Components of the Linear Accelerator (LinAc)

Comprehensive practice flashcards reviewing the core components, physics principles, beam steering mechanisms, treatment head elements, collimation systems, and verification imaging devices of a C-arm linear accelerator (LinAc).

What is the typical energy range of megavoltage (MV) photon beams utilized in external beam radiotherapy?

$$4\text{--}25\,\text{MV}$$

Why are conventional kilovoltage (kV) X-ray tubes unsuitable for treating deep-seated tumours?

Conventional X-ray tubes produce maximum beam energies in the kilovoltage region, which lack sufficient penetration depth to effectively treat tumours located deeper inside the body.

What are the three major core sections of a linear accelerator that produce high-energy electron beams?

An electron source, a source of radiofrequency (RF) electromagnetic waves, and an accelerating waveguide.

By what mechanism does the electron gun filament generate electrons, and what materials are commonly used?

Electrons are generated via thermionic emission by electrically heating tungsten or lanthanum hexaboride ($\text{LaB}_6$) to a high temperature.

Why is a triode electron gun structure preferred over a diode structure in modern LinAcs?

The triode structure features a smaller design, offers greater operational flexibility, and allows independent regulation of the gun current and beam energy.

How does the LinAc anode differ functionally from the anode in a conventional kV X-ray tube?

Rather than serving as an impact target for electron collision, the LinAc anode has a small aperture through which electrostatic-focused electrons pass to enter the accelerating waveguide.

What is the key functional difference between a magnetron and a klystron?

A magnetron directly generates high-power RF waves (typically used in low-energy units), whereas a klystron functions as an RF amplifier that amplifies a low-power RF oscillator signal using a high-power electron stream (used in high-energy units).

What are the two main purposes of the copper discs inside a travelling wave waveguide?

Copper provides high electrical conductivity to reduce power loss, and the discs slow the propagation velocity of the RF wave to match the velocity of the accelerating electrons.

How is a standing wave pattern produced in a standing waveguide structure?

Reflective discs at the end of the accelerating structure reflect the incoming wave backward, causing the forward and reflected waves to travel in opposite directions and interfere, establishing a standing wave pattern.

What insulating gas fills the waveguide, and what toxic decomposition byproducts can it yield?

Sulphur hexafluoride ($\text{SF}_6$) at approximately $2$ times atmospheric pressure; upon breakdown, it decomposes into toxic compounds including sulphur tetrafluoride and hydrogen fluoride.

Why does the accelerating waveguide require an active, temperature-controlled water cooling system?

Temperature variations cause the copper waveguide to expand or contract, altering its physical dimensions and severely diminishing the cavity's acceleration efficiency.

What level of vacuum is maintained inside the accelerating structure by ion pumps, and why?

A vacuum of approximately $10^{-7}\,\text{torr}$ is maintained to prevent accelerating electrons from losing energy through collisions with residual gas molecules.

What two physical factors cause electrons to diverge during acceleration, and how is this divergence counteracted?

Divergence is caused by the small radial component of the RF wave and mutual electrostatic repulsion between electrons; it is counteracted by magnetic fields generated by focusing coils surrounding the waveguide.

What is the function and layout of steering (centering) coils in a LinAc?

Steering coils exert electromagnetic forces to keep the electron beam centered along the waveguide axis against structural imperfections and the Earth's magnetic field; they are arranged in pairs along the x-axis and z-axis at both ends of the waveguide.

What occurs within the evacuated flight (or drift) tube?

Electrons exit the accelerating waveguide and drift into the bending cavity without undergoing any further acceleration.

Which three electron bending magnet configurations are depicted in the diagram, and what trajectory angle does the slalom system use?

The $90^\circ$ system, the $270^\circ$ system, and the slalom system (which directs electrons along an achromatic zigzag trajectory of $112.5^\circ$).

What materials are typically used for the primary X-ray target in a LinAc, and how is its thickness determined?

The target is made of high-atomic-number ($Z$) metals such as tungsten or gold; it must be sufficiently thick to interact with all incoming electrons while remaining thin enough to avoid attenuating the produced bremsstrahlung photons.

What is the purpose of incorporating an optional secondary target made from a low atomic number material?

A low-$Z$ target generates lower-energy photons, which improves soft-tissue contrast for verification imaging.

What LinAc treatment head component is shown here, and what is its specific clinical function?

The beam flattening filter; its conical shape (thickest at the central axis) attenuates the peaked central intensity of the forward-peaked photon beam to deliver a uniform dose distribution at depth.

When is Flattening Filter Free (FFF) mode used, and what replaces the filter?

FFF mode is used in intensity-modulated radiotherapy (IMRT) and small stereotactic radiation fields; a thin copper or brass foil may be substituted to absorb stray electrons emerging from the target.

What is the design advantage of a dual, parallel, sealed plate ionisation chamber in the LinAc head?

The chamber is sealed to provide constant readings irrespective of ambient temperature and pressure, while the dual-system redundancy ensures the beam terminates if monitor units (MUs) exceed a safety limit.

How does the optical field system replicate the radiation beam path without attenuating the treatment beam?

A lateral light bulb projects light onto a thin Mylar mirror coated with a few millimeters of reflective aluminium set at $45^\circ$, superimposing the visible light field onto the invisible X-ray beam path.

What are the three sets of collimation systems present within a linear accelerator head?

The primary collimators, secondary collimators (jaws), and multileaf collimators (MLCs).

Where is the primary collimator situated, and what parameter does it define?

It is positioned immediately downstream of the target and defines the maximum circular divergence angle of the exiting radiation beam.

What are the construction details and field size range of secondary collimators?

They consist of two pairs of adjustable lead blocks situated downstream of the mirror, defining square or rectangular field sizes from approximately $4\,\text{cm}^2$ to $40\,\text{cm}^2$ at a focus-to-skin distance (FSD) of $100\,\text{cm}$.

What are the standard leaf-shielded widths at $100\,\text{cm}$ FSD for standard, mini, and micro multileaf collimators (MLCs)?

Standard MLCs are $0.5\text{--}1\,\text{cm}$, mini MLCs are $0.3\text{--}0.5\,\text{cm}$, and micro MLCs are $0.25\text{--}0.3\,\text{cm}$.

How is radiation leakage between adjacent multileaf collimator (MLC) tungsten leaves minimized?

The leaves are engineered with stepped or overlapping tongue-and-groove profiles rather than straight, flush edges.

What type of beam-modifying accessories are shown, and what standard wedge angles are manufactured?

Manual (physical) wedges made of lead or steel, available in nominal wedge angles of $15^\circ$, $30^\circ$, $45^\circ$, and $60^\circ$.

How does a dynamic (virtual) wedge function, and what clinical purpose does it achieve?

A dynamic wedge moves one of the Y-axis secondary collimator jaws across the beam at a precalculated speed to achieve dose uniformity across tissue obliquity or closely intersecting beam fields.

What material is utilized for the treatment couch tabletop, and what 6 degrees of motion can modern support systems execute?

The couch top is made of carbon fibre (for low beam attenuation and rigid support); its 6 motions are longitudinal, vertical, lateral, rotation, roll, and pitch.

What are the two primary on-board verification imaging devices mounted on a C-arm LinAc, and what beam energies do they detect?

The Electronic Portal Imaging Device (EPID), which detects the megavoltage (MV) treatment beam, and the On-Board Imaging (OBI) system, which uses an auxiliary kilovoltage (kV) X-ray source and flat panel detector.

In this linear accelerator treatment head schematic, what component directly follows the target in the beam line?

The primary collimators.