
When trying to understand the optical design of an existing Medical Gantry for ProtonTherapy, then the
compactness (e.g. no spaceconsuming modular waist to waist transport system is acceptable here) and the large
90 to 135 degree bending magnet (focusing properties, momentumdispersion) at the end of the gantry is
usually causing some headache to newcomers in this field. Especially the fact, that the dispersion at the beginning
(matching or rotationpoint) and at the end (isocenter, location of the tumor to be irradiated) must be zero
(D=D'=0) or (R16=R26=0). The first order input files for the Graphic Transport Framework for three types of gantries
will be presented here. In order to ease the search for an optical solution of the problem, the principle of
beam line inversion (BLI) will be applied here. The reasons for this are the following facts: It is much
easier to start with the requested beam properties at the end (values for x , x', y=x, y'=x', dp/p and D=D'=0)
and then apply some amplitudelimiting constraints in backward direction (heuristic trial and error method)
along the beam line and again D = D' = 0 after the last bending magnet before the rotationpoint of the gantry.
Additional constraints are also x=y
and x'=y' (and preferably a doublewaist)
at the rotationpoint of the gantry. (To fullfil the later requirements see the notes about
Algebraic combination constraints).
Important inputs are the signs and strengths of the starting values (guesses) for the quadrupole lenses.
The different groups (2 to 5 quads) should always have alternating gradients. The sign of the 1st quad from the
end (near the 90 or 135 degrees bending magnet) is given by keeping primarily the most critical one of either
the xamplitude, the yamplitude or the dispersion nearby at a limited size. Of course, the different
parameters in these 3 files may be altered and adapted to your special needs. Care has then to be taken,
that the beam size values of the different fitconstraints (fit code digits i, j equal 1, 1 for x or 3, 3 for y)
are adapted manually and eventually moved to other zcoordinates.
References:
Other Proton Beam Therapy Application Examples: Degrader Design for a 250 MeV Medical Cyclotron Isotope Production Yield Optimization at PSI Double Scattering Computations for Proton Beam Spreading Last updated by Urs Rohrer on 13Feb2007 