P.J. Bryant – CERN, Geneva
JAI-2006- P.J. Bryant Slide 1
What is a gantry? Matching methods…
A more detailed classification… Symmetric beam.
Passive spreading gantries. Round beam.
Divergent-beam voxel- Rotator method.
scanning gantries. Equal sigmas method.
Parallel-beam voxel- Patents.
GSI therapy complex.
Geometry is not enough…
Gantry matching requirements.
Riesenrad gantry film.
JAI-2006- P.J. Bryant Slide 2
If you have a therapy centre,
then you will want a gantry or
JAI-2006- P.J. Bryant Slide 3
What is a gantry?
A gantry directs the beam onto the patient at
whatever angle is required by the treatment Accelerator plane
plan. Ideally, the full 2p should be available (horizontal) Target
about the gantry axis. Horizontal axis
Ideally one should also be able to rotate the Gantry plane
patient, so as to access the full 4p solid
The requested treatment field is 40 40 cm2.
The requested beam penetration 27 cm.
There are two broad classifications :
JAI-2006- P.J. Bryant Slide 4
A more detailed classification…
Beam diverges after last dipole:
Passive spreading gantries (protons).
Divergent-beam voxel-scanning gantries (protons).
Beam diverges within gantry lattice:
Parallel-beam voxel-scanning gantries (light-ions).
Exo-centric “Riesenrad” gantry (light ions).
There is also an extended category of novel
JAI-2006- P.J. Bryant Slide 5
Passive spreading gantries
The first passive spreading gantry built
was the “corkscrew” gantry at Loma
Loma Linda Corkscrew
Today the accepted conventional design
is the “conical” gantry as demonstrated
To understand these gantries it is useful to look
at the passive spreading technique.
JAI-2006- P.J. Bryant Slide 6
Double-scatterer system for protons
First scatterer significantly
increases angular divergence.
Second scatterer is shaped to
scatter the dense centre to the
edges while letting the edges pass
~60% of the beam will be lost.
Scatterers will be a high Z
material to favour scattering Quasi-uniform beam
(copper). (within 2%) over
20 20 cm2
JAI-2006- P.J. Bryant Slide 7
Proton beam preparation before the scatterers
“Double wedge system”
Beam E0 Beam E1 Beam E1 Emin- E1 Beam E1
Beam E1 Beam E3
fixed energy energy
energy E1 E1
E2 E acc. to
E4 thickness Emin
Adjust the beam Stepwise energy Fast modulation Static modulation
energy (Bragg-peak) modulation to by a rotating by a ‘ridge’ filter
to the maximum define the slices propeller to may be used to
tumour depth. in the tumour. create SOBP. replace propeller.
Low-Z materials preferred for less scattering (plexiglass).
JAI-2006- P.J. Bryant Slide 8
Passive spreading gantries continued
The Twiss functions and beam emittances at the entry to the
gantry are not critical because the beam is strongly scattered
after the last dipole largely destroying the memory of the
Similarly, the alignment of the incoming beam is not critical
because the beam will be spread out and collimated to the
correct shape and position just before the patient.
The magnets in the gantry can have normal apertures (i.e.
weight, cost) because the beam is spread out after the last
This type of beam delivery and gantry is only used for
protons. Light ions would fragment in the scatterers and the
beam would be heavily polluted by neutrons.
JAI-2006- P.J. Bryant Slide 9
Divergent-beam voxel-scanning gantries
PSI have a working voxel scanning system for
protons from a cyclotron.
IBA are promising a replacement nozzle for their
passive-spreading proton gantries that will perform
voxel scanning. (The ‘Nozzle’ contains the
spreading or scanning system and the collimator).
For voxel scanning, the tumuor is divided into layers and each layer is divided
into pixels. Each pixel has a width, height and thickness of a few mm, i.e. it is a
volume pixel or voxel. The beam energy and energy spread are set to match
the layer depth and thickness and the beam size is set to the voxel size.
