Implementation of a proton FLASH platform for pre-clinical studies using a gantry-mounted synchrocyclotron

<i>Objective</i>. External beam radiation therapy (RT) at ultra-high dose rate (FLASH RT) has shown promise for improving the therapeutic ratio; exploiting its full potential, however, requires systematic preclinical studies to unravel the underlying radiobiological mechanisms. We demonstrate a proton irradiation platform for pre-clinical FLASH studies using a gantry-mounted proton therapy system in clinical operation.<i>Approach</i>. An accessory comprising a transmission ionization chamber, a tray accommodating beam modifying elements, including range shifting blocks made of boron carbide (B₄C) and poly(methyl methacrylate) (PMMA), and brass apertures to shape the beam's lateral extent was attached to the nozzle. A range modulator composed of arrays of holes drilled in a PMMA slab was used to form a spread-out Bragg peak (SOBP). The integral depth dose (IDD) curves, lateral dose profiles, and dose rate were measured using existing dosimeters for different beam modifying material combinations.<i>Results</i>. The range modulator allowed achieving an SOBP with 14 mm modulation. The proton range was gradually reduced through adding B₄C and PMMA blocks in the beamline, while the beam spot's size gradually increased and became more symmetric as protons traveled through more material. The commercial scintillator screen showed a dose-rate-independent response for measuring lateral dose profiles. The representative IDDs of the FLASH beam can be measured with a commercial multilayer ionization chamber device at a low dose rate since the IDD did not depend on the dose rate.<i>Significance</i>. This work demonstrated a platform for delivering ∼70 Gy s^-1SOBP proton FLASH beams using a gantry-mounted synchrocyclotron clinical system. We showed the evolution of an asymmetric and small single proton spot to a more symmetric and larger spot after ranging and shaping through different components. Using dosimeters commonly employed for quality assurance purposes, we report an efficient method for the characterization of proton FLASH beams.