Author: Akbar Anvari, Carnell J. Hampton, Melanie R. Piantino, Kai Yang 👨🔬
Affiliation: Levine Cancer Institute, Atrium Health 🌍
Purpose: To evaluate the effectiveness of angular correction factor for output stability on Mevion S250i Hyper scanning machine, which allows user to apply correction factors for output at selected gantry angles relative to the absolute output set during machine installation.
Methods: A full TRS-398 was performed with two PPC05 chambers to calibrate machine to deliver 1Gy at depth of 5cm using standard 10cm x10cm 1681MU scanning map with 0.25cm spot spacing. Absolute output is subsequently confirmed by IROC TLD measurement. Gantry dependent output was measured with a gantry mounted device positioning PPC05 at depth of 5cm at isocenter. Output at 9 different angles is measured during acceptance of machine and iteratively adjusted by applying gantry dependent output factors at selective angles. Output stability was monitored by measuring output at all 9 angles at least once a month initially and reduced to cover all 9 angles in 3 months afterwards. Comprehensive daily QA including output measurement was performed for both gantry 0 degree and gantry 90 degree initially and reduced to cover both angles through the week.
Results: Absolute dose agrees between two PPC05 chambers and two setup positions within 0.5%, and within 1% of IROC measurement. 0.8% output spread across all 9 gantry angles is achieved by iteratively applying correction factor at selective angles during acceptance compared to +/-2% vendor tolerance. Subsequent monitoring the output stability revealed the interpolation of the output between gantry angles are not linear but based on threshold, which is sub-optimal. As a result, the interpolation is set to be between cardinal gantry angle rather than the nearest neighbor factor in next treatment console release (version 2.8.2.3).
Conclusion: Stable output across gantry angles is achievable with correction factors, and comprehensive routine QA process is needed to monitor the system to take advantage of this development.