Radiotherapy Linac

Overview

A radiotherapy linear accelerator (linac) is a precision medical device delivering high-energy electron or X-ray beams to kill cancer cells while minimizing dose to surrounding healthy tissue. Modern clinical linacs operate at 6–25 MeV, treating thousands of patients annually. The system integrates mechanical precision (±1 mm positioning), real-time imaging for patient verification, and software-driven dose calculation to achieve conformal (tumor-matching) radiation therapy.

The fundamental mechanism: a microwave-driven Accelerating Waveguide accelerates electrons to high energy; these electrons are then converted to X-rays via a tungsten target or used directly for electron therapy. A Bending Magnet sweeps the beam and filters contamination. Motorized leaf collimators shape the radiation beam to match the tumor outline.

How it works

Electron Acceleration

The Accelerating Waveguide is a ceramic-lined copper tube, typically 50 cm long, resonating at 3–10 GHz (microwave frequency). Electrons from a heated cathode are injected into this standing-wave cavity, gaining ~2.5 MeV per meter of waveguide. A solenoid Focusing Magnet focuses the electron beam on-axis.

Once electrons reach the target energy (6–25 MeV), they exit the waveguide through a thin titanium window, leaving the vacuum enclosure. The microwave source must maintain phase and amplitude stability within ±1% to ensure dose accuracy.

Beam Bending and Filtering

The Bending Magnet deflects the electron beam by 90–270° (typically 270° for medical linacs) and acts as a momentum selector: only electrons at the desired energy pass through the primary collimator aperture. This filtering removes low-energy contamination and electrons decelerated by Compton scattering in the target.

For X-ray mode, the Tungsten Target sits downstream. A 4–6 MeV electron stopping in tungsten generates bremsstrahlung photons: high-Z material (tungsten) maximizes X-ray yield. The Flattening Filter hardens the bremsstrahlung spectrum and creates a flat intensity profile across the field (unfiltered bremsstrahlung is peaked at the beam center).

Collimation and Shaping

The Primary Collimator is a fixed tungsten aperture (1–3 cm diameter) limiting initial beam divergence. The Multileaf Collimator is the critical component: 40–120 motorized tungsten leaves (2.5–5 mm wide each) move independently to block radiation, conforming the field boundary to tumor and organ-at-risk contours. Leaf positioning must be accurate to ±1 mm; modern linacs verify leaf positions with imaging or mechanical feedback.

Patient Positioning and Imaging

The motorized 6-axis treatment couch aligns the patient to the beam isocenter with <2 mm and <1° accuracy. The Imaging System may use:

• Portal imager: real-time MV camera capturing treatment-beam radiograph for intrafraction verification.
• kV/MV fluoroscopy: optional kV X-ray tube (40–150 kV) mounted perpendicular to the linac beam, providing orthogonal anatomy images for setup matching to planning CT.

Matching software (deformable image registration) automatically aligns the setup radiographs to digitally reconstructed radiographs from the treatment plan, guiding the therapist to reposition the patient if needed.

Intensity-Modulated Radiation Therapy (IMRT)

Modern linacs deliver intensity-modulated beams by having the Multileaf Collimator dynamic-shape the field as the gantry rotates. This allows dose sculpting to concave targets (e.g., tumor wrapped around the spinal cord) while reducing dose to critical structures.

Safety and Dose Monitoring

Dual ionization chambers in the head continuously monitor dose rate and dose-rate flatness. If either exceeds tolerance, the beam shuts off automatically. Hardwired safety circuits (not software-dependent) provide this redundancy.

Clinical Workflow

1. Treatment plan: oncologist defines tumor volume and critical organs on planning CT
2. Dose calculation: software computes beam angles, leaf positions, and monitor units (MU)
3. Patient setup: align using lasers and imaging on the treatment day
4. Verification: confirm imaging overlap with plan; reposition if needed
5. Delivery: linac executes pre-programmed beam sequence, monitoring dose in real-time

Modern linacs integrate DICOM networking, allowing seamless handoff from planning software to delivery, reducing setup errors and treatment time to 15–30 minutes per fraction.