JAI-2006- P.J. Bryant Slide 10
Parallel-beam voxel-scanning gantries
Parallel-beam scanning reduces the surface dose
given to a patient by divergent-beam scanning.
Parallel-beam scanning gantries solve the problem of
lack of space in divergent-beam gantries for the
scanning magnets for ion beams that have a higher
Gantries specially designed to give parallel scanning
are the “cylindrical” gantries.
The disadvantage of the cylindrical gantry is the large GSI Cylindrical
aperture needed in the final dipole, which increases
size, weight and power consumption.
JAI-2006- P.J. Bryant Slide 11
Highest precision, but the small beam size makes tumour movement a
Well suited to light ions that scatter less and therefore preserve the small
Synchrotrons offer the best flexibility.
From full-volume passive spreading through to voxel scanning, there has
been a reduction in the elementary volumes that are irradiated and a
corresponding increase of about 3 orders of magnitude in the speed
required from the on-line dosimetry system to maintain the treatment time
and accuracy. This makes voxel scanning the highest technology variant.
JAI-2006- P.J. Bryant Slide 12
Wobbling of an enlarged beam spot
Consider the above scheme, especially for ions.
Full passive spreading of ion beams is not recommended as the beam fragments
and the impurities have different penetrations and RBE values.
In the above, the scatterer/absorber produces an enlarged beam transversely with
a momentum spread, typically 2 cm 2 cm 1 cm (spread-penetration).
The enlarged spot (‘blob’) is rapidly ‘wobbled’ in a circular motion across the
collimator to give a uniform irradiation field.
JAI-2006- P.J. Bryant Slide 13
To avoid “hot” and “cold” spots in the treatment field, voxel scanning requires
sub-millimetre precision for the positioning and the size of the beam spot.
Similarly, it imposes the same precision on the immobilisation of the tumour.
Passive spreading avoids “hot” and “cold” spots within the treatment area. The
collimator before the patient “screens” the effect of upstream movements of
gantry elements and the scatterer “screens” the patient from changes in the
upstream beam parameters from for instance gantry angle changes.
Movements of the tumour appears as a blurred edge to the treatment volume.
Thus voxel-scanning gantries must be more rigid for all gantry angles and
the beam matching for changing gantry angles is more critical.
Divergent scanning gantries are limited to protons by the underlying
technological limits on the scanning elements and the gantry size.
JAI-2006- P.J. Bryant Slide 14
Some more comparisons
Cylindrical gantries for ion voxel scanning are of the order of 700 t
making the specific load on the rollers consequential.
All the iso-centric gantries roll on large support rings with diameters up
to 12 m. If a ring is damaged, which occurs, it is virtually impossible to
replace the ring without completely re-building the gantry.
Due to the constraints of weight and size, all the iso-centric gantries have
a limited patient space at the iso-centre.
In comparison, exo-centric gantries are lighter, less power
consuming, the support rings are smaller and can be replaced by
commercially available roller bearings for turrets and the patient
space is effectively unlimited.
(IBA have solved the problem of minor damage to the rings, by providing a second precision
surface on the inside of the ring. This surface is used to guide a mobile grinder that clamps on the
ring using the inner surface as reference.)
JAI-2006- P.J. Bryant Slide 15
This category is the “engineer’s” solution that
places the heavy equipment to be rotated
(magnets, counter-balance weight) on the axis
and the light equipment (patient, couch, robot
arm) off axis, BUT the medical community
does not like this solution.
Perhaps the first publication is by R.L. Martin.
JAI-2006- P.J. Bryant Slide 16
“Riesenrad” exo-centric gantry
The “Riesenrad” is named after the famous
“wheel” in Vienna.
The heavy dipole magnet (~70 t) is kept on axis
where it can be more easily balanced and
The patient is in a spacious room which must
provide a firm support, but need not be positioned
to high precision.
The patient’s couch is then aligned with respect to
the dipole by a robot arm and a photogrammetric
The key to this gantry is how to match the
JAI-2006- P.J. Bryant Slide 17
The criteria applied by the early gantry designers are not always clear.
