Quantification of radiation and imaging isocenter coincidence of a multi-room PBS proton therapy system

Alignment between the radiation isocenter and imaging isocenter is critical to ensure accurate proton dose deposition in the patient. The purpose of the current study is to perform a comprehensive evaluation of the coincidence (planar kV X-rays vs. pencil proton beam) among three, beam-matched pencil beam scanning (PBS) gantries of a ProteusPLUS proton therapy system. For proton radiation isocenter measurement, a cone-shaped scintillator detector, XRV-124, was utilized. To test the impact of gantry angle on the beam coincidence, measurements at 12 different gantry angles were performed in the study. Additionally, the energy dependency on beam coincidence was investigated by acquiring measurements for a total of 63 different energies ranging from 70-225 MeV at increments of 2.5 MeV. The measurements were performed in all three beam-matched gantries (GTR1, GTR2, and GTR3). A correction of <= 0.3 mm was applied in the imaging system to account for the imaging isocenter shift as the gantry rotates from 0 degrees to 330 degrees. The beam coincidences in X, Y, and Z were within +/- 1.0 mm for 63 different energies at 12 different gantry angles in all three beam-matched gantries. The beam coincidence results among three gantries (GTR1, GTR2, and GTR3) ranged from - 0.9 to 0.7 mm in the lateral (X), - 0.8 to 0.5 mm in the longitudinal (Y), and - 0.7 to 0.7 mm in the vertical (Z) directions. Overall, the mean isofocus of radiation isocenter was 0.4 +/- 0.2 mm in GTR1, 0.5 +/- 0.2 mm in GTR2, and 0.4 +/- 0.2 mm in GTR3. Given the consistent agreement (< 1.0 mm) between radiation and imaging isocenters for all three rooms, it appears feasible to confidently generate proton treatment plans for one treatment room and deliver in the other beam-matched rooms.