Enhancement of Adaptive Collimation Precision on Pencil Beam Scanning Proton System πŸ“

Author: Akbar Anvari, Carnell J. Hampton, Melanie R. Piantino, Kai Yang πŸ‘¨β€πŸ”¬

Affiliation: Levine Cancer Institute, Atrium Health 🌍

Abstract:

Purpose: To evaluate aperture collimation on a C-shape gantry-mounted proton accelerator. Mevion S250i Hyper scanning machine implemented aperture collimation utilizing adaptive aperture (AA). The AA is essentially a mini-MLC consisting of seven pairs of leaves made of nickel, which has a small neutron cross section. The AA is designed inside the nozzle to shape spot delivery to minimize penumbra at different energies.

Methods: AA interleaf leakage, alignment and positional accuracy were characterized. AA interleaf leakage was quantified using a scintillator detector by delivering full energy collimated and uncollimated beams. AA alignment was performed using the scintillating detector to assess accuracy of AA in trimming spots. Initially, alignment was carried out with extended nozzle position. AA alignment was verified by star-shot delivery using films sandwiched in a vendor provided phantom, delivering collimated beams with extended nozzle position. To investigate clinical scenarios, we evaluated AA alignment at different nozzle extensions to characterize impact of extension on AA alignment for all gantry angles. Lastly, AA leaves positional accuracy was characterized by delivering Picket Fence patterns using low and high energies at different gaps.

Results: Measured dose leaked through leaves was less than 1%, within the 2% recommended limit. After initially aligning the AA at fully extended nozzle, the extension was changed, and the AA alignment was degraded up to 2mm. We aligned AA at mid extension (0.7mm, -0.3mm) to align AA reasonably at small (1mm, -1.3mm) and large (-0.4mm, 0.8mm) extensions.

Conclusion: AA is very effective in trimming spots to reduce lateral penumbra; however, it requires high precision in AA leaf calibration by the vendor. Moreover, it requires special considerations of users to verify AA alignment at different nozzle extensions. We propose to align AA at the nozzle’s mid position to achieve acceptable accuracy at all extensions.

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