Center for Proton Radiation Therapy
FEASABILITY OF SIMULATED SCATTERING ON A
SCANNING GANTRY FOR PROTON RADIATION THERAPY
Silvan Zenklusen
Prof. André Rubbia, Doktorvater; Prof. Ralph Eichler, Co-refferent, ETHZ
Eros Pedroni, Ph.D., and David Meer, Ph.D., Supervisors, PSI
and the whole CPT team, PSI
X-ray and proton beams & applications, Ph.D. Student Seminar
June 4th, 2009
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Content
Proton radiation therapy – rationale
Making use of the physical properties of p+ for medical needs
Established proton beam delivery techniques and resulting dose distributions
Broad beams
Scanned beams
Proton radiation therapy at PSI
Discrete spot scanning using PSI’s compact gantry (Gantry 1)
Novel beam delivery techniques
Simulation of scattering
Theory
Experiment and first results
Open challenges
Conclusion & Outlook
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Proton radiation therapy – rationale
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Ballistic properties:
- Maximal dose at a well
defined depth (Bragg peak).
- No dose beyond Bragg peak.
- Include density of material in
case of a tumour in a body.
For simplicity this calculation
is for water only.
- Spread out Bragg peak
(SOBP) = linear combination
of single Bragg peaks.
relative dose
Why use of protons for radiation therapy?
As compared to photons lower
integral dose (2-5) to healthy
tissues.
15 MeV photons
proton SOBP
protons
tumor
depth [cm]
The use of multiple beam directions (fields) results in concentration of the high
dose in the tumour and reduction of dose outside the tumour –
(for photons and protons).
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Creation of a spread out Bragg peak (SOBP)
relative dose [-]



An SOBP is a linear
combination of different
single Bragg curves.
Usually the spacing in depth
is 0.45 cm
To achieve a 3-dim dose
distribution with spot
scanning the spots are
placed on a regular grid.
(0.5 x 0.5 x 0.45 cm3)
range [cm]
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Established proton beam delivery techniques and resulting
dose distributions
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Broad beams - scattering
spinal cord
range-shifter
wheel
collimator
lumbar spine
tumor
patient
scatter foils
compensator
entrance dose
100% dose
target volume
intestine & bowel,
sensitive to radiation dose
Traditional and established technique
since the 60’s.
Individual compensator, collimator for
every field.
Sharp dose conformation lateral and
distal.
02.06.2009
Scattered, broad proton beam
Dose distribution for treatment of a huge and
irregularly shaped abdominal tumor. Excellent
lateral and distal dose conformation, saving the
spine, spinal cord and bladder from radiation.
However, the radiation sensitive intestines
receive high dose levels due to suboptimal
proximal (= upstream) dose conformation.
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Scanned beams - scanning
spinal cord lumbar spine
90° bending
magnet
pencil beam
(σ = 3 mm)
sweeper magnets
(2 dimensions)
tumor
target
patient
intestine & bowel,
sensitive to radiation dose
Improved 3 dimensional dose
conformation.
Better dose conformation to irregular
shaped tumors – as compared to
broad beams.
No individual hardware required.
Fully automated dose delivery.
02.06.2009
Spot scanning proton beam
Dose distribution for the same abdominal tumor.
Comparable lateral and distal dose conformation,
protecting the spine, spinal cord and bladder.
However, the low plateau doses of each pencil
beam are resulting in better sparing the radiation
sensitive intestines from high dose (= prescribed
therapeutic dose to sterilize the tumor cells)
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Proton radiation therapy at PSI
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Proton radiation therapy at PSI – Gantry 1
Development started in early 90’s.
Successfully operating since 1996.
(~300 patients with deep seated tumors)
Discrete spot scanning.
sweeper magnet
90° bending
magnet
a
b rotation
14.09.07
Silvan Zenklusen, PSI/ETHZ
rotation
f rotation
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Center for Proton Radiation Therapy
Situation at PSI – PROSCAN
Expansion of radiation therapy
facilities at PSI
•
•
•
•
•
Dedicated superconducting
cyclotron → 250 MeV protons
4 beam lines 3 are for medical
use.
Deflector plate inside the
cyclotron for fast intensity
variations at 50 μs timescale.
Laminated beam line for
Gantry 2 together with
degrader system will allow for
energy changes within max.
80 ms (for 4.5 mm steps)
Gantry 2 has two sweeper
magnets corresponding to U
& T direction.
14.09.07
medical cyclotron
(COMET)
PIF
OPTIS 2
degrader
Gantry 2
Gantry 1
The completely new section from COMET to
Gantry 2 is designed for the development
of advanced scanning techniques.
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
The new PSI Gantry 2

A tool for developing advanced
beam scanning techniques

Iso-centric layout

Double magnetic scanning
(double-parallel)

Dynamic beam energy variations
with the beam line
28.04.2009

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
D. Meer: New fast scanning
techniques using a dedicated
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Center for Proton Radiation Therapy
Simulation of scattering
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Motivation to try to simulate scattering



Scattering is still the most common approach in proton therapy

Technique is from the 60/70’s.

