an overview of next generation gantries
TRANSCRIPT
An Overview of Next Generation Gantries
David Robin
Lawrence Berkeley National Laboratory
With input from
LBNL : S. Caspi, H. Felice, R. Hafalia, A. Sessler, C. Sun, W. Wan, Postech : M. Yoon, CNAO (ULICE, PARTNER) : V. Lante, M.M. Necchi, M. Pullia, S. Savazzi, J. Osorio Moreno, PSI : E. Pedroni, GSI : U. Weinrich, HIT: T. Haberer BNL: D. Trbojevic
Outline
• Some Gantry Proper.es and Considera.ons • Present State-‐of-‐the-‐Art
• Future Direc.ons
1. More Compact Gantries using Superconduc.ng Magnets
2. Mobile Isocenter Gantries 3. Fixed Field Alterna.ng Gradient Gantries
• Possible to access the full 4 π solid angle with a combina.on of gantry and pa.ent rota.on.
• A gantry is a beam line that directs and focuses the beam onto the pa.ent at whatever angle is required for the treatment plan op.miza.on.
What is a Gantry
Some Existing Proton Gantries
PSI Gantry 2
Heidelberg Ion Therapy Carbon Ion Gantry
Only Carbon Gantry Worldwide
Treatment with gantry possible by end of 2011
Gantries are Large
Weight Ø Proton gantries weight about 100 tons Ø HIT carbon gantry weighs 600 tons
~12 m
~8 m
~13 m
~22 m
HIT (Carbon)
PSI-‐2 (Proton)
1/10 of the Eiffel Tower
Proton Therapy Center
Gantries are larger than the accelerator
Heidelberg Ion Therapy (HIT) Facility
This is even more striking for carbon facili=es
Carbon Ion beams are harder to bend
• Difficulty to bend measured by magne.c rigidity, Bρ [T m]
• Bending radius, ρ, is : ρ = Bρ/BMag
• For the same penetra.on depth – Magne.c Rigidity is nearly 3 .mes higher for Carbon Ion Beams
Why Carbon Gantries Are Bigger
BMag
ρ
Magne.c Rigidity
0 2 4 6 8
Proton
Carbon
Bρ [T m]
Magne.c rigidity for 3 – 30 cm penetra.on depth
Example : HIT 90° Dipole Field Strength is 1.81 Tesla For Bρ is 6.64 Tm the bending radius is 3.67 m
Potential For Significant Improvement
• Gantries are expensive – Large structures – Requiring big heavily shielded (~5m thick) rooms – Substan.al opera.ng costs (up to 0.8 MW for HIT Gantry)
• Advantages of gantries are such that many new proton facili.es are being built with mul.ple gantries
• Gantry development has poten.al for achieving significant gains in cost and performance
Some Gantry Properties and Considerations
1. Active Scanning 2. Large Good-Field Region 3. Dose Uniformity 4. Parallel (or nearly) Scanning 5. Fast Depth Scanning 6. Fixed Versus Mobile Isocenter
1. Ac.ve Scanning
• Ac.ve spot or pencil scanning -‐ the desired dose is delivered by scanning a small (few millimeter) beam in all 3-‐dimensions
Will only consider ac=ve scanning gantries
Transverse (outward)
Lateral plane (Dispersive and Transverse Dimensions)
2. Good Field Region
Dispersive
Dispersiv
e
Transverse
Good field region is the lateral region that can be scanned without moving the gantry or pa.ent
Scanning Magnets
Field Patching
Move couch
Move couch
Large Field è Scan in one go
Small Field è Scan and move couch (PSI gantry 1)
• Field patching is slower • Large field may require larger magnet apertures and higher costs
What is an optimal size for the good field region?
Move couch
• Dose uniformity (~2.5%) à precise beam position and minimal beam distortion (when scanning or rotating)
• If not designed well magnetic elements between scanning magnets and patient can cause distortion (i.e. large T terms)
3. Acceptably Small Beam Distortion While Scanning
x
y
y
x
How much distor=on is acceptable?
Beam Distortion
Δx = kxθx + T122θ2x + T144θ2
y + … Δy = kyθy + T324θxθy + …
Scanning
θ
4. Parallel versus Angular Scanning
• To minimize normal .ssue dose à Parallel scanning is preferable
• Source to Axis Distance (SAD)
• But some small angle might be acceptable
Penalty of Angular Scanning
• Higher rela.ve skin dose 40% increase in skin dose for 2 meter SAD
What is the Minimal Acceptable SAD?
ULICE Study
Scanning Magnets Location
• Large aperture magnet • Emphasis on magnet
field quality • Higher operating costs
• Larger gantry radius • Larger room size
UPSTREAM
DOWNSTREAM
Upstream or Downstream of Final Bend?
