Shielding and Multi-Modality Compatibility of a Compact Proton System In a Standard Linac Footprint
Abstract
Purpose
The Mevion S250-FIT is the world’s first ultra-compact proton therapy system designed with a form factor compatible with existing LINAC vaults. This raises questions regarding radiation shielding adequacy and electromagnetic compatibility with adjacent radiotherapy modalities. This work evaluates: (1) shielding effectiveness of a standard LINAC vault housing a 230 MeV compact proton system, and (2) magnetic field interference from the 10T superconducting synchrocyclotron on adjacent therapy systems.
Methods
Measurements were performed throughout and around the S250-FIT vault. Radiation shielding was assessed using calibrated survey meters and dosimeters to measure gamma and neutron dose rates at 10 accessible locations during clinical operations (workload: 4 Gy/fraction, 3 patients/hour, 40 hours/week). Magnetic field mapping was conducted at 23 locations using a calibrated gaussmeter. Pre- and post-magnet ramp-up assessments evaluated impacts on adjacent ViewRay MR-Linac and CyberKnife systems. ViewRay evaluations included MR-conditional 2D ion chamber arrays for dosimetry and MagPhan RT phantoms for image quality. CyberKnife performance was verified using high-resolution CMOS detectors and orthogonal kV imaging for targeting accuracy.
Results
Radiation measurements confirmed dose rates below 2 mrem/week at all surveyed occupancy locations and satisfied NCRP 151 requirements. Magnetic field strengths ranged from 1.3–12.5 gauss at clinically relevant locations; the 5-gauss line extended approximately 8m laterally and 4m anterior-posterior. For adjacent systems, all parameters remained within institutional tolerances: ViewRay CAX dose differences ≤0.2%, field variations <0.1 mm, and no image degradation. CyberKnife dose variations were ≤0.01 cGy/MU with field variations and targeting accuracy within 0.2 mm.
Conclusion
We demonstrated that a compact superconducting synchrocyclotron could be safely installed in multi-vault facilities without compromising radiation safety or adversely affecting adjacent systems. The 5-gauss line is confined to the vault maze, requiring precautions for cardiac devices but not affecting other treatment areas. The self-shielded design provides adequate containment for typical clinical scenarios.