Gantry design and experience at PSI

Transcription

1 Gantry design and experience at PSI Eros Pedroni for the R&D Technology Team Center for Proton Therapy Paul Scherrer Institute Villigen-PSI SWITZERLAND Workshop on Hadron Beam Therapy of Cancer Erice April 2009

2 1. Experience of using GANTRY 1 A system designed in 1991 for protons Based on the scanning experience with pion therapy between Gantry 1

3 System characteristics of Gantry 1 Magnetic scanning started before the last bending magnet Upstream scanning (applied only in the dispersion plane of the beam) Excentric mounting of patient table on the gantry front wheel (with counter-rotation) Reduce gantry radius to 2 m Patient couch rotation in the horizontal plane by +, axial rotation (a mistake?) α rotation β rotation φ rotation

4 A very compact system Flexibility to apply the beam from any direction Patients safety concerns when treating with beam from below patient rescue in case of motors failures BODY α rotation HEAD α+β rotation

5 If I could do it again Eccentric mounting as with Gantry 1 (R = 2m) But with rotation only on one side (0 180 ) - as with the new Gantry 2 Floor underneath the patient table Room moving with the gantry With a counter-rotation 2 m compact eccentric gantry Gantry 3?

6 Dynamic pencil beam scanning Gaussian pencil beam: 3 mm σ in air Cartesian scanning (infinite SSD) step and shoot 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 E. Pedroni 100 us reaction time CPT - Paul Scherrer Institute - Erice

7 Clinical use of GANTRY 1 > 400 patients treated since 1996 Mainly skull, spinal chord and low pelvis Only non moving targets! First human patient treated in fractions (treatment lasting ~ 6 weeks) Capability Max 19 patients/day (2-3 per hour) 2.8 fields/fractions in average Multiple fields delivered without personnel entering the treatment room SKULL Weak points of Gantry 1 Table motion is part of scanning (3. axis) Not possible to use collimators Not possible to apply repainting PELVIS

8 Advantages of using scanning Automated treatments all by software Avoid the use of individualized hardware Save money and personnel to fabricate and mount field-specific equipment Apply dose fields in sequences in one go without personnel entering the treatment room To reduce treatment time Minimal neutron background (for the patient) Lower risk of second cancer than scattering (depends on sophistication) Important for pediatric treatments Less activation of equipment in the nozzle (for the personnel) Use of a lower extracted beam intensity Less activation of the accelerator Minimal simple equipment The (pencil) beam does it for you

9 Variable modulation of the range Avoid unnecessary 100% dose on the healthy tissues Especially relevant for large tumors Reduce skin dose

10 The possibility to deliver IMPT and IGPT IMPT (intensity modulated PT) The term of comparison with IMRT (conventional therapy) PSI: the only facility delivering routinely proton-imrt (30% of pts) 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

11 Major disadvantage: organ motion sensitivity Scanning is very sensitive to organ motion errors during beam delivery -> disturbance of the dose homogeneity At the moment we can treat at PSI only well 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: Repainting Gating Tracking? or a combination of those possible points to be developed with Gantry 2 The points to learn about

12 2. Next step in 2000 at PSI: the PROSCAN project Facility expansion in the NA- Hall Problems before 2007 Parasitic use of the beam in a physics research environment Split and degrade beam from the (2 ma) 590 MeV main beam A multi-user complex facility Long shut-down each year Beam available only from May until December OPTIS 1 NEW PROSCAN AREA GANTRY 1

13 Layout of the PROSCAN facility COMET - new dedicated superconducting cyclotron [ACCEL - H. Blosser design] Beam for Gantry 1 all the year through patients treatments restarted in February 2007 (no shut-downs since August 07) Next generation scanning gantry : Gantry 2 (1. patient planned for 2010)) Horizontal beam line for OPTIS 2 (1. patients in 2009 higher priority) Medical cyclotron Exp. Area (PIF) OPTIS 2 Gantry 2 Disconnect from Ring cyclotron Gantry 1

14 Dedicated accelerator COMET Super-conducting cyclotron Requirements for Gantry 2 Constant energy Constant intensity Stable beam at the ion source Aiming at 2-3% in a few 100 us scale Dynamic control of the beam intensity With a deflector plate in the first turn Beam intensity modulation Aiming at 100 µs time scale For fast direct control of the dose delivery while painting dose lines For delivering repainted scanning See talk of D. Meer tomorrow

15 Energy variations with a degrader Fast energy changes with degrader + beam line (GANTRY 2) Aiming at 100 ms for a 5 mm proton range step Carbon wedges moving against each other in the beam

16 3. Next generation scanning gantry: Gantry 2 A tool for developing advanced beam scanning techniques Iso-centric layout (~ 4 m radius) Double magnetic scanning (parallel) started upstream of the last 90 bend Dynamic beam energy variations with the beam (gantry beam line with laminated magnets) 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 Analogy with longitude and latitude in world-geography

17 Design started from the patient table Beam Line Support Bearing axle From -30 to +180 Patient table Room with fixed floor Services X-ray console 0 to +180 would have been a better choice (cheaper) Patient and doctor

18 without loosing any functionality Sliding CT (Siemens) Setup for treating moving targets (relation of the external motion surrogate with the internal true organ motion)

19 BEV X-ray: for QA of moving targets X-ray Hole in the return yoke of the 90 bending magnet imager The new idea to use BEV X-rays simultaneously to the proton beam to be investigated

