New Gantry Idea for H + /C 6+ Therapy G H Rees, ASTeC, RAL 4 th September, 2008

Traditional H + /C 6+ Gantry Dipoles: - 45°, + 45°, + 90° Quadrupoles: 11 or 12 units Pair of x, y scanning magnets Laser alignment, in-beam PET, X-ray imaging, and feedback. eg Heidelberg gantry has total length 22 m, Diameter & length of rotating part: 14 & 19 m, weight 630 tons, with 135 tons in magnets.

Gantry for Ring Scheme Outlined at FFAG07 Full tumour length scanned each cycle, using full current, with a single transverse scan obtained over the subsequent cycles. Achieved by foil stripping, ring extractions of Hˉ or C 4+ ions and by use of tracking or FFAG magnets in beam lines and gantry. Tracking is complex for many magnets of a traditional gantry. Hence, seek more compact gantry with simpler optical design. Gantry designed for tracking but may suit an FFAG design.

Fast scanning using Small Aperture Rings. Inner ring: Hˉ ions MeV/u Inner ring: C MeV/u C 1 = m C 2 = m Hˉ and C 4+ RFQ linac Outer ring: C MeV/u extraction stripping to C 6+ continuous extraction- stripping to protons 5 Hz Synchrotrons magnet apertures 42 x 60 mm 2

Basis of New Gantry Idea  No reverse bending magnet is used, allowing a compact form for a three-quarter ring gantry, as outlined overleaf  Magnets supported on both sides of a central, elliptically shaped structure (more symmetry than traditional gantry)  Simple optical design: 4, identical, BD-o-F-o, hybrid cells for a 2π achromatic section, with zero output dispersion  Each BD unit is a vertically focusing, combined function magnet of length 4 m and a bend angle of (270/4)°  The fourth F quadrupole (all 0.3 m long) is replaced by a quadrupole triplet for better adjustment of final beam spot

Conceptual Gantry Design ~10 m

Downstream End View of Conceptual Gantry

Gantry Features  The bend fields and gradients of the accelerator, beam line and gantry all have to track over the desired energy range  The F quads in the achromat and the F,D of the f-F-D output triplet all have normalised field gradients of m -2  The BD normalised gradients are B′/Bρ = m -2, with the maximum Bρ value for the C 6+ ions = T m.  A range of waists (β = 2.5 to 10.0 m) may be obtained at the gantry iso-centre by adjusting the input Twiss parameters  The normalised field gradient for the f unit is – m -2

More Gantry Features  Distances from the last BD magnet and the triplet lens to the iso-centre are m and 2.0 m, respectively  Scanning magnets may be located ahead of the triplet, so that there is no need for a large aperture, final BD unit  Gantry length, L = z (sin 67.5°  sin 45°)  2ρ, where z is 1.3 m and ρ is the BD orbit bend radius, m.  Thus, L = m (cf 19 m of the Heidelberg gantry), though elliptical, central structure has a large major axis  A vertical bend tracking magnet is needed at gantry input as beam entry is from below the patient platform  Stray field at the patient has to be at an acceptable level.

Tumour Doses for the Low Beam Currents  Assume: 2 nA and 0.1 pnA average currents of H + and C 6+, with overlap voxel scanning for sections 10 cm x 10 cm and with a beam spot diameter at the patient of ~ 1 cm  At 5 Hz, the full tumour is scanned over 200 pulses in ~ 40 s or, scanning during field rise & fall, over 100 pulses in ~ 20 s and, for sections ~ 20 cm x 20 cm, over 400 pulses in ~ 80 s  Average beam power at top energies for H + & C 6+ is ≤ ½ W So, if E av = ½ E max, up to 5(20) joules is delivered in 20(80) s If half reaches a litre tumour, dose received is 2.5 (10) Gray

Scanning Times/Doses for 1 Litre Tumours Length (cm) Section (cm 2 ) Min. Scan time (s) Dose (Gy) x x x x Max length dir’n gives fastest scan but most multiple scattering. More overlapping & scan time may be used to increase doses. Dose required is reduced by the number of gantry angles used.

Advantages over Traditional Gantry  Simpler beam dynamics design  Fewer number of magnet types  All magnets of small aperture  Shorter length for the structure  More symmetrical arrangement  Less flexing over angle range Notes: Angle range restricted to ~300° May also serve as a traditional gantry.

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Parameters for Synchrotron Rings 5 Hz Synchrotrons Inner Ring (Hˉ) Inner Ring (C 4+ ) Outer Ring (C 4+ ) Kinetic energy (MeV/u) 5.0 – – – Circumference (m) Gamma transition Minimum central field (T) Maximum central field (T) Maximum beta(v) value (m) Maximum beta (h) value (m) Maximum dispersion (h) (m) σ emittance ε n ((π) mm mr) Max. vertical beam size (mm) Max. horiz. beam size (mm) Max. aperture height (mm) Magnet v x h gap size (mm 2 ) 42.0 x x x 60.0

Features of 5 Hz Synchrotrons  Each ring has six, FODo, combined function lattice cells  Ring magnets have small (42 mm x 60 mm) apertures  Injection of Hˉ or C 4+ to Ring 1 is from a common RFQ  Ring 1 has 1-turn Hˉ injection and outward stripping ejection  Ring 1 has 1-turn injection for C 4+ ions and fast extraction  Ring 2 has fast inject of C 4+ and inward C 6+ stripping ejection  Max. field in Ring 1 is < 5 kG for low, Hˉ Lorentz stripping  Both rings require vacuum pressures of a few x Torr

Ring Acceleration Systems  Harmonic numbers are 14 for Ring 1, and 16 for Ring 2  High Q s values are favoured for accurate rf beam steering  Frequency range for Hˉ ions in Ring 1: to MHz  Frequency range for C 4+ in Ring 1: to MHz  Frequency range for C 4+ in Ring 2: to MHz  Ring 1 has two straights for rf cavities and Ring 2 has four  Broad band, 115°, 1 m drift tubes in ring 1 (~ 1.5 kV / turn)  Ferrite tuned drift tubes proposed for Ring 2 (~ 5 kV / turn)

Other Ring Features  Ratio for the radius of Ring 1 to that of Ring 2 is 7 : 8  RFQ beams chopped so that ≤12 (of 14) bunches formed  Betatron tunes are Q v = 1.44, Q h = 1.73, with γ- t = 1.57  Two dipole/quadrupole correctors are used for each cell  Accurate movement of beam to stripping foils required  Steering with dipole correctors & rf frequency modulation  Foils near end of BF magnets, for high dispersion points  Diff. height rings for outward/ inward ejection of H + / C 6+