• Depth Change : 5 mm (ΔL) • Field Change ~5% of Peak Field change in field (ΔB/BM) • Time (ΔT) - Synchrotron ~4 seconds - Cyclotron ~0.1 seconds - Cyclinac < 0.01 seconds
Higher field magnets require faster ramping rates just to maintain the same depth scanning rate
Depth Scanning Speed
!L!t[mm/s] "
16.65 L[mm]( )3/4
BM [T]!B!t[T /s]
Non Rela.vis.c Approx
Gantry Functional Specifications
• As part of the Union of Light Ion Centers in Europe (ULICE), physicians and medical physicists from CNAO and other European facilities completed a survey
• Some of the survey results are shown on the next page are useful in understanding the carbon ion gantry functional specifications
Gantry funcBonal specificaBons
Minimum good field size 15 x 15 cm2
Maximum number of fields per session
4
Penetration depth (range) 3 – 30 cm (corresponding energy: p = 60 - 220MeV; C ion = 120 – 430 MeV/u)
Voxel dose accuracy ±1%
Dose uniformity ±2.5%
Voxels characterization 3 x 3 x 3 mm3
Voxels out of range 1%
Field position accuracy ±0.5 mm
SAD 4 m
Maximum treatment time 30 min Minimum required space around isocenter
60 cm
Achieved beam directions 4π
Present State of the Art
• Fast Conformal Scanning with Volumetric Repainting • Active and Parallel scanning • Short (few minute) treatment time per field (including
repaintings) • Large Lateral Good-Field Regions > 10cm x 10cm • Fixed Isocenter • Field position accuracy ~1mm • Full 4π angular coverage
PSI Gantry 2 and the HIT Gantry are the state of the art
• Derivatives of the same design - Pavlovic Type
Fixed Isocenter
Fixed Isocenter -‐ Pa=ent does not move ver=cally when gantry rotates
Point is at the same posi.on for each gantry angle
Pavlovic Type Gantry Design
M. Pavlovic, et al, Nucl. Inst. and Meth A 545 (2005) 412-426
Radially very compact by having the last magnet a 90 degree magnet. All the other magnets – quadrupoles, dipoles, and steering magnets, do not take up additional radial space.
The radial size is determined by only three things – (1) the space between the patient and the last magnet, (2) the bending radius of the 90 degree dipole, and (3) the outer diameter of the magnet.
Pros and Cons of Pavlovic Design
Pros • Efficient use of Magnets • Radially compact • Parallel Scanning
Cons • Last bending magnet needs to have a large aperture to
accommodate the scanning • Strong requirements on field quality
– Heavy (~90 tons for the HIT gantry)
Example – PSI Gantry 2
• Good Field Region 12 cm by 20 cm
• Fast double parallel scanning – Lines ~10 ms – Lateral Plane (one energy) <200 ms (20 lines) – Beam Intensity Modulation (BIM)
• Quick Depth Change – 100 ms per 5 mm
T U
• Repainting of iso-layers (Goal) • ~ 6 seconds per cubic liter (20 energies)
• Volumetric repainting capability • 10 repain.ngs / liter in 1 (2) minutes
Lateral Scanning
Treatment with gantry within a year
Fast beam energy changes
Carbon wedges moving against each other in the beam
Fast energy changes with degrader and beam line (including GANTRY 2 magnets) Aiming at 100 ms dead .me for range steps of 5 mm
Achieved 80 ms !!
Future Direc.ons
1. More Compact Gantries Using Superconduc.ng Magnets
2. Mobile Isocenter Gantry for Poten.al Cost Savings
and/or Mul.ple Treatment Rooms
3. Faster and Easier Energy Scanning Using FFAG Magnets
More Compact Gantries
Weight • Proton 100 tons • Carbon 600 tons ~12 m
~8 m
~13 m
~22 m
HIT (Carbon)
PSI-‐2 (Proton)
What is possible? For instance can the size of a carbon gantry be reduced to about the size and weight of an exis.ng proton gantry?
Future Direction 1: Reducing the size of the Final 90° Bend?