Reducing the the axial length seems to be the most frequent aim and
surprisingly they seemed insensitive to weight.
The block of 4 drawings below are taken from the EULIMA study (~1991).
In general, no optical principles were published for matching the exotic
gantries apart from the dipole layout.
JAI-2006- P.J. Bryant Slide 18
Novel gantries continued
The “Planar Gantry” is proposed by M.M. Kats ITEP, Moscow.
The moving structure is eliminated, but the space around the patient is still
limited and the total bending is about twice that in the accelerator.
Two fixed beam lines (one horizontal and one at 60 deg.) is simpler and with
rotation and limited tilting of a supine or sitting patient nearly all requirements
can be reasonable met.
JAI-2006- P.J. Bryant Slide 19
Novel gantries continued
There are some other “mobile-magnet” geometries that
have been patented”, e.g. by Prof. G. Kraft...
JAI-2006- P.J. Bryant Slide 20
Novel gantries continued
Another idea is to reduce the overall gantry diameter and weight by inclining the
final beam at 60 degree (the “alternative gantry” proposed by Marius Pavlovic
and patented by GSI).
Note: If two fixed lines are used instead of a gantry then the combination of horizontal +
60 deg. gives access to more solid angle than horizontal + vertical because rotation about
the vertical beam brings no gain. If tilting the patient is allowed, then the access becomes
JAI-2006- P.J. Bryant Slide 21
Novel gantries continued
The “S.C. pipe gantry” that guides and focuses
a beam like a hose pipe guides water, suggested
by G. Benincasa.
The dream of having a flexible beam guide may
have started with S. Van der Meer, “The Beam
Guide”, CERN 62-16, 1962. Van der Meer
studied a coaxial system with the outer
conductor at infinity and mentioned the use of a
A PhD student, A. Maier, tried to improve the
focusing by shaping the X-section of the
conductor, adding the return conductor and
stepping the conductors to get focusing and
defocusing regions, but a practical scheme could
not be found.
In fact, the interest in making charged-particle
pipes is quite widespread. The next step is to
add some iron, which leads to the “Pipetron” an
extruded single or double channel magnet with
one S.C. cable.
JAI-2006- P.J. Bryant Slide 22
Geometry is not enough...
Up to this point we have concentrated on the dipole geometry of gantries
without explaining how the dispersion and focusing could be made to work.
In fact, many early gantry proposals ignored this point completely.
To match a gantry ones needs to take care of several aspects:
The rotational optics (see next slides).
In some cases, it is necessary to consider the transverse beam distributions, i.e. to
distinguish between beams from a resonant slow extraction in a synchrotron and a
beam from say a cyclotron.
For voxel scanning, it is necessary to design the optics of the line and the gantry as
an integrated whole e.g. beam size control may be in the line or in the gantry
according to the method applied.
JAI-2006- P.J. Bryant Slide 23
Gantry matching requirements
The shape and size of the beam spot at the patient must be totally
independent of the gantry angle.
There must be no correlation between momentum and position across the
For the purposes of scanning, the optics inside the gantry must be
independent of gantry angle.
JAI-2006- P.J. Bryant Slide 24
Symmetric beam method with zero dispersion (exact)
The beam must have zero dispersion and be rotationally symmetric i.e. the same
distribution (gaussian or KV) with equal Twiss functions and equal emittances in
both planes at the entry to the gantry.
The gantry must be designed with a closed dispersion bump in the plane of bending.
Round-beam method with zero dispersion (partial)
The beam must have zero dispersion, the same distribution (gaussian or KV) in both
planes with the condition Exbx=Ezbz at the entry to the gantry. It would also be
desirable but not absolutely necessary to have x=z=0.
The gantry must be designed with phase advances of multiples of p in both planes
(i.e. 1:1 or 1:n matrices) and a closed dispersion bump in the plane of bending.
