Room: Exhibit Hall | Forum 3
Purpose: To evaluate the dosimetric and mechanical performance of the Adaptive Aperture(AA) of the Hyperscan proton PBS system and to demonstrate its beam penumbra sharpening effects.
Methods: The AA is a mini-MLC consisting of 7 pairs of leaves made of nickel. The leaves follow and trim the spots at the field edge to sharpen the penumbra. Single-spot, spot maps, and single-field plans of cubic volumes were created with and without AA to investigate the penumbra sharpening effects. Specially designed leaf patterns were used to check its mechanical alignment accuracy. A PTW Octavius ion chamber array, calibrated against a PPC05, was used to measure absolute dose and dose profiles at the Isocenter plane. A test pattern with 10-spot was created to measure the inter-leaf leakage.
Results: The difference of absolute dose output with and without AA can reach 5%, which is energy, field-size and nozzle extension dependent, and is attributed to an overlap between the leaf and the low dose tail. AA can reduce the penumbra by 2mm to 11.3mm from the highest energy of 227 to 64MeV. The significant penumbra reduction makes it feasible to achieve conformal dose distribution without using an external range shifter at low energies. The inter-leaf leakage for the wider leaves is ~6.5% at an air gap of 2cm at phantom surface and is reduced to <1% at 2cm depth. The inter-leaf leakage for the thinner leaves is <1%. Design revision is under way to further reduce the wider leaf leakage to <2%.
Conclusion: As the first and the only commercially implemented penumbra trimming mini-MLC, the AA of the Hyperscan PBS proton system can effectively trim the penumbra of a proton beam to achieve sharper lateral dose fall-off with clinical potential for greater lateral OAR sparing.
Not Applicable / None Entered.