2.2 Radiation Production in Linear Accelerators & X-Ray Tubes

Radiation Production in Linear Accelerators & X-Ray Tubes

Quick Reference: Linear accelerators (linacs) generate megavoltage photon and electron beams for cancer therapy. Electrons are emitted thermionically, accelerated via radiofrequency microwaves, steered by achromatic bending magnets, and converted into Bremsstrahlung photons or scattered electron fields controlled by multi-leaf collimators.

Overview of Modern Linear Accelerator Components & RF Systems

A modern medical linear accelerator (linac) converts wall electric power into high-energy electron and photon beams ($4-25\text{ MV}$). The linac hardware is divided between the stationary drive stand, the rotatable gantry, the treatment head, and the control console.

Radiofrequency Microwave Generation: Klystron vs. Magnetron

To accelerate electrons to near the speed of light, linear accelerators utilize high-power radiofrequency (RF) microwaves operating at $2997\text{ MHz}$ (S-band). Microwave power is produced by one of two devices:

1. Klystron: A high-power microwave amplifier. It does not generate RF power independently; instead, it receives a low-power RF seed signal from an RF oscillator and amplifies it to $5-7\text{ MW}$ peak power. Klystrons are exceptionally stable, possess long operating lifespans, and are mandatory in high-energy linear accelerators ($>10\text{ MV}$).
2. Magnetron: A microwave oscillator that functions as both the RF source and amplifier. It contains a central cylindrical cathode surrounded by a copper anode with resonant cavities in a permanent magnetic field. Magnetrons are compact and lower in cost, making them ideal for low-energy single-photon linacs ($6\text{ MV}$) or portable units.

Microwave power travels from the klystron/magnetron through evacuated rectangular copper waveguides to the accelerating structure. A four-port circulator (waveguide isolator) is placed in the line to prevent reflected microwave energy from returning to damage the klystron.

Electron Gun & Accelerating Waveguide

• Electron Gun: Contains a heated barium aluminate cathode that releases electrons via thermionic emission. A grid pulse trigger applies a negative voltage pulse, injecting bunches of $50\text{ keV}$ electrons into the accelerating structure in phase with the incoming RF microwaves.
• Accelerating Waveguide: An evacuated copper tube containing disc-loaded resonant cavities. Accelerating structures utilize either traveling wave or standing wave designs. Standing wave structures use side-coupled non-accelerating cavities, allowing maximum energy gain per unit length and permitting a significantly shorter waveguide ($1.0-1.5\text{ meters}$) that can be mounted horizontally inside the gantry arm.

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Beam Steering, Target Selection & Beam Conditioning

Because the accelerating waveguide is oriented horizontally in high-energy linacs, the accelerated electron pencil beam must be bent $90^\circ$ or $270^\circ$ to point toward the patient treatment couch.

Bending Magnets and Achromatic Optics

• $90^\circ$ Bending Magnet: Causes chromatic aberration (energy dispersion), where higher-energy electrons bend less and lower-energy electrons bend more, broadening the focal spot size.
• $270^\circ$ Achromatic Bending Magnet: Uses a triple-focusing magnetic field loop. Electrons of varying energies exit the magnet at the exact same spatial point and trajectory, maintaining an ultra-fine focal spot diameter ($<2\text{ mm}$) on the target.
• Slalom Bending Magnet: Uses three sequential bending magnets ($45^\circ, -45^\circ, 270^\circ$) to achieve compact beam steering.

Target & Flattening Filter vs. Scattering Foils

The linac treatment head contains specialized components configured according to the selected treatment mode (Photon vs. Electron mode).

