Cyclinacs: Fast-Cycling Accelerators for Hadrontherapy

We propose an innovative fast-cycling accelerator complex conceived and designed to exploit at best the properties of accelerated ion beams for hadrontherapy. A cyclinac is composed by a cyclotron, which can be used also for other valuable medical and research purposes, followed by a high gradient linear accelerator capable to produce ion beams optimized for the irradiation of solid tumours with the most modern techniques. The properties of cyclinacs together with design studies for protons and carbon ions are presented and the advantages in facing the challenges of hadrontherapy are discussed.

💡 Research Summary

The paper introduces a novel accelerator concept called the “cyclinac,” which combines a conventional cyclotron with a high‑gradient linear accelerator (linac) to produce ion beams optimized for modern hadron‑therapy techniques. The authors begin by reviewing the rapid growth of hadron therapy, noting that over 55,000 patients have been treated with protons and about 7,000 with carbon ions, yet most facilities still rely on large, low‑flexibility cyclotrons or synchrotrons. These traditional machines suffer from slow energy‑selection (mechanical absorbers for cyclotrons, one‑second synchrotron cycle times) and complex extraction systems, limiting their ability to implement advanced dose‑delivery strategies such as respiratory gating, multi‑painting, and real‑time feedback for moving organs.

A cyclinac addresses these limitations by using the cyclotron as an ion source that delivers a low‑energy beam (30–62 MeV) to a 3 GHz side‑coupled linac (SCL). The linac provides high accelerating gradients (≈ 20–30 MV/m), allowing the beam to reach therapeutic energies of 200–250 MeV for protons or 3.5–4.5 GeV for carbon ions within a compact footprint of roughly 10 m. Crucially, the linac’s RF power can be modulated module‑by‑module, enabling electronic energy variation in about 1 ms—three to four orders of magnitude faster than synchrotrons. This rapid energy modulation, together with a pulsed beam structure (1.5–5 µs pulses separated by 2.5–5 ms at 200–400 Hz), matches the timing requirements of spot‑scanning and raster‑scanning delivery systems. The beam is present only ≈ 1 % of the time, which dramatically reduces background for in‑beam PET monitoring and simplifies quality assurance.

The cyclinac architecture includes: (i) a computer‑controlled ion source synchronized to the linac repetition rate, (ii) a cyclotron (or synchro‑cyclotron, with future FFAG possibilities), (iii) external beams for isotope production and research, (iv) a transport line to the linac, (v) the linac itself (often a drift‑tube section followed by an SCL), and (vi) a distribution system to treatment rooms equipped with gantries. Permanent‑magnet quadrupoles (PMQs) placed inside each linac module maintain beam focusing across the full energy range, ensuring loss‑free transport even when the downstream modules are switched off to lower the output energy.

The authors detail the historical development of the LIBO (Linac Booster) prototype, a 3 GHz SCL module built in the 1990s, and its successful RF testing at CERN. Two‑module LIBO prototypes were displayed at the CERN “Physics and Health” exhibition, demonstrating the feasibility of high‑gradient, low‑β acceleration for hadrons. Design studies for both proton and carbon‑ion cyclinacs are presented, showing that a 30 MeV cyclotron feeding a 10‑module SCL can deliver 250 MeV protons, while a 62 MeV cyclotron plus a drift‑tube section and SCL can reach 4500 MeV carbon ions. Beam currents suitable for therapy (≈ 2 nA for protons, 0.2 nA for carbon) are achieved by chopping the cyclotron beam in synchrony with the linac pulses, allowing precise control of the number of particles per voxel with 3 % accuracy.

The paper argues that the cyclinac’s fast electronic energy control, pulsed time structure, and high‑gradient acceleration make it uniquely suited to implement three advanced strategies for treating moving targets: (1) respiratory gating synchronized to the patient’s breathing phase, (2) multi‑painting of the tumor volume to average out residual motion‑induced dose errors, and (3) real‑time feedback loops that adjust transverse steering magnets and longitudinal energy (via RF module control) based on live imaging of tumor position. Because the beam can be turned off electronically in < 1 ms, in‑beam PET systems can acquire clean signals for range verification with ≈ 1 mm accuracy, a capability that is difficult to achieve with continuous‑extraction synchrotrons.

In summary, the cyclinac concept combines the high‑current, multi‑purpose capabilities of a cyclotron with the rapid, flexible energy modulation of a linac. It offers a compact, cost‑effective solution that can support both proton and carbon‑ion therapy, provide beams suitable for active scanning techniques, enable real‑time dose verification, and free up the cyclotron for ancillary applications such as isotope production. The authors conclude that cyclinacs represent a promising next‑generation platform for hadron therapy facilities, capable of meeting the clinical demands of precision, speed, and versatility required for modern oncological radiotherapy.