Magnetically focused 70 MeV proton minibeams for preclinical experiments combining a tandem accelerator and a 3 GHz linear post‐accelerator

Abstract

Purpose: Radiotherapy plays an important role for the treatment of tumor diseases in two-thirds of all cases, but it is limited by side effects in the surrounding healthy tissue. Proton minibeam radiotherapy (pMBRT) is a promising option to widen the therapeutic window for tumor control at reduced side effects. An accelerator concept based on an existing tandem Van de Graaff accelerator and a linac enables the focusing of 70 MeV protons to form minibeams with a size of only 0.1 mm for a preclinical small animal irradiation facility, while avoiding the cost of an RFQ injector. Methods: The tandem accelerator provides a 16 MeV proton beam with a beam brightness of B ¼ 4 nA mm 2 mrad 2 as averaged from 5 µs long pulses with a flat top current of 17 µA at 200 Hz repetition rate. Subsequently, the protons are accelerated to 70 MeV by a 3 GHz linear post-accelerator consisting of two Side Coupled Drift Tube Linac (SCDTL) structures and four Coupled Cavity Linac (CCL) structures [design: AVO-ADAM S.A (Geneva, Switzerland)]. A 3 GHz buncher and four magnetic quadrupole lenses are placed between the tandem and the post-accelerator to maximize the transmission through the linac. A quadrupole triplet situated downstream of the linac structure focuses the protons into an area of (0.1 × 0.1) mm 2 . The beam dynamics of the facility is optimized using the particle optics code TRACE three-dimensional (3D). Proton transmission through the facility is elaborated using the particle tracking code TRAVEL. Results: A study about buncher amplitude and phase shift between buncher and linac is showing that 49% of all protons available from the tandem can be transported through the post-accelerator. A mean beam current up to 19 nA is expected within an area of (0.1 × 0.1) mm 2 at the beam focus. Conclusion: An extension of existing tandem accelerators by commercially available 3 GHz structures is able to deliver a proton minibeam that serves all requirements to obtain proton minibeams to perform preclinical minibeam irradiations as it would be the case for a complete commercial 3 GHz injector-RFQ-linac combination. Due to the modularity of the linac structure, the irradiation facility can be extended to clinically relevant proton energies up to or above 200 MeV.

Concept and performance evaluation of two 3 GHz buncher units optimizing the dose rate of a novel preclinical proton minibeam irradiation facility

“…In case of the 4-gap concept the maximum input power variation of about ±0.4% (at P in = 795 W) results in a variation of the buncher amplitude of Δ U b = ±0.1kV. The TRAVEL simulation of the presented preclinical proton minibeam irradiation facility shown in [ 18 ] were performed for the corresponding buncher amplitude U b of 41.9 kV and 42.1 kV. This results in a variation of the maximum beam current of the irradiation facility (19 nA) of ±0.08%, which does not limit the planned preclinical experiments.…”

“…However, the maximum proton energy and thus the range of the protons is limited there by the terminal voltage of the 14 MV Van-de-Graaff accelerator to 28 MeV (about 6 mm proton range in water) [ 7 ]. In order to demonstrate the advantages of pMBRT with transverse beam dimensions σ < 90 μm at higher proton energies and thus for deeper lying tissue, a new concept for a preclinical irradiation facility (shown in Fig 1 ) was designed and characterised by simulations [ 18 ]. A tandem accelerator generates a 16 MeV proton beam of high brilliance that is further accelerated by a 2997.92 MHz linear post accelerator (linac) system to 70 MeV (about 40 mm proton range in water) and focused by a quadrupole triplet on the target.…”

“… a) longitudinal phase space at z1 (see Fig 1 ) coming from the tandem ( E kin = 16 MeV, Δ E kin = 0.01%) [ 18 ] b) longitudinal phase space in front of the linac at z2 (see Fig 1 ) (colored dots) and accepted phase space of the linac (grey dots). The red curves along the x- and y-axes are projections of the particle distributions.…”

“…Fig 3 shows the percentage of protons transmitted from the tandem through the linac into an area of 0.1 x 0.1 mm 2 as a function of U b . An optimal buncher amplitude of U b = 42 kV results in a maximum proton transmission of 47%, which is a factor 3 higher than the transmission without a buncher unit [ 18 ]. For an average tandem beam current of 40 nA (5 μs at 200 Hz), this results in a beam current of I B = 19 nA at the focus which is more than sufficient for preclinical experiments.…”

Conceptual Design of a Novel Nozzle Combined with a Clinical Proton Linac for Magnetically Focussed Minibeams

“…However, beam energies there are currently limited to 20 MeV which is only suited for the irradiation of superficial lesions. An update of the facility is planned which would allow it to reach energies of up to 70 MeV, however the focus will remain on preclinical experiments and the irradiation of small animals [ 39 ]. Thus, the generation of magnetically focussed proton minibeams in a clinical context remains a challenge.…”