The Flattening Filter

Bremsstrahlung x-ray production at megavoltage energies is intensely forward-peaked. To produce a uniform dose distribution across a $40 \times 40\text{ cm}^2$ field, a conically shaped high-$Z$ (lead/tungsten/copper) flattening filter is placed in the beam. The filter is thickest in the center, absorbing more central photons (differential attenuation/beam hardening) and creating a flat beam profile at a standard reference depth ($10\text{ cm}$). Unflattened beams (Flattening Filter-Free / FFF) remove this filter, yielding ultra-high dose rates for SRS and SBRT.

Scattering Foils

In electron mode, the raw electron pencil beam ($0.5-2\text{ cm}$ diameter) would deliver an extremely concentrated, destructive dose. Dual metallic scattering foils (thin gold, brass, or aluminum foils) scatter the electrons elastically, expanding the beam into a uniform clinical treatment field.

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Collimation Systems & Beam Monitoring Control

Beam shaping and dose delivery precision are governed by primary collimators, secondary jaws, multileaf collimators, and transmission ionization chambers.

Collimation Assemblies

1. Primary Collimator: A stationary tungsten block located immediately below the target that defines the maximum conical beam divergence (typically a $45^\circ$ cone producing a $50\text{ cm}$ circular field at $100\text{ cm}$ SAD).
2. Secondary Collimating Jaws: Two pairs of movable tungsten blocks ($X_1, X_2$ and $Y_1, Y_2$) that define rectangular field sizes from $0.5 \times 0.5\text{ cm}^2$ up to $40 \times 40\text{ cm}^2$ at $100\text{ cm}$ SAD.
3. Multileaf Collimators (MLC): Composed of $60$ to $120$ individual computer-controlled tungsten leaves ($2.5-5\text{ mm}$ leaf width at isocenter). MLCs shape complex IMRT and VMAT fields dynamically. Interleaf transmission leakage is maintained below $0.5-1.5%$.

Dual Transmission Ionization Chambers

Located below the flattening filter/scattering foil are two independent, sealed transmission ionization chambers (Chamber 1 and Chamber 2). They continuously monitor:

• Integrated Dose (Monitor Units / MU): Chamber 1 counts primary MUs and terminates the beam when the prescribed dose is reached. Chamber 2 acts as an independent backup, set to terminate the beam if Chamber 1 fails (typically set $2-5%$ or $25\text{ MU}$ higher).
• Dose Rate: Real-time feedback controls gun grid pulsing to maintain constant output.
• Beam Flatness & Symmetry: Transverse and longitudinal segmented chamber quadrants monitor beam symmetry (tolerance $\pm 2%$) and flatness (tolerance $\pm 2%$), instantly tripping a beam interlock if asymmetry occurs.

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Physics of Radiation Production: Bremsstrahlung vs. Characteristic

When high-energy electrons strike a high-$Z$ target (such as Tungsten, $Z=74$), two distinct x-ray production processes occur.

Bremsstrahlung (Braking Radiation)

• Mechanism: An incoming fast electron passes close to the strongly positive tungsten nucleus. The electrostatic attraction decelerates and deflects the electron. The lost kinetic energy is emitted directly as an x-ray photon.
• Spectrum: Continuous spectrum ranging from near zero up to the maximum kinetic energy of the incident electron ($E_{\max} = h\nu_{\max} = e \cdot V$). The average photon energy of a Bremsstrahlung spectrum is approximately one-third of the peak energy ($E_{\text{avg}} \approx \frac{1}{3} E_{\max}$).

Characteristic X-Ray Production

• Mechanism: An incoming electron collides with an inner-shell target electron ($K$-shell), imparting energy greater than the shell binding energy ($69.5\text{ keV}$ for tungsten) and ejecting it. An outer-shell electron ($L$ or $M$ shell) falls into the vacancy, releasing a photon with energy equal to the exact difference between the two shell binding energies: $E_{\text{characteristic}} = E_{K,\text{shell}} - E_{L,\text{shell}}$
• Spectrum: Discrete monochromatic energy lines ($K_\alpha \approx 59.3\text{ keV}$, $K_\beta \approx 67.2\text{ keV}$).