FEASABILITY OF SIMULATED SCATTERING ON A SCANNING GANTRY FOR PROTON RADIATION THERAPY

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



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


Proton radiation therapy – rationale




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


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)


range [cm]

rela

tive

do

se [

-]


Established proton beam delivery techniques and resulting dose distributions




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.



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)


Proton radiation therapy at PSI




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



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.


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


Simulation of scattering




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!



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.



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



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.


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



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.



First measurements on Gantry 2 with a collimator/compensator

Experimental setup



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



12cm Plexiglas

14 cm Plexiglas

16 cm Plexiglas

10 cm Plexiglas


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.




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.

Documents

FEASABILITY OF SIMULATED SCATTERING ON A SCANNING GANTRY FOR PROTON RADIATION THERAPY