systemVarianLinear Acceleratorfamily: clinac-ex-legacy

Varian Clinac 21EX

Family: Varian Clinac EX (legacy) · Modality: Linear Accelerator

Dual-energy C-arm linear accelerator in Varian's Clinac EX-series. Direct predecessor to the Clinac iX family — Clinac iX is essentially the EX chassis with upgraded electronics, On-Board Imager, and RapidArc-capable control system; the mechanical head, gantry, and accelerator structure are largely shared. Very large global installed base through the late 1990s and 2000s; still in active clinical service at many radiation-oncology departments.

Platform highlights

  • Dual-energy photon configuration — 6 MV low-energy beam plus a high-energy photon (typically 10 MV or 15 MV depending on order). The 6 MV beam is the daily workhorse for most treatments; the high-energy beam addresses deeper-seated disease and dose falloff considerations.
  • Electron energy suite — 6 / 9 / 12 / 15 MeV typical. Some 21EX configurations shipped with up to 18 MeV.
  • Millennium MLC — 120-leaf (40 central 5 mm leaf pairs covering a 10 × 10 cm central field + 20 outer 10 mm pairs) or 80-leaf variants. The 120-leaf Millennium is the IMRT / VMAT-capable MLC; 80-leaf is the earlier 3D-CRT-era MLC.
  • Klystron-based RF — Varian's signature Clinac-era RF source. Klystron end-of-life is the dominant service-capex event on 21EX.
  • 600 MU/min maximum dose rate.
  • On-Board Imager (OBI) — kV imaging arm available on late-production 21EX units; standard on the iX upgrade path.
  • Head geometry — shared with Clinac iX, Trilogy, and Novalis TX. A meaningful service-parts-compatibility advantage.

Specs

  • Dual photon energies: 6 MV + 10 MV or 15 MV
  • Electron energies: 6 / 9 / 12 / 15 MeV (configuration-dependent)
  • Millennium 120-leaf or 80-leaf MLC
  • 600 MU/min dose rate
  • Klystron RF source · SF6-insulated waveguide
  • OBI (late-production configurations)
  • ±185° gantry rotation · 100 cm SID

Clinical positioning

  • 3D-CRT and IMRT — the clinical workhorse use cases of the Clinac EX era.
  • IGRT (late-production OBI-equipped 21EX) — kV-kV orthogonal imaging, kV fluoro, CBCT on upgraded configurations.
  • Conventional radiation therapy — breast, prostate, head-and-neck, thoracic.
  • SRS / SBRT capability on late-production 21EX with 120-leaf MLC and SRS-capable control software; Trilogy / iX became the preferred SRS platforms as 21EX production wound down.

Service reality

  • Klystron is the dominant wear item — klystron replacement is a standing budget event.
  • Waveguide vacuum discipline — pressure and arcing history matter.
  • Thyratron switching-tube replacement cadence.
  • MLC leaves — Millennium 120 leaf service on IMRT-era units.
  • Head-geometry parts compatibility with Clinac iX means a broader parts aftermarket than less-shared platforms.

Market position (secondary / refurb)

  • Large global install base. 21EX is a common legacy linac in the secondary market, especially internationally where IMRT / VMAT workflows are established but TrueBeam / Edge capital is out of scope.
  • Service ecosystem — Varian OEM service; mature multi-vendor service on the EX-era chassis; klystron / waveguide / thyratron aftermarket is well-developed.
  • Upgrade pathway — Clinac iX in the Clinac family; TrueBeam for a chassis replacement.

Relationship to siblings and successors

  • Clinac 23EX — higher-energy sibling (6 + 18 MV).
  • Clinac 2100C / 2300C — predecessor-class (shared Clinac architecture).
  • Clinac iX — direct successor (EX → iX digital / imaging upgrade).
  • Trilogy — IGRT-era sibling.
  • TrueBeam — current-generation chassis replacement.

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