Commissioning and beam characterization of the first gantry-mounted accelerator pencil beam scanning proton system

Purpose: To present and discuss beam characteristics and commissioning process of the first gantry-mounted accelerator single-room pencil beam scanning (PBS) proton system. Methods: The Mevion HYPERSCAN employs a design configuration with a synchrocyclotron mounted on the gantry to eliminate the traditional beamline and a nozzle that contains the dosimetry monitoring chambers, the energy modulator [energy selector (ES)], and an adaptive aperture (AA). To characterize the beam, we measured the integrated depth dose (IDD) for 12 energies. from the highest energy of 227 MeV down to 28 MeV with a range difference similar to 2 cm between the adjacent energies, using a large radius Bragg peak chamber; single spot profiles in air at five locations along the beam central axis using radiochromic EBT3 film and cross compared with a scintillation detector; and determined the output using a densely packed spot map. To access the performance of AA, we measured interleaf leakage and the penumbra reduction effect. Monte Carlo simulation using TOPAS was performed to study spot size variation along the beam path, beam divergence, and energy spectrum. Results: This proton system is calibrated to deliver 1 Gy dose at 5 cm depth in water using the highest beam energy by delivering 1 MU/spot to a 10 x 10 cm(2) map with a 2.5 mm spot spacing. The spot size in air at isocenter for a maximum range beam of 227 MeV is 4.1 mm. This system is able to reduce the beam range all the way to the patient surface; the lowest energy beam measured was 28 MeV which has a spot size of 15.7 mm. The beam divergence is 2.4 mrad at 227 MeV and 52.7 mrad for the superficial 28 MeV beam. The binary design of the ES has resulted in shifts of the effective SSD toward the isocenter as the energy is modulated lower. The pristine Bragg peaks have a constant 80%-80% width of 8.4 mm at all energies. The interleaf leakage of the AA is less than 1.5% at the highest energy. The AA reduces field penumbras. For a 10x10 optimized field, the 227 MeV beam penumbra measured at isocenter with a 5 cm air gap went from 6.8 to 4.7 mm and the 28 MeV beam penumbra went from 21.4 to 7.5 mm. Conclusions: The HYPERSCAN proton system has a unique design, which is reflected in the Bragg peak shapes, the variation of spot sizes with energy and the penumbra sharpening effect of the AA. The combination of the ES and AA makes PBS implementation possible without using a beam transport line and supplemental range shifter devices. In commissioning the TPS and designing plans these differences need to be considered. (C) 2020 American Association of Physicists in Medicine