Large energy acceptance gantry for proton therapy utilizing superconducting technology

Abstract

When using superconducting (SC) magnets in a gantry for proton therapy, the gantry will benefit from some reduction in size and a large reduction in weight. In this contribution we show an important additional advantage of SC magnets in proton therapy treatments. We present the design of a gantry with a SC bending section and achromatic beam optics with a very large beam momentum acceptance of ±15%. Due to the related very large energy acceptance, approximately 70% of the treatments can be performed without changing the magnetic field for synchronization with energy modulation. In our design this is combined with a 2D lateral scanning system and a fast degrader mounted in the gantry, so that this gantry will be able to perform pencil beam scanning with very rapid energy variations at the patient, allowing a significant reduction of the irradiation time.We describe the iterative process we have applied to design the magnets and the beam transport, for which we have used different codes. COSY Infinity and OPAL have been used to design the beam transport optics and to track the particles in the magnetic fields, which are produced by the magnets designed in Opera. With beam optics calculations we have derived an optimal achromatic beam transport with the large momentum acceptance of the proton pencil beam and we show the agreement with particle tracking calculations in the 3D magnetic field map.A new cyclotron based facility with this gantry will have a significantly smaller footprint, since one can refrain from the degrader and energy selection system behind the cyclotron. In the treatments, this gantry will enable a very fast proton beam delivery sequence, which may be of advantage for treatments in moving tissue.

Technical challenges for FLASH proton therapy

“…There are broadly two types of design: smaller-acceptance designs that have an energy acceptance sufficient for a single treatment depth range, in which the gantry optics are often composed of several conventional magnetic achromats [41][42][43]; and large-acceptance designs which aim to allow any energy from an accelerator source (say, from 70 MeV to 250 MeV) with no magnetic adjustment at all. For the latter case, FFAG (fixed-field, alternating gradient) optics, or achromatic multipole-based bending systems, are often proposed and since the magnets are fixed one may use more compact designs that utilise either permanent magnets or superconducting magnets to limit complexity and mass [44][45][46][47].…”

“…An alternative would be to use a beamline with a suitably wide energy acceptance such that all magnetic fields vary at the same speed during beam delivery, or such that the magnets could be set to accept the complete range of energies for a given treatment plan. Such designs have already been investigated for compact, large energy acceptance gantries [50,47] but prototype clinical systems have not yet been realised.…”

FLASH Irradiation with Proton Beams: Beam Characteristics and Their Implications for Beam Diagnostics

“…Energy change with a degrader typically takes at least 100 ms [26]; thus, sequential energy changes might be too slow for future FLASH treatments. There are concepts of diminishing this time to about 10 ms [27], which is still long for FLASH applications. As previously mentioned, energy degraders also limit beam intensity transported to the treatment area and, thus, the dose rate.…”

Uncertainty quantification analysis and optimization for proton therapy beam lines

“…Normal conducting gantries have a momentum acceptance limited to ±0.5-1% Δp/p, defined by the aperture of ESS slit. Achieving larger momentum acceptance (⩾10%) is one of the main potential offered by the SC gantries [22]. Since few years PSI researchers are Fig.…”

“…Within each energy range the magnetic fields of the SC bending section are optimized for a reference energy only (185 MeV for the first setup and 115 MeV for the second). The other energies in each sub-range are transported through the gantry thanks to the large momentum acceptance [22,24]. The development of the beam dynamics model of the SC bending section requires a combination of three codes: COSY-Infinity for a preliminary lattice optimization [25], OPERA-3D to design a realistic configuration of the SC magnetic field components [26] and OPAL for a precise particle tracking through the 3D-magnetic field maps exported from OPERA-3D.…”