Pedroni Eros Paul Scherrer Institute - Villigen PSI, Switzerland

Transcription

1 Pedroni Eros Paul Scherrer Institute - Villigen PSI, Switzerland Center for proton therapy - PSI Gantry 2, the next generation gantry of PSI: a new system for promoting pencil beam scanning as a universal beam delivery technique IOP Half-Day Meeting on Hadron Therapy Friday 10 June 2011 Manchester

2 Layout of the presentation Gantry 1 experience Facility expansion at PSI - new SC cyclotron The new Gantry 2 of PSI Layout Beam optics and beam size Energy variations with the beam Sweeper magnets calibration Advanced beam scanning techniques Possible future clinical use of Gantry 2 Scanning as a universal beam delievery Page 2

3 Early 90's GANTRY 1 USED FOR PATIENT TREATMENTS SINCE 1996 GOAL in 1989 SHOW THE BASIC FEASIBILITY OF PENCIL BEAM SCANNING Page 3

4 The long term experience of PSI of using scanning Gnatry 1 designed in 1991 for protons On the basis of the scanning experience with pion therapy using inverse planning - based on CT data Gantry 1 Page 4

5 Upstream scanning System characteristics of Gantry 1 Magnetic scanning started before the last bending magnet Eccentric mounting of patient table on the gantry front wheel (with counter-rotation) Gantry radius reduced to 2 m Still the smallest proton gantry in the world α rotation β rotation φ rotation Page 5

6 Pencil beam scanning Small pencil beam: 3 mm σ (7-8 mm FWHM) Cartesian scanning (infinite SSD) Discrete spot scanning step and shoot,method - on a 5 mm grid From 1996 until May 2008 the only scanning gantry in the world Elements of scanning: Dose Monitor+Kicker X Sweeper magnet 5 ms / step fast Y Range shifter 30 ms average Z Patient table 1 cm/s slow 100 us reaction time Page 6

7 Clinical use of GANTRY 1 In use since1996 Full fractionation ~30 fractions Treatments 8:00 16:00 Max 19 patients/day (2.5 per hour) 2.8 fields/fractions in average 1/3 of patients are children Under anesthesia 1/3 of treatments are IMPT Courtesy of B. Timmermann Weak points of Gantry 1 Table motion is part of scanning Not possible to use collimators Not possible to apply repainting We treat only non moving targets! Page 7

8 Main goal: Delivery of IMPT and IGPT Dose shaping within the target IMPT (intensity modulated PT) The term of comparison with IMRT (conventional therapy) Biological targeting (image guided proton therapy) Intentional non-homogeneous dose distributions Dose proportional to the tumor activity (biological signal) The topic for the future Courtesy of A. Lomax PSI Page 8

9 Major disadvantage of scanning : organ motion sensitivity Interference of organ motion with the scanning sequence M.Phillips PSI 1992 Disturbance of the dose homogeneity within the target With Gantry 1 we treat only immobilized lesions attached to bony structures Tumors in the head, spinal chord and low pelvis We accept only movements < + -2 mm for treatments with full fractionation! Can we overcome this drawback? Possible remedies : Fast Scanning (Repainting) Gating Tracking? or a combination of those the possible points to be developed with Gantry 2 The point of scanning to be improved Page 9

10 2000 EXPANSION OF THE PROTON FACILITY AT PSI SUPERCONDUCTING CYCLOTRON FAST DEGRADER LAMINATED BEAM LINES GOALS: Stable DC beam Modulation of the beam intensity at the ion source Very fast energy changes Page 10

11 Layout of the newly expanded proton facility COMET - dedicated superconducting cyclotron [ACCEL - design H. Blosser] Beam for Gantry 1 all the year through patients treatments restarted in February 2007 no shut-downs since August 07 Horizontal beam line for OPTIS 2 transfer from OPTIS 1 last year Next generation scanning gantry : Gantry 2 1. patient planned for 2012 Medical cyclotron Exp. Area (PIF) OPTIS 2 Gantry 2 Disconnected from PSI ring cyclotron in 2006 Gantry 1 Page 11

