Transition from passive scattering to pencil beam scanning: Physics commissioning of a proton radiosurgery system

Our institution has provided proton stereotactic radiosurgery (PSRS) since 1961 using passive scattering (PS). A recent transition to pencil beam scanning with a new treatment planning system required extensive validation, particularly for small field applications.

This study details the clinical adaptation of our PSRS program to pencil beam scanning (PBS), emphasizing small field dosimetry and treatment delivery accuracy.Methods: Patient alignment via implanted fiducials was validated using a head phantom. Test fields were created to span clinically relevant parameters including distal target depth (3.4-17.9 cm), target thickness (1.0-3.0 cm), aperture diameter (0.9-3.0 cm), and aperture aspect ratios (1.0-3.0). Dosimetric validation of the in-house, vendor co-developed planning system included: (1) central axis depth-dose measurements in water with a micro-diamond detector; (2) absolute point dose measurements in solid water; and (3) two-dimensional relative dose distributions with Gafchromic film. Validation was supplemented by a commercial high-resolution ion chamber array.

End-to-end alignment using a hidden target approach showed a mean deviation of 0.6 ± 0.3 mm. Agreement with the treatment planning system (TPS) for 90% distal and proximal central axis depths was within 0.1 ± 0.7 mm and 0.7 ± 0.5 mm, respectively. Absolute isocenter dose agreement in water was within 1.6%. Gamma analysis (2%/1 mm, 10% threshold) yielded a 97.6% average pass rate. A patient-specific QA protocol for small fields was established and validated.

We successfully transitioned our PSRS program to PBS, achieving high geometric and dosimetric accuracy suitable for small field proton radiosurgery.