Linear Accelerator (Linac)

Radiation therapy device. Accelerates electrons to MeV energies through a waveguide; electrons either strike a target to produce MV photon (X-ray) beams for deep treatment, or exit as electron beams for superficial treatment. Delivers prescribed dose to tumor volumes under image guidance.

Therapy device, not diagnostic. Different regulatory, safety, and operational posture than any imaging modality. Downtime cascades into cancer-treatment schedules; daily output QA is mandatory; interlocks are not bypassed.

Physics

1. Electron gun emits electrons.
2. Klystron or magnetron generates RF microwave power.
3. RF accelerates electrons through the waveguide to MeV energies.
4. Bending magnet directs beam to the treatment head.
5. Target produces bremsstrahlung X-rays; flattening filter evens the dose profile (or is removed for FFF high-dose-rate modes).
6. Multi-leaf collimator shapes the beam to the planned aperture.
7. Patient treated per plan; MU delivery measured by an ionization chamber in the treatment head.

History

• 1946 — Donald Kerst (Illinois) builds the first medical betatron.
• 1953 — first clinical linac treatment (Stanford).
• 1970s — commercial clinical linacs proliferate.
• 2000s — IMRT becomes clinical standard.
• 2004 — Elekta Synergy ships with integrated kV CBCT — IGRT inflection point.
• 2010 — Varian TrueBeam released; FFF beams reach clinical scale.
• 2018 — Elekta Unity ships — first commercial 1.5T MR-Linac.

Key specs

• Photon energies — typically 6 MV and 10 MV; some platforms include 15 / 18 MV. Electron energies — 6 / 9 / 12 / 15 / 20 MeV.
• Dose rate — 600 MU/min flattened; FFF plans run 1200–2400 MU/min.
• MLC class — Millennium 120 (5 mm), HD120 (2.5 mm central leaves), Agility (5 mm Elekta).
• OBI + CBCT — IGRT capability, sub-millimeter setup accuracy at SRS programs.
• 6 DoF couch — PerfectPitch (Varian) / HexaPOD evo RT (Elekta) — mandatory for modern SRS.
• Licensed software — RapidArc, HyperArc, SRS / SBRT packages, FFF entitlement.

Systems

• Varian — Clinac iX, Trilogy, TrueBeam, TrueBeam STx, Edge, Halcyon
• Elekta — Synergy, Versa HD, Infinity, Unity (MR-Linac)
• Accuray — CyberKnife, TomoTherapy / Radixact

Clinical applications

• IMRT
• VMAT
• SBRT
• Brain SRS
• Brachytherapy (sibling rad-onc modality, distinct architecture)

Service and refurb reality

• Dominant cost items — klystron / magnetron replacement, MLC leaf service, thyratron / modulator consumables, waveguide vacuum integrity, OBI / kV imager flat-field calibration.
• Annual TG-142 QA — daily / monthly / annual cadence; signed by a Qualified Medical Physicist.
• Commissioning for new chassis is a months-long event regardless of platform — beam-data collection, beam-model tuning, end-to-end SRS test.
• Refurb economics dominated by license tier (RapidArc / HyperArc / FFF / SRS), MLC class (Millennium 120 vs HD120 vs Agility), 6 DoF couch presence, and imaging chain. See Clinac iX Field Guide and TrueBeam Field Guide.
• Tube vendor lead time for klystrons / magnetrons is non-trivial; sites without parts agreements wait days to weeks.

Regulatory

• State radiation-producing-machine license for the linac itself.
• NRC or Agreement-State byproduct license if brachytherapy sources or calibration sources are co-located.
• Qualified Medical Physicist (QMP) and Radiation Safety Officer (RSO) required at the program level.
• IEC 60601 + AAPM TG-142 commissioning and ongoing QA framework.
• ACR accreditation for clinical accreditation in many U.S. payer contexts.

Related

• MR-Linac (sibling — MR-guided RT)
• CyberKnife (robotic-arm sibling)
• TomoTherapy (helical sibling)
• Gamma Knife (cranial sealed-source sibling)
• Proton Therapy (particle-therapy sibling)
• Varian
• Elekta
• Accuray
• Rad Onc Physicist
• Linac Decommissioning