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.

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 treatment head.
5. Target produces bremsstrahlung X-rays; flattening filter evens dose profile.
6. Multi-leaf collimator shapes the beam.
7. Patient treated per plan; MU delivery measured by ionization chamber.

History

• 1946 — Donald Kerst (Illinois) builds the first medical betatron.
• 1953 — first clinical linac treatment (Stanford).
• 1970s — commercial clinical linacs proliferate.
• 2000s — IMRT clinical standard.
• 2010 — Varian TrueBeam released.
• 2018 — Elekta Unity ships — first clinical MR-Linac.

Key specs

• Photon energies (MV) + electron energies (MeV)
• Dose rate — MU/min
• MLC — leaf count + resolution
• OBI + CBCT — IGRT capability
• RapidArc / VMAT — licensed

Systems

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

Service reality

Dominant cost items: klystron replacement, MLC leaf service, annual TG-142 QA. Downtime affects cancer patients whose schedules are time-sensitive. A medical physicist, not an imaging engineer, signs off on the machine.

Regulatory

State radiation facility license. NRC if brachytherapy sources on-site. Qualified Medical Physicist and Radiation Safety Officer required. IEC 60601 + TG-142 commissioning.