KEY POINTS
- The MedAustron team commissioned a synchrotron-based scanned helium ion beamline across energies of 54.6–402.8 MeV/u. A dedicated RayStation model was created for the clinically relevant range of 65.4–220.8 MeV/u, corresponding to water-equivalent ranges of approximately 30–300 mm.
- Measured beam ranges showed excellent agreement with the accelerator energy tables. The maximum deviation from nominal range was −0.1 mm, and the mean absolute deviation was 0.06 ± 0.06 mm; agreement with independent GATE/Geant4 simulations remained within the predefined ±0.3 mm tolerance.
- Spot full width at half maximum decreased from 17.8 mm at 54.6 MeV/u to 4.8 mm at 402.8 MeV/u. Spot-size differences between 1-, 5-, and 10-second spills remained below 1.7%, while nearly all measured spot positions were within ±0.5 mm of their nominal coordinates.
- During 10-second spills, spot-position variations remained ≤0.2 mm, and relative spot-size variations were ≤5.4%. Two-dimensional field homogeneity worsened from approximately 2% to 12% as energy increased because the fixed spot spacing became large relative to the decreasing spot size.
- In monoenergetic two-dimensional dose validation, treatment-planning system calculations agreed with ion-chamber measurements within 0.1% across all investigated clinical energies.
- For three-dimensional spread-out Bragg peak fields in water, median gamma pass rates were 100% at isocentre and at least 97.6% at the second measurement distance using 5%/1.5 mm criteria. Most measured dose differences remained within 2%, although deviations reached 12% in shallow, high-gradient fields using a range shifter.
- Absolute dose agreement in homogeneous water was approximately −0.31 ± 0.35% without a range shifter and 0.25 ± 0.08% with one. In the heterogeneous head phantom, the treatment-planning system overestimated measured dose by approximately 2.2–3.2%, highlighting residual limitations of the analytical pencil-beam algorithm at bone–soft-tissue interfaces.
CLINICAL TAKEAWAY
This work demonstrates that scanned helium beams can be delivered reproducibly and modeled with accuracy approaching established proton and carbon-ion workflows. It supports further biological and translational development, but does not yet establish clinical effectiveness; helium-specific relative biological effectiveness, fragmentation modeling, and dose calculation in heterogeneous anatomy remain important unresolved issues.