12 Facility specifications were derived for the new Gantry 2 Super-conducting cyclotron Very stable beam at the ion source Aiming at 2-3% at the µs scale Deflector plate in the first orbit Dynamic control of the beam intensity µs time scale Fast degrader (moving carbon wedges) Continuous choice of the beam energy Beam line with laminated magnets Providing fast changes of beam energy Page 12

13 Excellent beam current stability an example Change of paradigm of testing the beam monitors Use the inherent stability of the beam to check the fine tuning of the electronics (capacitive matching) Result: improved linearity of the dose < 0.5% down to a 0.1 Gy dose Ideal beam for scanning with 100% duty factor Dose measurements for diff. Plugin configurations 600 µs 33 nf 1.01 m e a s u re d D o s e /m e a s u re d D o s 1 G y applied Dose [cgy] ISO, 22pf ISO, 47 pf ISO, 33 pf ISO, 39 pf 22 nf Page 13

14 2005 A NEW PSI GANTRY - GANTRY 2 MAIN GOAL DEVELOP FURTHER SCANNING TO BECOME A UNIVERSAL BEAM DELIVERY TECHNIQUE Page 14

15 GANTRY 2 TOPIC 1: INNOVATIVE GANTRY LAYOUT READINESS FOR IMAGE GUIDED PROTON THERAPY VERY EFFICIENT PATIENT HANDLING Page 15

16 Design started from the patient table ANIMATION Beam Line Support Bearing axle From -30 to +180 Patient table Room with fixed floor 0 to +180 would have been a better choice (cheaper) Page 16

17 Layout of the Gantry 2 room: patient table, compact nozzle Small compact nozzle Same patient table as at RPTC in Munich (Schär Engineering) Easy access to the patient table on fixed floor Fixed walls and ceiling for mounting commercial equipment (Vision RT?) A system open on both sides - lateral and front - Optimal for using an in-room sliding CT Page 17

18 In-room sliding CT - within reach of the patient table Beginning of IGPT? Patient positioning (tumor in soft tissues region) Setup for treating moving targets - 4D CT (relation external gate - internal motion) Page 18

19 BEV X-ray - synchronized with proton beam delivery BEV imaging - equivalent to portal imaging with photons Very large field-of-view (26 cm x 16 cm) not masked by equipment or collimators in the beam path QA control of gating and tracking (scanning + pulsed X-rays) Fluoroscopy mode Beam guidance? Proton beam Sweepers UPSTREAM SCANNING Bend and scan X rays tube Yoke hole Bending magnet Scan and bend Sweeper or Scatterer IGPT nozzle Patient Imager a) compact gantry b) long throw gantry Collimator Page 19

20 BEV - imager Check patient position at the isocenter Photograph of the retractable arm for holding the X-ray panel behind the patient on the side opposite to the nozzle (BEV X-ray). Page 20

21 GANTRY 2 TOPIC 2: PENCIL BEAM AND BEAM OPTICS GOALS: SMALL PENCIL BEAM FOR PRECISION THERAPY PARALLELISM OF SCANNING Page 21

22 GANTRY 2 beam optics X M A 3 2 Q 6 W P T A 3 P M 1 Q C Q 4 S Q 5 Q 3 M2 H Q 7 W U 3.2 m S Q 1 Q 2 A 1 SPECIFICATIONS Rotational symmetric (large) phase space +- 3mm mrad % dp/p Complete achromatism Scanning-invariant beam focus Orthogonal (x-,y-) focal planes in T and U Focus to focus with 1:1 imaging from the coupling point of the gantry to the iso-center 2D- parallelism of scanning (in T and U) (on Gantry 1 only U) Page 22