The problem in this case is that the optics inside the gantry changes with rotation
angle. However, it is often possible to “freeze” the last section so that the scanning
JAI-2006- P.J. Bryant Slide 25
Matching methods continued
Rotator method (exact and completely general)
This method will rigorously map all Twiss functions and the dispersion
functions into the gantry coordinate system independent of the gantry
The gantry must be designed to give zero dispersion at the exit, but note
that it can be finite at the entry. This will be illustrated later with the
This method is essential for slow extracted beams that have extremely
unequal emittances and control their beam sizes in unconventional ways.
The rotator appears in a Loma Linda patent, but is not explained!
JAI-2006- P.J. Bryant Slide 26
Quadrupole lattice v
p=3600 q=1800 x
cos 0 sin 0 cos 0 sin 0
1 0 0 0 2 2
1 0 0 0
0 cos 0 sin 0 1 0 0 0 cos 0 sin 0 1 0 0
M0 2 2 2 2
sin 0 cos 0 0 0
1 0 sin 0 cos 0 0 0 1 0
0 sin 0 cos 0 0 0 1 2 2
0 sin 0 cos 0
0 0 1
2 2 2 2
Maps the beam 1:1 to the gantry independent of the angle.
This is the rotator solution and it maps the dispersion
function and the Twiss functions rigorously to the gantry
JAI-2006- P.J. Bryant Slide 27
Appearance of rotator in a Loma Linda patent
The “rotator” quadrupoles are clearly
labelled, but the patent does not explain
the theory or function of the device
(omission of a non-trivial step should
invalidate the patent). It is not sure that
the patent writer realised that the
“rotator” rotates .
The “rotator” was invented by Lee Teng of
Fermilab, but he did not publish the
design. The detailed derivation appears in
an internal Loma Linda report and his
private laboratory notebook.
The Rotator design shown is clearly a
FODO design which is not ideal as it
exhibits large beam changes inside the
lattice during rotation.
JAI-2006- P.J. Bryant Slide 28
Matching methods continued
Equal sigma method (partial)
This method is used in the GSI ion gantry.
The method has been patented by GSI (EP 1 041 579 A1).
The validity of this method has not been fully demonstrated.
The problem can be understood intuitively by first considering the beam
spot size in terms of sigma and then in terms of the FWHH, for example.
Although the sigma values may be equal, the FWHH values will be
different in the two planes because of the different beam distributions.
Thus, the beam spot will be distorted and its orientation will depend on the
This distortion may, or may not, be acceptable after smoothing by
scattering in the patient’s body.
JAI-2006- P.J. Bryant Slide 29
Equal sigma method
The beam is represented by its sigma
x2 xx xz xz 1,1 1, 2 1,3 1, 4
xx x2 xz xz 2,1 2, 2 2,3 2, 4
zx zx z2 zz 3,1 3, 2 3,3 3, 4
2 4, 2 4,3 4, 4
zx zx zz z 4,1
The sigma matrix translates as:
σ 2 M Overallσ 0 M Overall
M Overall M Gan M Rot r1,1 r1, 2 r1,3 r1, 4
r2,1 r2, 2 r2,3 r2, 4
Diagonal terms gives beam widths, e.g.: M Gan
r r3, 2 r3,3 r3, 4
r r4, 4
(2)1,1 r1,12 [ (0)1,1 cos 2 (0)3,3 sin 2 ] 4,1 r4, 2 r4,3
2r1,1r1, 2 (0)1, 2 cos 2 cos 0 sin 0
r1, 2 [ (0) 2, 2 cos 2 (0) 4, 4 sin 2 ]
2 0 cos 0 sin
sin 0 cos 0
2r1,1r1, 2 (0)3, 4 sin 2
sin 0 cos
JAI-2006- P.J. Bryant Slide 30
Equal sigma method continued…
The angular dependence in the beam widths can be removed.
