Ring-gantry linacs (Halcyon, Ethos)
A radiotherapy machine built like a CT scanner: the accelerator spins inside an enclosed ring, must image the patient before every dose, and treats fast. Ethos adds software that redraws the plan to the anatomy of the day while the patient lies on the couch.
Overview
Varian's Halcyon, cleared in 2017, put a single-energy flattening-filter-free 6 MV linac and a fast dual-layer multileaf collimator inside an enclosed ring that rotates several times a minute. Image guidance is not optional: every fraction starts with megavoltage or kilovoltage cone-beam CT, and the newer HyperSight imager produces CT-quality images in seconds. Because there is no C-arm, the machine is quicker to install and commission, needs less bunker space in some layouts, and delivers VMAT plans in a few minutes, which suits high-volume departments and lower-resource settings. Ethos is the same platform with an adaptive workflow: artificial-intelligence contouring and a fast optimiser create a new plan on that day's images and the clinician chooses between the scheduled and adapted plans before treating.
The trade-offs are the absence of electron beams and higher photon energies, no non-coplanar beams for brain radiosurgery, a bore that constrains some immobilisation devices, and, for adaptive treatment, extra minutes per session and heavy reliance on automated contouring. Elekta's compact Harmony linac addresses the same market with a conventional C-arm.
- Linac head + MLC
- Target (PTV)
- Gantry arc
- Cone-beam CT guidance
How it works
A flattening-filter-free 6 MV linac and dual-layer multileaf collimator rotate on an enclosed ring with mandatory cone-beam CT before each fraction; the Ethos variant re-optimises the plan on the daily image before delivery.
- Fast treatment and set-up
- Mandatory image guidance every fraction
- Online adaptive replanning on Ethos
- No electrons or higher photon energies
- No non-coplanar beams
- Adaptive sessions take longer and rely on auto-contouring
Latest papers
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