Has less problems with organ motion.

Sharp lateral dose confirmation due to collimators.
Scanning is only used at very few facilities

Real 3D dose conformation.

Less neutron production directly in front of patients.

Possibility to reduce/optimize scan-field size.

Proof of principle!
Both techniques can be done with one machine!
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Motivation: Beam scanning and organ motion
•


The effect of organ motion:
The lateral dose conformation can not be
guaranteed (scattering and scanning)
Disturbance of the dose homogeneity
(only scanning)
This makes spot scanning very sensitive
to organ motion during beam delivery

With Gantry 1 we can treat only immobile
lesions. On Gantry 1 we accept only
movements <1-2mm with full fractionation

BUT: On Gantry 2 we plan to treat
mobile tumors using repainting and
gating.
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Scattering on a scattering machine
• Scattering
– Use scatter foils to broaden up the
beam
beam
→ high neutron production
→ higher risk of secondary tumors
– range shifter wheel to create
SOBP
→ more neutrons…
02.06.2009
Silvan Zenklusen, PSI/ETHZ
range shifter wheel
scatter foils
divergent beam
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Center for Proton Radiation Therapy
Simulate scattering on a scanning machine
= continuous scanning at maximal speed
• Scanning
– Use sweeper magnets to
broaden up the beam by
continuous fast motion
(requires fast magnets:
10 x 10 cm2 in 100ms)
→ no neutrons
beam
– At PSI we use a degrader
system far away from the
patient (requires fast beam
line: 4 MeV steps in 80ms)
→ no neutrons to patient
parallel beam
BUT: In both cases there will be neutrons delivered to the patient originating
from collimators and compensators, which is not the case for spot scanning.
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
•
•
•
•
•
Use of FPGA based control system to paint
meander pattern
Vertical deflector is used to cut of edges (switch
off/on the beam in less than 50 ms)
Repainted, homogeneous area of 6 x 8
cm2
# repaintings
Beam delivery: Continuous scanning
500 iso-energy planes painted in less than 1
minute
120
80
40
122
144
167
Energy [MeV]
SOBP is created using different numbers of layer
repetitions per energy
28.04.2009
D. Meer: New fast scanning
techniques using a dedicated
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Center for Proton Radiation Therapy
Optimize scan-field size to avoid unwanted entrance dose
actual scan/scatter field
collimator
compensator
100% dose
Normal scattering:
beam
100% dose outside
target region due to
too big scatter field
entrance dose
target
Simulated scattering:
no 100% dose
outside target
region since scan
field is smaller and
shaped proximally.
02.06.2009
beam
scan path
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
First measurements on Gantry 2 with a collimator/compensator
Experimental setup
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Results: Difference between ‘Box’ scan fields and ‘Shrinked’ field, for a
better dose control they are delivered using spot scanning technique.
6 cm Plexiglas
6 cm Plexiglas
10 cm Plexiglas
10 cm Plexiglas
•
‘Shrinked’-field is very
sensitive on correct
alignment whereas
‘Box’-field is not.
•
Reduction of entrance
dose is clearly visible, up
to 15 %.
•
Same coverage within
the target volume.
9 cm Plexiglas
9 cm Plexiglas
12cm Plexiglas
12 cm Plexiglas
16 cm Plexiglas
14 cm Plexiglas
14 cm Plexiglas
02.06.2009
16 cm Plexiglas
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
The challenge of the dose control in continuous mode


Requires a very stable beam.
Constant beam intensity is demanded at the Gantry for all
energies between 100 and 200 MeV. (transmission drops by a
factor of 50.)

Tuning the beam line, focusing/defocusing on collimators for a coarse
balancing of the beam intensity. (done)

Feedback-loop between dose monitors and vertical deflector (within
cyclotron) for additional online correction. (on the way, but was not working
yet while data taking.)
→ real simulation of scattering.

Absolute dose control using the monitors.
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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Center for Proton Radiation Therapy
Conclusion & Outlook

The use of collimators and compensators on Gantry 2 is possible.




Fixation is foreseen and will allow much better alignment.
To simulate real scattering on a scanning gantry a fast scanning
and energy variation system is mandatory.
Obtain relative dose control, having a very constant beam
intensity. (soon)
Obtain absolute dose control.
02.06.2009
Silvan Zenklusen, PSI/ETHZ
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