A double multi-turn injection scheme for generating mixed helium and carbon ion beams at medical synchrotron facilities

TL;DR

This paper tackles the challenge of delivering a mixed helium and carbon beam for hadron therapy by proposing and demonstrating a double multi-turn injection scheme that sequentially injects He and C from separate ion sources into a medical synchrotron. The approach aligns the injected species in a single RF-harmonic lattice by adopting energy-adaptation at injection and maintaining a fixed magnet program, enabling simultaneous acceleration and extraction of both species to 262.3 MeV/u. Experimental results at MedAustron confirm the first mixed ^4He^2+/^12C^6+ beam delivery, with detection via radiochromic film and a silicon LGAD detector showing distinct Bragg peaks and LET signatures; the observed He/C separation matches expectations, and the mixing ratio can be tuned by the second-bump amplitude, though substantial optimization remains for stable, spill-wide control. The work demonstrates the feasibility of mixed-beam operation at state-of-the-art medical synchrotrons without major infrastructure changes, and identifies key challenges and pathways for future improvements, including injection-energy offset mitigation, capture efficiency enhancement, burn-in of the dual-species spill, and the development of clinically applicable imaging and planning strategies, all while acknowledging practical considerations like energy costs and contamination risks. The known range advantage of helium, $R_{He} \approx 3\,R_C$ at the same $E/m$, underpins the diagnostic potential of helium downstream of the patient for range verification in carbon therapy.

Abstract

The low relative charge-to-mass ratio offset of 0.065% between fully ionized helium-4 and carbon-12 ions enables simultaneous acceleration in hadron therapy synchrotrons. At the same energy per mass, helium ions exhibit a stopping range approximately three times greater than carbon ions. They can therefore be exploited for online range verification downstream of the patient during carbon ion beam irradiation. One possibility for creating this mixed beam is accelerating the two ion species sequentially through the LINAC and subsequently "mixing" them at injection energy in the synchrotron with a double multi-turn injection scheme. This work reports the first successful generation, acceleration, and extraction of a mixed helium and carbon ion beam using this double multi-turn injection scheme, which was achieved at the MedAustron therapy accelerator in Austria. A description of the double multi-turn injection scheme, particle tracking simulations, and details on the implementation at the MedAustron accelerator facility are presented and discussed. Finally, measurements of the mixed beam at delivery in the irradiation room using a radiochromic film and a low-gain avalanche diode (LGAD) detector are presented.

Figures (14)

• Figure 1: Schematic mixed beam irradiation. The carbon beam is used for tumor treatment while the residual helium energy is measured downstream of the patient for diagnostic purposes.
• Figure 2: Layout of the MedAustron accelerator adapted from pivi_commissioning_2024. The availability of proton (p), carbon (C), or helium ions (He) is indicated for each irradiation room.
• Figure 3: Dispersive beam orbits $x(s)=D_x(s)\frac{\Delta B\rho}{B\rho}$ and RMS envelopes $\sigma_x=\sqrt{\frac{\epsilon_{n,x}\beta_x(s)}{\beta\gamma}+\left(D_x(s)\sigma_{\Delta B\rho/B\rho}\right)^2}$ for different ion pairs after the multi-turn injection at $E/m\approx7$ MeV/u according to the parameters presented in Section \ref{['ch:injectionSystem']}. The offset in $B\rho$ was chosen such that the ions exhibit the same revolution frequency (single-harmonic RF operation).
• Figure 4: Schematic illustration of the double multi-turn injection scheme. Top: Time evolution of the orbit bump amplitude at the injection septum. Bottom: Horizontal phase space distributions during the double multi-turn injection.
• Figure 5: Analytic approximations and comparative simulations of the final helium (top) and carbon (bottom) distribution in normalized phase space $(\overline{X}, \overline{X}')$. For the helium ions, the distribution is observed at the turn at which the second (carbon) injection bump amplitude reaches its maximum. For the carbon ions, the observation turn is chosen directly after the second injection bump has fully decayed. Left: Evolution of the second injection bump, indicating the varying aperture constraints $\Delta \overline{\bm{X}}$ as distances of beam center to injection septum blade for an arbitrary turn $i$ and the observation turn $j$. Right: Normalized phase space plots displaying the analytically estimated acceptance and simulated beam distributions after the second injection bump. The total area occupied by the carbon ions is not primarily defined by the losses but is limited by the injection window (gray-shaded area). Note the different axis scaling in the top and bottom plots.