345 - Commissioning of the First Synchrotron Based Upright Proton Therapy System

Purpose/Objective(s):

We report the first clinical implementation integrating a 6D upright treatment chair and a vertical CT system for image guided proton therapy on a synchrotron based fixed beamline. This configuration preserves small spot pencil beam scanning (PBS) performance, ensuring that treatment quality is not compromised while enabling an improved workflow, reduced facility footprint, and expanded clinical accessibility. This is especially impactful for patient populations who would benefit from upright positioning and for centers seeking more compact proton therapy solutions.

Materials/Methods:

A 6D upright chair was integrated with a vertical CT system to provide diagnostic quality imaging in the treatment position. Small spot PBS performance was preserved by tuning to match our gantry room with identical source to axis distances (SAD). A dedicated compact nozzle allows the snout to rotate toward the ceiling, providing sufficient clearance for patient CT simulation and alignment. An upright specific coordinate and scan direction convention was implemented, with scanning magnet directions reversed relative to the gantry room. Upright beam commissioning included MU/spot and output calibration, SAD verification, spot profile measurements, and integrated depth dose (IDD) curves for energies 70.5-228.9 MeV. Two range shifter solutions were implemented to support variable air gaps and field sizes while maintaining spot quality.

Results:

Beam commissioning was successfully completed across 98 discrete proton energies (70.5-228.8MeV). The system achieved ~1 mm range interval between adjacent energies at low energies, increased to a maximum of 6 mm spacing at higher energies. Bragg peak widths (R80-R80) ranged from 1.2 to 8.4 mm across the energy spectrum with the distal R80-R20 ranging from 0.7-4.6 mm. Spot profile measurements using the LOGOS XRV-2000 demonstrated isocenter plane spot sizes (s) of 2.0-5.5 mm, confirming the performance of the small-spot PBS was preserved as the gantry room. The PBS spot positioning accuracy was found to be within 0.5mm. Two complementary range shifter solutions were implemented to support clinical flexibility. A fixed 4 cm WET range shifter at a fixed 39 cm snout position supports a maximum field size of 30×40 cm². A snout mounted range shifter with 10 discrete positions enables smaller air gaps to maintain a sharper lateral penumbra, particularly for lower energy beams and for a field size of 26×18cm2. Output calibration using R25M10 conditions confirmed accurate dose delivery with 200 MU corresponding to 200 cGy at mid-plane under a SAD setup.

Conclusion:

Proton beam quality was maintained for high quality treatment delivery, including the largest 30 × 40 cm² field size, in the upright configuration. This integrated synchrotron and vertical CT based system in conjunction with the 6D upright chair provides a practical foundation for broader future clinical deployment and continued development of upright proton therapy.