20 BEV X-ray images simultaneous to proton beam 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) Single image in the beam direction Scan and bend Proton beam Bend and scan Sweepers X rays tube Yoke hole Bending magnet Sweeper or Scatterer nozzle Patient Imager Collimator a) compact gantry b) long throw gantry

21 Remote control of patient table motion Expand range of magnetic scanning Beyond 20 cm x12 cm Taking advantage of the parallelism of the beam Field patching independent of treatment planning Example: paediatric cases treated in 2004 with Gantry 1 Recurrent Medulloblastoma, 5 year old boy (with Anaesthesia)

22 GANTRY 2 beam optics S1y H1 Q4 A2 M2 S2y S2x H2 WT WU P1 A3 Sy QC Q3 Q5 Q6 Q7 M3 3.2m 0 M1 REQUIREMENTS: A1 K 7.9m 11.7m Q1 Q2 Rotational symmetric (large) phase space +- 3mm mrad Complete achromatism 2D- parallelism of scanning (in T and U) (on Gantry 1 only U) Scanning-invariant beam focus Orthogonal (x-,y-) focal planes in T and U 1:1 imaging from the coupling point of the gantry to the iso-center

23 Beam optics confirmed by RAY-TRACING Harald Enge raytrace code Action of the U-sweeper Action of the T-sweeper Dispersive plane Transverse plane

24 Advantage of using parallel scanning Simplify treatment planning Dose homogeneity achieved with standard SOBP rules Simplify dosimetry and QA control No sensitivity to the distance from the apparent source (small on a gantry) Dose value and dose distribution in the patient and in the phantoms are the same Dose homogeneity preserved Easier field patching (expansion of the range for treating long targets) Can be done without optimization within treatment planning Just exchange magnetic scan position with patient table offset Compensators (simulated scattering) No dose errors due to inverse R-square effects Collimators No tapered faces necessary

25 4. Gantry 2: aiming at new faster scanning modes Gantry 2 improvements With respect to Gantry 1 Double parallel magnetic scanning One magnetic axis + patient table Energy variations with the beam Range shifter before patient Modulation of the beam intensity Discrete spot scanning Single pencil beam Lateral scanning Beam intensity modulation Repainting Dynamic beam energy Gating Multi-gating Tracking

26 Aiming at the highest scanning speed Painting of lines (contours) At max. velocity (~1-2 cm/ms) Dose shaping with Beam Intensity Modulation (I.M.) < 10 ms per line (10cm + line change) Painting of energy iso-layers 200 ms per plane (20 lines x 5 mm) Change of energy (100 ms - 5mm range) Repainting of iso-layers 6 s per liter (20 energies at 5mm steps) Volumetric repainting capability (aim) repaintings / liter in 2 minutes See talk of David Meer tomorrow

27 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 10cm Telescopic motion of the nozzle To reduce air gap (keep patient at isocenter) Option to add collimator (compensator) 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)

28 5. Initial commissioning of Gantry 2 Only beam line completed First beam through the new PSI Gantry 2 on May 2008

29 Results: pencil beam size (on axis) Minimize material in the nozzle for keeping the beam size between 3 and 4 mm sigma at all energies ( MeV) To have a sharp lateral fall-off Adding piece by piece the materials in the nozzle

30 For all energies Parallelism Max deviation ~4 mrad 100 MeV 120 MeV 140 MeV 160 MeV 180 MeV 200 MeV

31 Intensity vs. beam energy 600 Rate Monitor 1 Equalization of the intensity losses of the degrader Goal By detuning quadrupoles in the beam line (gain a factor of 50:1) Constant beam intensity at patient location [0.3 na] With constant extracted intensity [120 na] Independently of the energy setting Reserve the use of the modulation of the beam intensity at the ion source for dynamic dose painting! khz Energy [MeV] down up

32 Taking into account hysteresis effects Integral depth dose curves measured with a wide integrating (8-cm diameter) ionizations chamber in a water phantom ( + measurements, curves dose model calculation) Energy loop Red: up-down Blue: down-up

33 Range control with an ionization chambers stack Verification with a stack of cm-broad ionization chambers We can measure the whole range curve on a spot by spot basis (Torino TERA chip) Energy of a tunes defined within an hysteresis loop of MeV Blue curves up/down down/up correct order Red curves - up/down down/up - wrong order We should be able to scan with energy steps up-down and down-up

34 With very fast energy changes. 80 ms Example : Time pattern of scanning devices of a scan for a 65 mm dose box (2940 Spots) Degrader 90 Bending magnet 80 ms Beam monitor An order of magnitude faster than any other system

35 Planned clinical use of Gantry 2 The new instrument for treating moving targets with IMPT With Repainting Gating Tracking (?) multiples gates(?) New indications Moving targets: Liver, Lung, upper GI Large Targets: Craniospinal axis (medulloblastoma), Central Thorax (Mesothelioma), Abdominal and Pelvic Lymphnodes Small Targets: Integrated boost, Retinoblastoma, Proton Radiosurgery (AVM etc) Breast cancer

36 Conclusions: Beam optics commissioning of Gantry 2 successfully completed The beam spot size is small as planned The double parallel scanning is satisfactory We can apply very fast dynamic beam energy changes with the beam line We hope to show the feasibility of delivering highly repainted scanning for treating moving target (with and without gating) Still missing: control of the patient table, the patient handling equipment and the finishing of the area And the detailed precision work! VIDEO OF THE PROTON BEAM OF GANTRY 2 Scintillator block the beam of Gantry 2 seen with a TV camera THANK YOU