For example, if the gantry matrix is arranged to give r1,1=0
and the incoming beam is adjusted to give (0)2,2=(0)4,4, then
(2)1,1 r1, 2 2 (0) 2, 2
The vertical plane and the correlation terms can be similarly
Despite it not being possible to patent equations, this method
has been patented,
See European Patent Application EP 1 041 579 A1.
If one reads carefully, “round” is defined as equal sigmas,
which does not necessarily mean rotationally symmetric.
JAI-2006- P.J. Bryant Slide 31
You are probably familiar with the web site:
But maybe you are not familiar with:
Try entering for GSI’s therapy system :
EP 0 986 070 A1
EP 0 986 071 A2
JAI-2006- P.J. Bryant Slide 32
GSI therapy complex
GSI have recently revamped the patent for their therapy
complex. The “gist” is quoted below:
“The new set of claims 1 to 22 concentrates on the
inventive idea to use a special designed delivery system
for high energy particles from an accelerator system to
the isocenter of a treatment field.
The claimed invention is based on the split of the high
energy beam transport system into a high energy beam
transport line and gantry. The high energy beam
transport line delivers a beam to a coupling point of the
gantry with special beam properties, so that the gantry
is able to guide the beam for all angles to the treatment
field having a circular beam without the need of GSI gantry matched by the
adjusting the high energy beam line.” equal-sigmas method
The above is based on the “Equal sigma” method, but the
patent writer does not understand what “round” really
Note. This is the opposite of the PIMMS method (next
slide) that leaves the gantry unchanged and puts the
adaptive elements in the line.
JAI-2006- P.J. Bryant Slide 33
PIMMS (Proton Ion Medical Machine Study)
PIMMS, CERN 2000-006, Volume II is a design for a synchrotron-based, light-ion
therapy centre with various gantries.
The transfer lines and gantries were designed as an integrated system. The lines are
assembled from basic modules with 1:1 or 1:n transfer matrices. The phase shifter
and betatron stepper are placed upstream and can act for all gantries. The rotator can
usually be placed upstream, but for the “Riesenrad” it is needed next to the gantry.
JAI-2006- P.J. Bryant Slide 34
Horizontal beam size control
The extracted segment (in blue) is
called the ‘bar of charge’.
The bar of charge is about 10 mm
long and has a very small angular
Horizontally the distribution is quasi
rectangular and vertically it is
for extraction X´ Z´
channel Fitting an ellipse to this narrow bar is
Instead the bar is regarded as a
diameter of a larger unfilled ellipse.
The bar turns at the rate of the phase
Beam width Beam height A phase shifter can be used to turn the
bar and change the projected beam
JAI-2006- P.J. Bryant Slide 35 size.
Vertical beam size control
The vertical beam distribution is the usual gaussian and the beam size is controlled
in the classic way by varying the betatron amplitude function.
Classic beam size control The stepper is a 1:n telescope module that
keeps all beam parameters constant except
the vertical betatron amplitude function.
In the PIMMS design, the phase shifter and beta stepper are combined
in the lattice module illustrated. This module can keep all parameters
constant while varying x and bz in any desired combination.
JAI-2006- P.J. Bryant Slide 36
Scanning with the spot shape from a slow
The spot in PIMMS is NOT round.
Horizontally the distribution is quasi-
rectangular and vertically it is
It is important that the spot always has
the same orientation in the gantry
frame, so that the spot moves with the
sharp-edged distribution in the
direction of motion and the gaussian
tails overlap from row to row.
The unevenness in the spill in time can
be smoothed (within limits) by a feed-
forward that acts on the scan velocity.
JAI-2006- P.J. Bryant Slide 37
“Riesenrad” gantry matching
Since the “Riesenrad” has only one dipole it excites dispersion, but cannot close the
dispersion bump to zero within the gantry itself.
In the PIMMS design, the bend from the main extraction line excites the dispersion
function and the gantry is used to close the bump, thus delivering zero dispersion to
the patient. This is possible because the rotator turns the dispersion function to
match the gantry’s coordinate system.
