External beam radiotherapy

External beam radiotherapy (EBRT (EBRT) is a form of ionizing radiotherapy delivered by a medical linear accelerator (or historically a Cobalt-60 source) to a patient lying on a treatment bed.  Mega Mega-voltage x-rays (4-25 MV) are the most commonly used.  Alternatives include electron therapy, orthovoltage x-rays (0.1-1 MV) and particle therapy (proton therapy, neutron therapy and carbon ion therapy).

Conventional (2D) radiation therapy

Prior to the ubiquitous availability of CT scanners, treatments would be planned by defining a limited number of beams, the boundaries of which were delineated on orthogonal x-rays of the patient.  Beam Beam shaping was limited and typically simple square or rectangular beams were used.  A A typical beam arrangement is the four field box.  Due to the low conformity of these treatments, adjacent tissues/organs often fall into the high dose region resulting in treatment side effects.  2D 2D radiotherapy is now rarely used, but still has a role in palliative treatments which use generous margins and where the simplicity of the planning process allows same day-day treatment.

3D conformal radiation therapy (3D-CRT)

Modern radiotherapy involves CT planning where the volume to be treated is defined on a 3D data set.¹. Organs at risk can also bybe delineated with the aim of shielding these and reducing treatment side effects.  Radiotherapy planning software is used to design complicated beam arrangements and to assess dose volume-volume histograms for the tumour and organs at risk.

In its simplest form, 3D-CRT uses a multi-leaf collimator (MLC) in a step-and-shoot technique.  An An MLC typically consists of 40-80 tungsten leaves which can move independently into the beam path and thus create nearly limitless beam shapes.  Several of these 'conformal' beams would then be delivered from different angles to treat the tumour volume.

Intensity modulated-modulated radiation therapy (IMRT)

IMRT is a type of conformal radiation therapy where not only the shape but also the intensity profile (i.e. fluence) of each beam is varied.¹.

Early implementations involved building up a fluence profile by summing smaller beam 'segments' until the desired profile was achieved.  The The beam was turned off while the MLC leaves were in motion and only turned on for a specific period of time to deliver each segment.

Modern 'sliding-window' techniques use carefully calibrated MCLs which slide across the beam path while the beam remains on.  The The variable speed of each MLC leaf allows the creation of hot and cold spots in the beam profile.

Rotational/helical/arc IMRT

The most advanced form of IMRT is conceptually similar to helical or cone-beam CT.². The radiation beam remains turned on throughout the treatment while the gantry is moved around the patient at variable speed and the MLC simultaneously varies the shape of the beam.

Several competing implementations are available:

• Tomotherapytomotherapy (i.e. 'slice' therapy)
  • modulated fan beam delivers radiation in a helical pattern
  • several rotations of the beam are delivered while the patient moves through the machine
• Volumetric Arc Therapyvolumetric arc therapy (VMAT)
  • broad beam is shaped by an MLC as it goes around the patient in an 'arc'
  • typically only a single arc is used, more complex treatments may use 2-3 arcs
  • patient does not move during each arc, but may be re-positioned between arcs

Stereotactic techniques

Patient immobilisation techniques, such as head framesheadframes, allowed the development of stereotactic radiosurgery (SRS).  'Stereotactic' refers to the high precision of the treatment system which allows smaller margins and higher doses per treatment.³.  SRS involves a smaller number of treatments (typically just one) compared to conventional treatment schedules which require up to 30 treatments a(a.k.a. 'fractions'). 

SRS is defined as the treatment to the brain or spine, while stereotactic body radiotherapy (SBRT or SABR) refers to stereotactic treatment elsewhere in the body. ^3,4.

Stereotactic treatments are delivered with IMRT linear accelerators (requiring more advanced patient immobilisation or image guidance) or dedicated systems such as the Gamma knife^® or CyberKnife^®.

Proton therapy

The most common type of particle therapy is proton therapy.⁵.  The advantage of proton therapy is the reduction in beam entry and exit dose due to the physical properties of the Bragg peak. This allows higher dose gradients to be delivered and thus can increase tumour dose while reducing dose to adjacent organs at risk.

Proton therapy systems use cyclotrons which are an order of magnitude more expensive than mega-voltage linear accelerators.  Hence Hence, proton therapy is generally reserved for only the most critically conformal treatments such as paediatric cancers or re-treatment of previously irradiated tumours.