 2
Energy Filter
Energy Absorber
Iso
Center
A major problem with spot scanning:
The target can move during treatment leading to
Performance
Range
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)
dose errors!
Remedies:
Rescanning (spot, layer, volume)
ΔD/D ∝ 1/√n, where n = number of scans
Beam Gating
Real time tracking with markers
Irradiation time < 1.5 min to
deliver 2 Gy to 1 liter at any
depth.
Hitachi Spot Scanning Nozzle
Ionization Chamber Array
Martin Bues
Mirror
M D Anderson Cancer Center
Water column with 26 small
ionization chambers of 0.1 cm3
CCD
Scintillating Plate, Mirror and
Camera
CCD Camera used for pencil
beam scanning QA.
Dose box
Scintillating Plate
Beam
PTCOG 46
Educational Workshop
Spot Pattern Test
Uniform Field Scanning Test
Pedroni, PSI, Switzerland
Pedroni, PSI, Switzerland
Orthogonal IC array measurements performed at different water
depths using a computer controlled water column and
compared with calculations.
Scintillating screen viewed with a
CCD through a 45° mirror
WER
6.65
CM
– ideal for non homogeneous
dose distributions
1. Proton Accelerators
2. Isocentric Gantries
3. Typical Facility
W= 6.65 cm
‘Beam’s-eye-view’ of dose
in water
U axis profile
WE
R
7.82
CM
T axis profile
Measurement vs.
PTCOG 46
W= 7.82 cm
Calculation
Educational Workshop
Pedroni, PSI, Switzerland
7
Accelerators used in proton therapy facilities
Hitachi 250 MeV synchrotron
IBA 230 MeV Cyclotron
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
63 tons
8 m dia.
•
220 tons
Varian/ACCEL Superconducting Cyclotron
250 MeV; 90 tons; 3.2 m dia.
Still River Superconducting Synchrocyclotron
250 MeV; 20 tons; 1.7 m dia.
M. D. Anderson Gantry
Gantries
Proton
190 tons
120 tons
Hitachi
600 tons
Carbon
Siemens
Heidelberg
8
Still River
Typical Proton Therapy Facility
Proton Therapy System
2
• Accelerator mounted on Gantry
• Entire system contained in one
room
• Multiple independent rooms can be
installed
1
3
4
5
6
1. Accelerator
4. Gantry room
2. Beam transport line
5. Fixed beam room
3. Gantry room
6. Patient support area
A Proton + Light Ion Facility built in two phases
Phase I
Protons only
Robotic Applications
PHASE ll
Add Light ions
9
New Technologies
Laser Accelerated Proton Beams
Proton acceleration is achieved by focusing a high-power laser on a thin target.
Laser accelerated proton therapy has a time
frame of 5 – 10 years.
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.
Proton beam source
∼ 5 m laser system
10
Dielectric Wall Proton Accelerator (DWA)
The goal is to have a full scale prototype in
~ 4-5 years, which will be installed at UC
Davis CC.
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.
Thomotherapy is the private sector partner
Artist Concepts for DWA
clinical installation
Thank You!
11