The most common radiation therapy treatment modality is external beam radiotherapy, and in the developed world these treatments are most commonly delivered with linear accelerators (International Atomic Energy Agency 2008). These devices are able to provide accelerations of 2 to 25 MV, and can often produce both photon and electron beams.
Clinical photon and electron beams are both produced using a stream of accelerated electrons. The energies required for conventional external beam treatments exceed those of conventional x-ray tubes, so an accelerating waveguide structure is used. The acceleration is provided by electric fields, E, associated with resonating microwaves inside the tuned cavity. Steering and focusing solenoid coils are used to control the direction and position of the stream within the guide (Metcalfe et al 2007). These accelerating waveguide systems are frequently too large to be located on the treatment axis (that is, orthogonal to the patient) and so the beam must be “bent” after acceleration. The bending magnet system responsible for this refocuses the beam, such that the final output is near-monoenergetic (no greater than 5% of nominal peak energy) and near-monodirectional.
This stream of electrons enters the treatment head of the linear accelerator, pictured below, which is responsible for adapting it into a therapeutically useful beam. The treatment head is contained within the gantry, and is capable of rotating about the patient, who lies on the couch during treatment. A number of the components contained in the treatment head are fixed on carousels, which rotate in and out of the beam path depending on the treatment modality (that is, whether a photon or electron beam is desired, and at which energies). A number of the components are fixed, or patient independent, and are only changed between different treatment modalities. Others are patient dependent and are unique to each treatment.
Photon Beams
Where a photon beam is desired the stream of electrons from the accelerating waveguide are incident on a target – a thin layer of a high-Z material (such as tungsten). A broad high-energy photon beam is produced as electrons are decelerated in this medium (due to charged particle deflection – the effect is known as Bremsstrahlung production). The divergence of the resultant beam is restricted by a conical aperture known as the primary collimator, the dimensions of which define an approximately 60 cm diameter circular field at 100 cm from the source (that is, from the target, at a point known as the isocenter and located in the patient during most treatments).
The fluence of the broad photon beam is non-uniform: spatially the photon beam is forward-peaked, and since photon energies are inversely proportional to the angular deviation from the stream of electrons incident on the target; the net effect is a prevalence of higher energy photons concentrated along the central axis of the beam. To obtain a uniform field (which here means having a flat dose profile at 10cm depth in water), which is useful for treatment, a flattening filter is used. The conical filter is thickest along the central axis, so that particles in the centre of the beam experience a greater attenuation. This is often made out of tungsten or steel. The design of the flattening filters vary according to beam energy (as the required attenuation varies).
The next component downstream, the monitor chamber, does not significantly alter the beam. It typically consists of two parallel plate ionisation chambers: radiation detectors designed to measure the intensity of ionisation in the gas medium between the plates. Ions and dissociated electrons are produced when the high energy photons interact with the gas and attracted towards the polarised plates, resulting in a measurable current. This current indicates the ‘output’ of the accelerator, providing feedback that can be used to monitor beam production. There are two chambers orientated such that one is rotated 90 degrees to the other: this allows monitoring of beam symmetry and provides redundancy in case of error. Unlike the vented chambers commonly used in quality assurance measurements, these chambers are sealed so that the response is not dependent on temperature and pressure.
The broad photon beam is, at this point, uniform and monitored, and suitable for therapeutic use. The secondary collimators, including opposing pairs of metal blocks known as jaws, are responsible for restricting the size and shape of the incident radiation field. The jaws are typically thick enough to limit radiation transmission (for example, 8 cm thick tungsten) and are often designed such that the edge of the jaw matches the angle of beam divergence. The two opposing pairs of jaws can be rotated within the treatment head. The opposing jaws in each pair are independently driven, allowing asymmetric fields. The field can be further shaped using a multi-leaf collimator (MLC) system, an array of narrow interleaved collimators that can be driven separately, enabling irregular field shapes. This allows what is known as 3D conformal therapy, where fields are collimated to the projection of the treatment volume in the beam’s eye view (that is, the view from the target source).
Electron Beams
Where an electron beam is desired the stream of electrons from the accelerating waveguide is incident on a scattering foil – which is responsible for converting the thin monodirectional beam into a larger field. Electron beams are generally used for localised superficial treatments, and to minimise the scatter of electrons outside the treatment site the beam collimation is performed as close as possible to the patient using attachments called electron applicators.
Naming Conventions
Photon treatment beams are conventionally described by the acceleration voltage, for example, “6 MV” or “6X” refers to a photon beam produced by the acceleration of electrons to 6 MeV. The photons produced by a linear accelerator treatment head are smaller than the nominal acceleration value – that is, for a 6 MV beam, the majority of photons have energies between 1 and 2 MeV. Electron beams are conventionally described by the approximate electron energies.
References
- International Atomic Energy Agency, 2008. Setting up a radiotherapy programme: clinical, medical physics, radiation protection and safety aspects.
- Metcalfe P, Kron T and Hoban P, 2007. The Physics of Radiotherapy X-Rays and Electrons.