23 GANTRY 2 beam optics (TRANSPORT and RAYTRACE) Q C A 1 A 2 W T W U H S Double parallel scanning A 3 Point to point focus Achromatic Q 1 Q 2 Q 3 Q 4 Q 5 Q 6 Q 7 Double parallel scanning TRANSPORT beam envelopes through the Gantry 2 Page 23

24 The difficult piece: the last 90 bending magnet A big beast - 34 tons 15 cm gap To accommodate a scan area of 20 x 12 cm Lamination leaks but in the end Vacuum problems well solved! Photograph of the mounting of the 90 bending magnet. Integrated vacuum chamber embracing the poles of the magnet 180 tons mechanical isocenter within mm Page 24

25 Compact optimized nozzle Vacuum up to the patient Sharp pencil beam - 3 mm sigma Two monitors and a strip monitor 2 mm strips (TERA collaboration) Removable pre-absorber IN and OUT of beam For ranges below 4 cm Telescopic motion of the nozzle To reduce air gap (keep patient at isocenter) Option to add collimators and compensators To shield OAR on top of scanning To simulate passive scattering with a scanning beam Collision protection to treat patients remotely (multiple fields in one go) Page 25

26 Results: pencil beam size (on axis) Minimize material in the nozzle for having a sharp lateral fall-off beam size between mm sigma at all energies ( MeV) Region MeV 0.6 Adding piece by piece the materials in the nozzle Page 26

27 GANTRY 2 TOPIC 3 : ENERGY VARIATIONS WITH THE BEAM LINE COMPENSATED INTENSITY-ENERGY LOSSES VERY FAST ENERGY CHANGES Page 27

28 Compensation of beam losses in the range MeV na Constant ratio SC cyclotron Degrader Beam analysis Gantry 2 Coupling point na Rate Monitor Collimators for Intensity suppression khz Goal : reserve the use of the deflector plate for intensity modulated dose painting Energy [MeV] Page 28

29 Taking into account hysteresis effects ( MeV) Energy loop Red: up-down Blue: down-up Uncorrected 1 mm Corrected Page 29

30 With very fast energy changes. 80 ms But - we observe Slow change of lateral beam position after big energy changes in the range MeV 1-3 mm shift with exponential decay with decay time of ~2 s < 0.3 mm position change after small energy changes within the SOBP We plan to correct these shift with sweeper offsets as a function of the time after the last energy change Strategy: Fixed targets - range precision of < 0.5 mm while working with full ramp MeV Moving targets - repainting - precision ~ 1mm repaint only the SOBP up and down VIDEO Scintillator block the beam of Gantry 2 seen with a TV camera Page 30

31 GANTRY 2 TOPIC 4 : SWEEPER MAGNETS COMMISSIONING ISSUES NON LINEARITIES OF THE SWEEPERS Page 31

32 Need of a very precise mapping of the sweeper's action MEV 210 MeV MeV 150 MEV T (cm) 0 T (cm) U (cm) 80 MeV 8 80 MEV T (cm) U (cm) U (cm) Measured (red) and calculated (blue) spot maps of the (linear input) action of the sweeper magnets on the scanned beam position. The non linearities are due to a curvature of the effective boundary of the magnetic field of the 90 bending magnet which is changing with energy (changing sign) Page 32

33 After a proper mapping of the sweepers 70 MEV 120 MEV 170 MEV 220 MEV Beam spots (2 cm steps) at the isocenter covering the scan region of 12 cm x 20 cm Page 33

34 GANTRY 2 Topic 5 : the main goal of Gantry 2 NEW ADVANCED BEAM DELIVERY TECHNIQUES TO PROMOTE SCANNING AS A UNIVERSAL BEAM DELIVERY TECHNIQUE Page 34

35 Aiming for highest repainting From spots For reducing organ motion errors Goal - Fast painting with volumetric repainting Painting lines < 5-10 ms per line (10cm + line change) Painting energy layers 200 ms per plane (20 lines x 5 mm) Change of energy (100 ms - 5mm range) Painting of volume 6 s per liter (20 energies at 5mm steps) Volumetric repainting capability (aim) repaintings / liter in 2 minutes To lines To contours Page 35

