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

1. Silvan Zenklusen, PSI/ETHZ 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

2. Silvan Zenklusen, PSI/ETHZ 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

3. Silvan Zenklusen, PSI/ETHZ Proton radiation therapy – rationale

4. Silvan Zenklusen, PSI/ETHZ 15 MeV photons proton SOBP protons relative dose tumor depth [cm] 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).

5. Silvan Zenklusen, PSI/ETHZ relative dose [-] range [cm] 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)

6. Silvan Zenklusen, PSI/ETHZ Established proton beam delivery techniques and resulting dose distributions

7. Silvan Zenklusen, PSI/ETHZ range-shifter wheel collimator patient scatter foils compensator entrance dose target volume 100% dose Broad beams - scattering spinal cord lumbar spine tumor bladder 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. 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.

8. Silvan Zenklusen, PSI/ETHZ spinal cord lumbar spine 90° bending magnet tumor pencil beam (σ = 3 mm) sweeper magnets (2 dimensions) target patient bladder intestine & bowel, sensitive to radiation dose 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. 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)

9. Silvan Zenklusen, PSI/ETHZ Proton radiation therapy at PSI

10. Silvan Zenklusen, PSI/ETHZ sweeper magnet 90° bending magnet a rotation f rotation b rotation 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.

11. Silvan Zenklusen, PSI/ETHZ 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. medical cyclotron (COMET) OPTIS 2 PIF degrader Gantry 2 Gantry 1 The completely new section from COMET to Gantry 2 is designed for the development of advanced scanning techniques.

12. D. Meer: New fast scanning techniques using a dedicated cyclotron at PSI 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 sideby -30° to 185° Flexibility of beam delivery achieved by rotating the patient table in the horizontal plane The new PSI Gantry 2

13. Silvan Zenklusen, PSI/ETHZ Simulation of scattering

14. Silvan Zenklusen, PSI/ETHZ 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!

15. Silvan Zenklusen, PSI/ETHZ 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.

16. Silvan Zenklusen, PSI/ETHZ 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… beam range shifter wheel scatter foils divergent beam

17. Silvan Zenklusen, PSI/ETHZ 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 sweeper magnets degrader beam 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.

18. D. Meer: New fast scanning techniques using a dedicated cyclotron at PSI 122 167 144 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 ms) • 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 120 80 # repaintings 40 Energy [MeV]

19. Silvan Zenklusen, PSI/ETHZ collimator compensator actual scan/scatter field 100% dose beam entrance dose target beam scan path Optimize scan-field size to avoid unwanted entrance dose Normal scattering: 100% dose outside target region due to too big scatter field Simulated scattering: no 100% dose outside target region since scan field is smaller and shaped proximally.

20. Silvan Zenklusen, PSI/ETHZ First measurements on Gantry 2 with a collimator/compensator Experimental setup

21. Silvan Zenklusen, PSI/ETHZ 6 cm Plexiglas 9 cm Plexiglas 10 cm Plexiglas 12 cm Plexiglas 14 cm Plexiglas 16 cm Plexiglas 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 10 cm Plexiglas 12cm Plexiglas 16 cm Plexiglas 14 cm Plexiglas

22. Silvan Zenklusen, PSI/ETHZ 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.

23. Silvan Zenklusen, PSI/ETHZ 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.