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

Abstract

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 70MeV protons to form minibeams with a size of only 0.1mm for a preclinical small animal irradiation facility, while avoiding the cost of an RFQ injector. The tandem accelerator provides a 16MeV proton beam with a beam brightness of as averaged from 5µs long pulses with a flat top current of 17µA at 200Hz repetition rate. Subsequently, the protons are accelerated to 70MeV by a 3GHz 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 3GHz 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)mm2 . 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. 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 19nA is expected within an area of (0.1×0.1)mm2 at the beam focus. An extension of existing tandem accelerators by commercially available 3GHz 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 3GHz 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 200MeV.