topics covered proton physics and technology · topics covered • rationale for ... electronic...
TRANSCRIPT
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
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.
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
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
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
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.
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
8
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
9
• 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
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.
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!