Center for Proton Radiation Therapy FEASABILITY OF SIMULATED
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 4 th, 2009 02. 06. 2009 Silvan Zenklusen, PSI/ETHZ 1
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 2
Center for Proton Radiation Therapy Proton radiation therapy – rationale 02. 06. 2009 Silvan Zenklusen, PSI/ETHZ 3
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 Me. V 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 4
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. 45 cm 3) range [cm] 02. 06. 2009 Silvan Zenklusen, PSI/ETHZ 5
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 Broad beams - scattering spinal cord range-shifter wheel collimator lumbar spine tumor patient scatter foils dd e r target volume bl a compensator entrance dose 100% dose Traditional and established technique since the 60’s. Individual compensator, collimator for every field. Sharp dose conformation lateral and distal. 02. 06. 2009 intestine & bowel, sensitive to radiation dose 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 7
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 bl ad de r 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 8
Center for Proton Radiation Therapy Proton radiation therapy at PSI 02. 06. 2009 Silvan Zenklusen, PSI/ETHZ 9
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 10
Center for Proton Radiation Therapy Situation at PSI – PROSCAN Expansion of radiation therapy facilities at PSI • Dedicated superconducting cyclotron → 250 Me. V 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 degrader OPTIS 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 11
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) 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 Dynamic beam energy variations with the beam line 28. 04. 2009 D. Meer: New fast scanning techniques using a dedicated 12
Center for Proton Radiation Therapy Simulation of scattering 02. 06. 2009 Silvan Zenklusen, PSI/ETHZ 13
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 3 D 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 14
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 -2 mm with full fractionation § BUT: On Gantry 2 we plan to treat mobile tumors using repainting and gating. 02. 06. 2009 Silvan Zenklusen, PSI/ETHZ 15
Center for Proton Radiation Therapy 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… 02. 06. 2009 Silvan Zenklusen, PSI/ETHZ range shifter wheel scatter foils divergent beam 16
Center for Proton Radiation Therapy Simulate scattering on a scanning machine = continuous scanning at maximal speed ne ts r ag pe m ee sw ad gr de – Use sweeper magnets to broaden up the beam by continuous fast motion (requires fast magnets: 10 x 10 cm 2 in 100 ms) → no neutrons er • Scanning beam – At PSI we use a degrader system far away from the patient (requires fast beam line: 4 Me. V steps in 80 ms) → 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 17
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 cm 2 # repaintings Beam delivery: Continuous scanning 500 iso-energy planes painted in less than 1 minute 120 80 40 122 144 167 Energy [Me. V] SOBP is created using different numbers of layer repetitions per energy 28. 04. 2009 D. Meer: New fast scanning techniques using a dedicated 18
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 20
Center for Proton Radiation Therapy First measurements on Gantry 2 with a collimator/compensator Experimental setup 02. 06. 2009 Silvan Zenklusen, PSI/ETHZ 21
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 10 cm Plexiglas 9 cm Plexiglas 12 cm Plexiglas 12 cm Plexiglas 16 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. 14 cm Plexiglas 02. 06. 2009 16 cm Plexiglas Silvan Zenklusen, PSI/ETHZ 22
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 Me. V. (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 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 24
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