MR-Linac (MR-Guided Radiotherapy)
Hybrid radiotherapy device combining a linear accelerator with an integrated MRI scanner. Enables real-time soft-tissue imaging during treatment — so the therapist can see the tumor + critical organs moving in real time rather than inferring position from bony anatomy via CBCT. Supports adaptive radiotherapy — the treatment plan is modified daily based on that day's anatomy, not the simulation scan from weeks earlier.
The first clinical MR-Linac was Elekta Unity (FDA cleared 2018). ViewRay MRIdian followed a different architectural path (lower field strength) and preceded Unity to market with an earlier 0.35 T Cobalt-60 version.
Clinical significance
MR-Linac addresses specific limitations of conventional image-guided RT:
- Moving targets — pancreas, liver, kidney, bladder-adjacent prostate. Daily bowel / bladder filling changes geometry substantially.
- Soft-tissue targets against soft tissue — bony anatomy-based IGRT (CBCT) cannot distinguish prostate vs bladder vs rectum reliably; MRI can.
- Dose-escalation opportunities — adaptive planning lets centers push dose higher with confidence that OARs stay below constraints.
Clinical outcomes data still accumulating. Pancreatic SBRT and stereotactic prostate boost have the best-characterized benefit so far.
Two competing architectures
- Elekta Unity: 1.5 T MRI + 7 MV FFF linac. Linac rotates around the magnet. Higher field = better image quality; more engineering complexity in the beam-magnet interaction physics.
- ViewRay MRIdian: 0.35 T MRI + 6 MV linac. Lower field simplifies the beam-magnet interaction (electron return effect is smaller). Weaker image quality than 1.5 T; fewer RF safety considerations for implanted devices.
Key specs
- Field strength — 0.35 T (MRIdian) or 1.5 T (Unity)
- Linac energy — 6 MV (MRIdian) or 7 MV FFF (Unity)
- MLC — Agility (Unity, 160 leaves), MRIdian-specific (MRIdian)
- Real-time cine MRI during delivery — both
- Adaptive planning — both, with different workflows
- Gating — both systems support beam gating based on MRI cine tracking
Dosimetric physics
In a 1.5 T field, secondary electrons produced by the beam curve in the magnetic field (Lorentz force) — the "electron return effect" produces dose perturbations at tissue/air or tissue/bone interfaces. Treatment planning must account for this; beam data + TPS commissioning are not interchangeable with conventional linac commissioning.
Install reality
MR-Linac install is extreme — combines:
- RT vault shielding (linac beam) — concrete walls, lead cladding
- MRI RF shielding (Faraday cage inside the vault)
- Helium management (cold head, quench pipe to outside)
- 5-gauss-line containment + ferromagnetic detection
- Dual-power architecture (linac + MRI compressor on emergency power)
Multi-month site prep; install cost in the eight figures. Site count globally in the hundreds.
Systems
Regulatory
Full RT facility license + MRI safety program + state radiation license + QMP + RSO. Combines the full regulatory burden of both parent modalities.
Related
- Linear Accelerator (conventional RT)
- MRI (diagnostic MRI)
- PET/MR (other MRI hybrid)
- SBRT