Purpose: To investigate penumbra sharpening by adaptive aperture (AA) for a Mevion S250i Hyperscan proton pencil-beam scanning (PBS) system.
Methods: Penumbrae (width of 20%â€“80%) were measured in air with and without AA using an IBA Lynx system for three energies (33.04, 147.36, and 227.15 MeV), three snout positions (5, 15, and 33.6 cm), and five field sizes (3Ã—3, 5Ã—5, 10Ã—10, 15Ã—15, and 18Ã—18 cmÂ²). Penumbrae were also simulated in water with and without AA using a RayStation treatment planning system for two targets (3Ã—3Ã—3 and 10Ã—10Ã—10 cmÂ³), three depths (5, 10, and 15 cm), and four air gaps (5, 10, 15, and 20 cm). Finally, three clinical cases (brain, lung, and prostate) were evaluated by comparing dose sparing with and without AA in critical structures using 2-beam plans with the same planning parameters.
Results: AA sharpened penumbrae in air by an average of 12.8, 6.7, and 3.6 mm for 33.04, 147.36, and 227.15 MeV, respectively. AA also substantially sharpened penumbrae in water (average penumbra: 9.8 mm with AA vs. 18.4 mm without AA). The penumbra sharpening was relatively higher for larger air gap, shallower target, and smaller target. Dose reductions of mean dose by AA were 11.8% & 22.1% of prescribed dose in chiasm & right optic nerve for brain (~4 cm target, 4 cm air gap, and ~7 cm depth), 15.9% & 2.8% in ipsilateral lung & heart for lung (~3 cm target, 12 cm air gap, and ~8 cm depth), and 4.6% & 6.1% in bladder & rectum for prostate (~7 cm target, 12 cm air gap, and ~20 cm depth), respectively.
Conclusion: AA can effectively spare critical structures for the Mevion Hyperscan proton PBS system. AA has potential to improve plans better when the target is shallower and smaller and the air gap is larger.