36 Use of FPGAs for dose painting Vertical deflector plate for intensity modulation Installed inside accelerator after the first turn close to the ion source Fast intensity control at the time scale of 100 µs Requires flexible control system Synchronous control of fast actuators (sweepers, deflector plate) with 100 khz Tabulated dose delivery based on state-of-the-art electronics (FPGA) Example: Painting shaped energy iso-layer Page 36

37 Dose delivery as a function of the real time Time driven beam delivery Beam path downloaded as tables of the sweepers U - T and of the beam intensity I Dose control with a feed-back loop: Monitor 1 required dose -> vertical deflector plate A) If sweeper speed is the limit we use variable intensity (IM) B) If beam dose rate is the limit we use variable sweeper speed (SSM) C) or we use both! Full range of dynamic dose control from zero IM to any dose SSM Delivered MUs Dose linearity of simple T-lines Required Dose 5ms - 10 cm lines - painted with IM 23 times Max T speed Variable intensity Page 37

38 feed-forward only Video showing fast conformal line scanning VIDEO Page 38

39 GANTRY 2 POSSIBLE CLINICAL USE OF GANTRY 2 GOAL: PROMOTE SCANNING AS A UNIVERSAL BEAM DELIVERY TECHNIQUE make scattering obsolete Page 39

40 Often required Improve lateral fall-off for treating static targets Prostate sharp lateral fall-off at the boundary between tumor and sensitive structure at high energy scanning alone is better (edge enhancement with pencil beam) at low energy scanning with added collimation better (beam size limitations) Scanning with varying beam energy superior to scattering? less material in the beam path Brain stem FWHM mm Idealized scattering (zero phase space) Realistic scanning Factor 1.4 gain Difference Gauss to error-function (1.7) Scanning with collimation better Scanning alone better Range mm Page 40

41 Very big tumors Combine the use of fast scanning with patient table displacements Needs remote control of the patient table (collision detection) Take advantage of the parallelism of the beam trivial patching - shift table - and apply intensity filter to spot pattern Medulloblastoma Dorsal irradiation VIDEO Example: a paediatric case treated in 2004 with Gantry 1 Page 41

42 Moderately moving targets (~5 mm) Discrete spot scanning applied with iso-layered repainting (max dose per spot visit) Treatments in the trunk (pancreas, cervix, colon), breast, lymph nodes, etc. Advantage of having fast energy changes -> Volumetric repainting FAST SCANNING IS MORE THAN JUST TARGET REPEATING COMPARE G1 spots, scaled G2 spots, scaled G2 spots, iso-layer G2 lines, scaled Pancreas Rectum Breast From Simulation of repainting strategies S. Zenklusen's thesis work PMB 55 (2010) Page 42

43 Largely moving targets Volume painting within a single breath hold? (repeated) Using Conformal line painting Speed of painting Gy in a sphere of ½ liter (17 layers) in 7 secs If not working, Gating Lung liver 7s Page 43

44 Simulated scattering Dose shaping using compensators But applying uniform scanning Energy layer with homogeneous fluence Magnetic line scanning at max. speed Very high repainting number Collimator is optional As a sub-mode of conformal scanning To simulate scattering on a scanning-gantry To render scattering based gantries obsolete Retinoblastoma? Page 44

45 Feasibility demonstration of "simulated scattering" 8 cm diameter sphere Field shape = target projection minimal neutron background Variable modulation of the range Layer shrinking Parallel beam no compensator dose errors Highly repainted Distal layer: repainted 48 times in 30s From S. Zenklusen PhD - Medical Physics 2011 Page 45

46 Gantry 2 coming soon On behalf of the Gantry 2 team D. Meer, C. Bula, S.Safai, S. König, M.Rejzek, S. Zenklusen. THANK YOU Page 46