Purpose: To quantify lateral penumbra reduction of field with adaptive aperture (AA) compared to the same field without AA for the Mevion Hyperscan proton system under different energies, air gaps, field sizes, and depths.
Methods: RayStation Monte Carlo proton TPS was used to optimize various single energy layer plans without AA to thin target of size of 3x3, 10x10, and 18x18 cm at ISO. The same plans were re-calculated with AA that conforms to the known field size. EBT3 film and a PTW Octavius 1500XDR detector were used to measure lateral penumbra at the depth of Bragg peak for energies of 28, 120, 180, and 227 MeV. For 10x10 field of 120 MeV, plans with varied air gaps of 1.4, 5, 10, 15, 20 cm were created and measured. For 10x10 fields of 120 and 227 MeV, we also measured penumbra at surface and other 3 intermediate depths.
Results: Penumbra reduction decreases greatly from 78% to -4% as energy increases from 28 to 227 MeV. A change of 55% to 47% is observed for air gap of 1.4 to 20 cm for 120 MeV. There is no clear field size dependence except for 227 MeV where the change of 3.5% to -11% for field of 3 to 18 cm indicates no benefit with AA for the field larger than 6 cm. The smallest penumbra of 1.2mm is observed at surface of 227 MeV field with AA where the spot size is the smallest too. Consistent with energy change, penumbra reduction deceases linearly as depth (or range) increases. There exists a turning point of 29.5cm (216MeV) after which the penumbra with AA becomes worse than without AA.
Conclusion: The significant penumbra reduction is observed for low energy or shallow depth. A mild advantage with AA is demonstrated by reducing air gap.