Clinical commissioning of a novel compact multi-room pencil beam scanning proton therapy system

To present the clinical commissioning of the world's first multi-room Varian ProBeam 360° proton pencil beam scanning system.

The state-of-the-art system includes two clinical treatment rooms with 360° rotating gantries, a superconducting cyclotron, an energy selection system, a beam transport system and scanning nozzles. These components deliver proton spots ranging from 3.97 to 30.03 g/cm² to arbitrarily shaped target volumes over a scanning area of 25 cm × 25 cm at isocenter. Proton beam ranges (R₈₀) were measured and verified independently in both gantry rooms to ensure agreement with beam specifications. Dosimetric parameters, including depth dose curves, in-air spot profiles and dose per monitor unit (MU) as a function of energy, were measured and used to create a dose calculation model in the RayStation treatment planning system (TPS). Treatment plans with various sizes of spread-out Bragg peaks (SOBPs) and simulated patient plans for a range of clinical sites were created and measured at various depths to validate the TPS's beam model accuracy. Additionally, beam matching between the two rooms was performed and validated.

Across the full energy range of 69-218 MeV, the measured R₈₀ in both rooms were identical within 0.4 ± 0.2 mm deviation from the expected nominal range. In-air spot sizes agreed within 5.7 ± 1.8% while outputs matched within 1.0 ± 0.5%. The average point dose difference was 0.2 ± 0.7% for over 80 measurements of SOBP validation plans from the TPS calculations and all planar dose measurements matched TPS calculations with over 90% of data passing a 2mm/2% gamma criterion. All patient plans were validated at a 3mm/3% criterion and demonstrated over 90% of data passing.

Both gantry systems were successfully commissioned for clinical use. Accurate measurements with robust validation ensured essential parameters for beam delivery and dosimetry were characterized for safe patient treatments.