Helium pencil beam commissioning and beam modeling

Helium ions combine reduced lateral scattering compared to protons and a lower fragmentation tail than carbon ions, enabling sharp dose gradients and improved normal tissue sparing. This study reports the commissioning of a scanned helium pencil beam line and validation of its corresponding beam model.

Synchrotron-based helium ion beams were commissioned covering energies from 54.6 to 402.8 MeV/u. Depth-dose curves were measured and absolute dose calibration was performed. Beam optics (spot size, position, and intraspill stability) were evaluated for various spill lengths. A beam model was implemented in the RayStation treatment planning system (TPS) and validated through 2D absolute dose measurements in homogeneous and heterogeneous phantoms and 3D measurements of cubic spread-out Bragg peak fields. Gamma-index analysis and Monte Carlo (MC) simulations with GATE/Geant4 were performed for benchmarking.

Measured ranges agreed with MC simulations within $\pm$ 0.3 mm. Spot sizes decreased with energy, independently of the spill length. Spot positions remained within $\pm$ 0.5 mm and intraspill variations were $\le$ 0.2 mm (position) and $\le$ 5.4 % (size). TPS-predicted doses agreed within 0.1 %. For 3D validations in homogeneous phantoms, the dose differences were generally within 2 %. Median gamma pass rates exceeded 90 % for 3 %/1.5 mm and 95 % for 5 %/1.5 mm. For the heterogeneous phantom, differences were within -3.2 %.

Stable scanned beam delivery with helium ions was established. Validation demonstrated strong agreement between measurements, TPS calculations, and MC simulations, supporting research and future clinical application.