JAI-2006- P.J. Bryant Slide 38
Gantry construction – “Riesenrad”
The central cage supports
the three scanning
magnets (1.5 t) and the
large 90° dipole (62 t).
The total weight is ~127 t,
of which 23 t are due to
the counterweight. The
design of the central cage
is driven by the desire to
minimise sagging of the
dipole no matter what
gantry position is
JAI-2006- P.J. Bryant Slide 39
Gantry construction continued
I so c e n tr e d e fo r m a ti o n s d u e to e l a sti c
d e fo r m a ti o n s o f th e g a n tr y str u c tu r e
Top graph shows the
mechanical movement of the
exo-centre due to elastic
D e fo rm a ti o n [m m ]
deformations of the gantry while 0 Y
rotating –p to +p.
-9 0 -4 5 0 45 90 Z
-0 , 0 5
Z-c o rr
-0 , 1
-0 , 1 5
-0 , 2
G a n tr y r o ta ti o n a n g l e [d e g ]
Lower graph shows the shift of 0.2
the exo-centre in the transverse 0.175
plane due to the optical errors 0.15
caused by the movements of the 0.125
magnets during rotation. y(iso)_local [mm] All Quads 0.1
-0.15 -0.1 -0.05 0 0.05 0.1 0.15
-0.025 Ideal isocentre
JAI-2006- P.J. Bryant Slide 40
It is frequently suggested that S.C. magnets should be used to reduce gantry weight
In practice, it is not trivial:
To build large bending angle S.C. dipoles, especially with large apertures.
To either use flexible cryogenic lines that roll on and off a drum on the gantry axis, or to
mount the helium liquefier on the gantry.
To ensure the continuous flow of cryogens at all gantry angles.
To ramp S.C. magnets quickly.
If the S.C. dipole is iron-free, then the stray field is problematic for the detectors.
If the S.C. dipole has an iron yoke, then the weight is not so strongly reduced.
There are also security risks:
Quenching could frighten patients when the release valve opens.
There is a risk of oxygen deficiency if large quantities of helium are released.
There is a risk of cold burns (especially for the lungs) if large volumes of vapour are
A reduction in magnetic rigidity or treatment field would bring a more direct
JAI-2006- P.J. Bryant Slide 41
On the one hand… On the other hand…
Iso-centric gantries are the preferred Severe engineering problems render the
solution of the medical community. ‘ideal’ gantry expensive.
And there is considerable reluctance to Some argue that:
relax the specifications i.e. 90% of patients require a treatment
Treatment field: 40 x 40 cm2. field of no more than 10 x 20 cm2.
Magnetic rigidity corresponding to A penetration depth of 20 cm is largely
penetration up to 27 cm. sufficient.
An exo-centric gantry is superior.
Sufficient angular access can be
obtained with horizontal and 60 degree
fixed beam lines by turning, tilting and
sitting the patient.
JAI-2006- P.J. Bryant Slide 42
JAI-2006- P.J. Bryant Slide 43
The design of the gantry depends on:
The method chosen for the rotational optics matching.
The type of beam delivery: passive spreading, wobbling or voxel scanning.
The transverse beam distributions: slow extracted beams or fast extracted beams.
Whether the beam is spread before or after the last dipole.
The magnetic rigidity: protons or light ions.
Check the patent situation for the design chosen.
Be critical of the size of the treatment field and the maximum beam
penetration, as these two parameters are very expensive.
JAI-2006- P.J. Bryant Slide 44
The CD-ROM contains the full CERN Report 2000-006 of the Proton
Ion Medical Machine Study (PIMMS).
The optics program used for the design can be copied from the CD-
ROM and the lattice files for the various gantries can be found in the
folder “Winagile/Lattices/Optical/Extrline/ for the individual modules
and “Winagile/Lattices/Engineer/Extrline/ for the gantries set in their
The AutoCad concept design drawings of the elements can be found in
the Hardware folder.
JAI-2006- P.J. Bryant Slide 45
Loma Linda IBA IBA
JAI-2006- P.J. Bryant Slide 46