Design and <i>in-silico</i> evaluation of a novel beamline for precision small animal pencil beam scanning delivery at clinical proton therapy facilities

<i>Objectives.</i>Preclinical small animal proton beam irradiation systems are increasingly in demand. However, the absence of dedicated systems comparable in precision to those in clinical settings presents a considerable hurdle to investigations in this field. To address this need, the SIRMIO project has developed a novel compact beam transport system configured to degrade and focus clinical proton beams. The beamline, about 1 m long, housed in an environment to minimize scatter, includes degraders, collimators, and a permanent magnet quadrupole triplet to focus protons degraded from clinical energies. It is tailored to transport focused proton beams within the energy range of 20 to 50 MeV, ideal for small animal preclinical studies. The flexibility of this beamline design allows achieving beam-spot sizes of 1 mm sigma at the isocenter for all focused energies, with the particle fluence and spot sizes being variable through dynamic adjustment of the collimator and magnetic lattice. 3D scanning of the target volume is possible due to lateral beam scanning integrated into this design, without the use of additional scanning dipole magnets.<i>Approach.</i>The beamline was optimized using an accelerator beam optics code, followed by a Monte Carlo (MC) model to account for beam-matter interactions. Using an experimentally validated clinical proton beam phase space as input, degraded beams are transported through the MC model. Outcomes are assessed for beam characteristics and dosimetric properties.<i>Main results.</i>Beams transported by our proposed beamline design are shown to result in dosimetric properties suitable for preclinical studies, while also emulating realistic clinically relevant beam delivery scenarios like pencil beam scanning. Compared to a similar-sized collimator-only beamline, this design enhances transmission and reduces secondary dose at the target due to absence of scattering elements nearby.<i>Significance.</i>The portable SIRMIO beamline offers a flexible, precise alternative to passive methods for adapting clinical proton beams for small animal irradiation, enhancing preclinical research with clinically relevant beam delivery and enabling experiments in conditions more closely matching clinical practice.