HIT 90◦ Weighs 90 tons (65% of all rotating beamline
components)
• The Final 90° dipole is one of the main drivers of the size, weight and cost of the gantry
Many Requirements
• Bend the beam by 90◦ • Large aperture to accommodate scanned
beam • Rapidly ramp the field • Large SAD: Point-‐to-‐Parallel focusing
from the scanning magnets to the pa.ent
• Minimize beam distor.on while scanning
Superconducting Magnets
• Attraction – Superconducting magnets can achieve many times
the magnetic field strength compared with normal conducting magnets
– more compact lighter systems
• Some major challenges such as – Rotatable cryogenic systems – Rapid field changes for scanning depth
Superconducting Magnets (Cont)
• Several groups are looking at Superconducting Gantry Magnets (CEA, HIMAC, LBNL, and INFN Genova)
• Talks – Noda “The rota.ng gantry for the HIMAC facility” – Kircher “Cryogenic gantry at CEA“ – Caspi “A SC dipole magnet for a carbon therapy gantry”
• Posters – Felice/Caspi “Conceptual design of a curved superconduc.ng combined func.on magnet” – Fabbricatore “Superconduc.ng Magnets for Accelerator Gantries for Hadron Therapy ”
ETOILE (Superconduc.ng 90 degree Bend, CEA)
• 3.2 Tesla Superconduc.ng Dipole • 2 meter bending radius • Significantly lighter than HIT dipole • See talk by F. Kircher
(17tons for magnet)
M. Bajard, EPAC08, (2008 )1779-‐1781
Superconduc.ng (5 T) Combined Func.on Toroidal Magnet (Lawrence Berkeley Lab)
Work of Caspi, Robin, Arbelaez, Sessler, Sun, Robin, Hafalia, Yoon, Wan, and Robin
• Double .lted solenoid coil winding
More in talk by S. Caspi • Compact (1.3 m radius) • Light (~14 tons)
• Large good field region (26 cm diameter)
Scanning Mag. Pos.
Magnet Structure
Coil
Iron
• Minimized beam-‐shape distor.on
Future Direction 2: Mobile Isocenter Gantry
Fixed Isocenter Mobile Isocenter
Different Gantry typologies 1) Fixed isocenter gantry: Heidelberg (2007)
2) Mobile isocenter gantry: PSI -‐ 1 (1992)
3) Mobile isocenter gantry: PIMMS study (1993): Riesenrad gantry
Beam delivery system rotates around the pa.ent
that is not moved
Pa.ent and magnets rotate around the central axis – smallest possible radius
Isocenter moves according to the beam direc.on – only one 90°
bending magnet
Mobile Isocenter Gantries
Potential Advantages • More compact
• Serve more than one patient room
• Fewer rotating magnetic components
Not a New Concept
• PSI Gantry 1 (1993) – More compact that Gantry 2
Example 1. Moving Isocenter Gantry serving 4 rooms
(A. Brahme, Erice 2009)
Example 2: ULICE Workpackage 6 Riesenrad Like Gantry
• Rota.ng structure (room with the pa.ent) significantly lighter • 90° magnet alignment is easier in case of magnet rota.on around its axis rather
than in case of complete rota.on of the line;
V. Lante, M.M. Necchi, M. Pullia, S. Savazzi, J. Osorio Moreno
ULICE Workpackage 6 Riesenrad Carbon Ion Gantry
Reasons for the final choice (ULICE meeting 10th March 2011)
“The aim of the ULICE WP6 is to analyse the various aspects that influence the design of a carbon-ion gantry and to produce a design of the device. Both medical/functional and technological issues shall be considered to obtain a design that controls costs without limiting significantly the functionalities. The final target is the production of the conceptual design of an optimised gantry”
Geometry if conventional magnets are considered, the gantry layout has to be
innovative
mobile isocentre
PSI1-like Riesenrad-like
mechanical structure cost ~ 105% wrt HIT
mechanical structure cost ~ 70% wrt HIT
concrete & excavation cost ~ 1,377 k€
concrete & excavation cost ~ 1,963 k€
Future Direc.on 3: Fixed Field Alterna.ng Gradient Gantry
• Small aperture high gradient dipoles • See talk by D. Trbojevic
Dejan Trbojevic, Workshop on Hadron Beam Therapy of Cancer, Erice 2009
Future Direc.on 3: Fixed Field Alterna.ng Gradient Gantry
A Poten.al Major Advantage • Large Energy/Momentum Acceptance - May not need to change field while scanning depth - Poten.al to enable very fast depth scanning
• Ideally like to have scanning magnets ater the FFAG - FFAGs become very interes.ng if small (~2m SAD) - If not FFAG gantries will be radially very large
!L!t[mm/s] "
16.65 L[mm]( )3/4
BM [T]!B!t[T /s]
These three direc.ons are not mutually exclusive
• For instance one can conceive of FFAG gantries built up of superconduc.ng magnets
• Or combining superconduc.ng magnets in a Riesenrad design
Summary
• State of the art gantries are delivering impressive performance
• Several exci.ng direc.ons being explored that have the poten.al for achieving significant gains in cost and performance
• Some requirements, such as acceptable minimum SAD, are important to understand
` THANK YOU
Superconduc.ng Magnet (IRFU Saclay)