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FEASABILITY OF SIMULATED SCATTERING ON A SCANNING GANTRY FOR PROTON RADIATION THERAPY. Silvan Zenklusen Prof. Andr é 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. - PowerPoint PPT Presentation
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Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 1
FEASABILITY OF SIMULATED SCATTERING ON A SCANNING GANTRY FOR PROTON RADIATION THERAPY
Silvan ZenklusenProf. 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
Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 2
Content
Proton radiation therapy – rationaleMaking use of the physical properties of p+ for medical needs
Established proton beam delivery techniques and resulting dose distributionsBroad beamsScanned beams
Proton radiation therapy at PSIDiscrete spot scanning using PSI’s compact gantry (Gantry 1)Novel beam delivery techniques
Simulation of scattering TheoryExperiment and first resultsOpen challenges
Conclusion & Outlook
Center for Proton Radiation Therapy
Proton radiation therapy – rationale
02.06.2009 Silvan Zenklusen, PSI/ETHZ 3
Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 4
Why use of protons for 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.
As compared to photons lower integral dose (2-5) to healthy tissues.
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).
depth [cm]re
lativ
e do
se
15 MeV photonsproton SOBP
protons
tumor
Center for Proton Radiation Therapy
Creation of a spread out Bragg peak (SOBP)
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)
02.06.2009 Silvan Zenklusen, PSI/ETHZ 5
range [cm]
rela
tive
do
se [
-]
Center for Proton Radiation Therapy
Established proton beam delivery techniques and resulting dose distributions
02.06.2009 Silvan Zenklusen, PSI/ETHZ 6
Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 7
range-shifter wheel
scatter foils
collimator
compensatorentrance dose
100% dose target volume
patient
Traditional and established technique since the 60’s.
Individual compensator, collimator for every field.
Sharp dose conformation lateral and distal.
Broad beams - scattering
tumor
spinal cord lumbar spine
blad
der
intestine & bowel,
sensitive to radiation dose
Scattered, broad proton beamDose 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.
Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 8
Scanned beams - scanning
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.
sweeper magnets(2 dimensions)
target
pencil beam (σ = 3 mm)
patient
90° bending magnet
spinal cord lumbar spine
tumor
blad
der
intestine & bowel,
sensitive to radiation dose
Spot scanning proton beamDose 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)
Center for Proton Radiation Therapy
Proton radiation therapy at PSI
02.06.2009 Silvan Zenklusen, PSI/ETHZ 9
Center for Proton Radiation Therapy
14.09.07 Silvan Zenklusen, PSI/ETHZ 10
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.
rotation
rotation rotation
sweeper magnet 90° bending magnet
Center for Proton Radiation Therapy
14.09.07 Silvan Zenklusen, PSI/ETHZ 11
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.
OPTIS 2
medical cyclotron (COMET)
Gantry 2
Gantry 1
degrader
PIF
The completely new section from COMET to Gantry 2 is designed for the development of advanced scanning techniques.
Center for Proton Radiation Therapy
28.04.2009 D. Meer: New fast scanning techniques using a dedicated
cyclotron at PSI
12
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
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
Center for Proton Radiation Therapy
Simulation of scattering
02.06.2009 Silvan Zenklusen, PSI/ETHZ 13
Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 14
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!
Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 15
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.
Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 16
Scattering on a scattering machine
• Scattering
– Use scatter foils to broaden up the beam
→ high neutron production→ higher risk of secondary tumors
– range shifter wheel to create SOBP→ more neutrons… divergent beam
scatter foilsrange shifter wheel
beam
Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 17
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
– 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
degr
ader
swee
per
mag
nets
parallel beam
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.
Center for Proton Radiation Therapy
28.04.2009 D. Meer: New fast scanning techniques using a dedicated
cyclotron at PSI
18
Beam delivery: Continuous scanning
• 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 s)
• Repainted, homogeneous area of 6 x 8 cm2
• 500 iso-energy planes painted in less than 1 minute
• SOBP is created using different numbers of layer repetitions per energy
Energy [MeV]167122 144
# re
pain
tings 120
40
80
Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 20
Optimize scan-field size to avoid unwanted entrance dose
Normal scattering:
100% dose outside target region due to too big scatter field
target
compensatorcollimator
entrance dose
100% dose
scan path
beam
beam
actual scan/scatter field
Simulated scattering:
no 100% dose outside target region since scan field is smaller and shaped proximally.
Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 21
First measurements on Gantry 2 with a collimator/compensator
Experimental setup
Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 22
Results: Difference between ‘Box’ scan fields and ‘Shrinked’ field, for a better dose control they are delivered using spot scanning technique.
• ‘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.
6 cm Plexiglas 9 cm Plexiglas
12 cm Plexiglas10 cm Plexiglas
14 cm Plexiglas 16 cm Plexiglas
6 cm Plexiglas 9 cm Plexiglas
12cm Plexiglas
14 cm Plexiglas
16 cm Plexiglas
10 cm Plexiglas
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 23
Center for Proton Radiation Therapy
02.06.2009 Silvan Zenklusen, PSI/ETHZ 24
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.
Recommended