What are the components and operating principles of a linear accelerator (LINAC) used for external‑beam radiotherapy as described in the standard Khan textbook?

Linear Accelerator (LINAC) Components and Operating Principles

Core Technology and Definition

A linear accelerator (LINAC) is a microwave-powered electron accelerator that has become the dominant radiotherapy treatment unit, using radiofrequency energy to accelerate electrons through an accelerating structure (waveguide) to produce high-energy photon or electron beams for external beam radiotherapy. 1

Major Components

1. Radiofrequency (RF) Power Source

• Magnetron or klystron generates microwave power, typically operating at S-band frequency (approximately 3 GHz) in conventional systems 1, 2
• Newer compact systems utilize X-band technology (9.3 GHz), allowing for miniaturization while maintaining therapeutic beam quality 2
• The RF source provides the electromagnetic energy needed to accelerate electrons through the waveguide 1

2. Electron Gun

• Produces the initial electron beam through thermionic emission 2
• Operates in a constant emission region to maintain stable beam current 2
• Typical beam currents exceed 90 mA in modern systems 2

3. Accelerating Structure (Waveguide)

• Side-coupled or standing-wave structures are used to efficiently accelerate electrons while reducing RF power loss 2
• Modern X-band systems achieve therapeutic energies with accelerating tubes as short as 37 cm, compared to longer S-band structures 2
• High shunt impedance and electric field strength enable acceleration to >6 MeV even in compact designs 2

4. Beam Transport System

• Bending magnets guide the accelerated electron beam from the accelerating structure to the treatment head 1
• Focusing magnets maintain beam collimation and prevent divergence 1

5. Treatment Head Components

Primary Collimator

• First-stage beam shaping device 3
• Removing this component increases both photon and neutron fluences at isocenter 3

Target (for Photon Mode)

• High-Z material (typically tungsten) where electrons strike to produce bremsstrahlung X-rays 3
• Generates the therapeutic photon beam through electron-photon conversion 4

Flattening Filter

• Creates a uniform dose distribution across the treatment field 3
• Modern flattening filter-free (FFF) beams are increasingly used for stereotactic treatments, resulting in higher dose rates but different beam characteristics 2
• Removal increases both neutron contamination and photon fluence 3

Secondary Collimator (Jaws)

• Provides rectangular field shaping 3
• Works in conjunction with the MLC for field definition 2

Multi-Leaf Collimator (MLC)

• Enables complex field shapes for conformal and intensity-modulated radiotherapy 3
• Contributes to neutron production at energies >10 MV 3

Operating Principles

Energy Production and Beam Characteristics

• Clinical LINACs typically operate at 6,10,15,18, and 24 MV photon energies 3
• Widely variable energy linacs allow selection of different energies for optimal treatment 1
• Electron beams (when available) range from 4-20 MeV for superficial treatments 5
• Modern systems achieve dose rates of 820 cGy/min or higher at standard distances 2

Beam Quality and Spectrum

• The photon energy spectrum is complex and influenced by all treatment head components 3, 4
• Percent depth dose (PDD) ratios at 10 cm and 20 cm depth characterize beam energy (e.g., PDD₁₀/PDD₂₀ ≈ 0.572 corresponds to 6 MV) 2
• Beam quality can be verified through transmission measurements using graphite and lead attenuators 4

Neutron Contamination

• Photonuclear reactions produce neutron contamination at energies >10 MeV 3
• All treatment head components contribute to neutron production, with removal of any component increasing neutron fluence 3
• Requires additional shielding considerations compared to lower-energy systems 5

Clinical Implementation Standards

Treatment Planning and Delivery

• 3D CT-based treatment planning with dose-volume histograms enables precise targeting 6
• Intensity-modulated radiation therapy (IMRT) uses multileaf collimators for complex dose distributions 6
• Image-guided radiotherapy (IGRT) with fiducial markers or ultrasound improves accuracy 6

Dose Delivery Requirements

• For lung cancer, minimum 60 Gy with classical fractionation is standard for curative intent 6
• Dose escalation to 75-79 Gy has been safely achieved with 3D conformal techniques for prostate cancer 6
• Stereotactic body radiation therapy (SBRT) delivers high doses in 5 or fewer fractions using LINAC-based systems 6

Quality Assurance

• Beam parameters (energy, dose rate, flatness, symmetry, penumbra) must meet AAPM TG-51, TG-106, TG-142, and IAEA TRS-398 protocols 2
• Flatness and symmetry specifications ensure uniform dose delivery 2
• Periodic verification of beam quality through transmission measurements maintains dosimetric accuracy 4

Comparison with Alternative Technologies

LINAC vs. Cobalt-60

• LINACs offer higher beam energy, modulated dose rate, and smaller focal spot size for optimized conformal treatments 5
• Infrastructure and maintenance are more demanding for LINACs due to complex electrical components 5
• Life cycle costs are higher for LINACs, especially multi-energy systems 5
• LINACs enable more complex treatment techniques including IMRT and SBRT 5

Stereotactic Radiosurgery Applications

• LINAC-based stereotactic systems deliver radiosurgery by moving the gantry to change beam delivery angles 7
• For tumors >3 cm diameter, hypofractionated stereotactic radiotherapy using LINAC systems is generally preferred over single-fraction Gamma Knife 7
• CyberKnife systems (LINAC-mounted on robotic arms) achieve submillimeter accuracy without stereotactic frames 7
• No platform has demonstrated superiority; success depends on correct application by treating physicians 7

Advanced Applications

Emerging Technologies

• FLASH radiotherapy using modified LINACs achieves ultra-high dose rates >160-200 Gy/s, potentially reducing normal tissue toxicity while maintaining tumor control 8
• X-band compact LINACs enable integration with MRI or CT imaging systems in limited spaces 2
• Tracking systems with robotic arms allow energy delivery from any location around the patient 6

Critical Safety Considerations

Radiation Protection

• Concrete bunker shielding requirements are substantial, with additional neutron shielding needed for high-energy (>10 MV) LINACs 5
• Staff training requirements are more extensive for LINACs compared to simpler technologies 5

Heart-Sparing Techniques

• Deep inspiration breath-hold or respiratory gating shields the heart during left breast irradiation 6
• Multiple or rotational beam arrangements minimize cardiac dose 6
• Modern planning systems with dose-volume histograms enable precise cardiac dose limitation 6