First Clinical MR-Linac (2018)
2018 — Elekta Unity ships, the first commercial 1.5T MRI-guided linear accelerator. Combines a diagnostic-grade 1.5T MR magnet with a 7 MV FFF photon-therapy delivery system in one gantry. ViewRay MRIdian had reached clinical use earlier with a 0.35T MRI paired with cobalt-60 sources (2014) and later a 6 MV linac (2017), establishing the MR-IGRT category at lower field strength; Unity defined the high-field clinical MR-linac.
What MR-linac changes
Conventional radiotherapy plans the patient's tumor and surrounding anatomy from a CT simulation acquired days before the first treatment. Anatomy changes between simulation and delivery — bladder fills, rectum moves, tumor shrinks, weight changes — but the plan does not. MR-linac brings simultaneous high-quality MRI into the treatment session:
- Adaptive radiotherapy on the table — re-contour and re-plan against the MR-of-the-day, treating the actual anatomy rather than the simulation snapshot.
- Real-time motion management — beam-on visualization of moving / deformable targets.
- Dose escalation in moving organs — pancreas, liver, prostate, gyne where motion margin previously forced under-treatment.
Clinical adoption arc
- 2014 — ViewRay MRIdian Cobalt-60 system (0.35T) reaches clinical use; established the MR-IGRT clinical category.
- 2017 — MRIdian Linac (0.35T + 6 MV) replaces the cobalt configuration.
- 2018 — Elekta Unity (1.5T + 7 MV) ships; high-field MR-linac inflection.
- 2020s — clinical literature for pancreatic SBRT establishes MR-linac value most clearly; growing roles in adrenal SBRT, oligometastatic ablative RT, prostate / gyne adaptive workflows.
- Adoption volume remains small — dozens of installs globally as of mid-2020s; concentrated at major academic and tertiary cancer centers.
Engineering challenges
- Beam–magnet interaction — secondary electron paths bend in the magnetic field, creating the "electron return effect" at tissue boundaries; planning systems compensate but the physics is non-trivial.
- MRI safety in the therapy environment — ferromagnetic-screening discipline, cryogen safety, beam-on imaging RF interference all combine in one suite.
- Commissioning — combines MRI commissioning, linac commissioning, and end-to-end MR-IGRT validation. Months-long event.
- Service model — hybrid magnet + accelerator service is its own discipline; not interchangeable with conventional MRI or linac service practice.
Descends to
Related
- First Human MRI (1977)
- Varian TrueBeam Launch (2010) — conventional photon-linac inflection
- Linear Accelerator
- MRI
- SBRT
- Elekta
- ViewRay
- Rad Onc Physicist
- AAPM TG-142