11.07 - Field Junctioning

Many treatments require fields to be junctioned, due to contour irregularity, different treatment volumes in adjoining areas, or size of treatment field.

Photon-photon junctions

Photons beams usually have a sharp penumbra and reduce dose with depth. When two photon beams are placed side-by-side with parallel central axes, the following features are seen:

  • A cold spot at surface, above the point of intersection of the two beam edges. This is not seen if fields edges are aligned at or above the surface
  • A hot spot at depth, below the point of intersection of the two beam edges. This hot spot can be up to 40 - 50% higher than the prescribed dose. This hot spot becomes less problematic as the depth of beam junctioning increases.

If this type of junction is unavoidable, a decision must be made about the point to junction the beam. If a superficial site requires treatment, then beam junctioning should be done at surface to avoid underdosing the tumour. This has to be balanced against the size and location of the deeper hot spot. If the target is located deep in the body, then junctioning can occur at a greater depth to avoid a large hot spot, at the expense of a larger cold spot superficially.

Methods to solve photon-photon junctioning issues

There are two primary methods of avoiding the above issues.

  1. Align the divergent edges of the beam. If it is possible to align the divergent edges of the beams, there will be hot or cold spots generated as the penumbra of each beam will 'cancel out' the other. The negative aspect of this option is that the beams will have an oblique incidence on the target surface (usually a minor effect) and at the other side of the field the beam will travel more deeply into the patient at depth (due to increased divergence).
  2. Use a half beam block. By moving one of the independent jaws to midline, a half beam block can be created. This forms a non-divergent field edge centrally. This method is best used when the reason for junctioning is due to contour irregularity or different target volumes (eg. breast tangents and supraclavicular fossa field). The half beam block functions in a similar method to the aligning of divergent beams, but is easier to set up (less movements of the couch/gantry) and means that the beam is not oblique on the skin surface.

Junctioning through critical volumes

Despite the best efforts of radiotherapists and the use of immobilising devices, there is still the potential for over- or underdosage at the junction between two fields, even with a non-divergent edge. This can be a problem for both tumour (underdosage) or normal structures (overdosage). In these cases, a feathered junction is employed. A feathered junction is when the junction between the two beams is moved on a daily or weekly basis in an effort to avoid hot or cold spots from being focused on one point.
The benefits of junctioning include:

  • Less consequence of hot or cold spots

The disadvantages are:

Electron-electron junctions

Electron junctioning different to photon junctioning. Electrons tend to have a larger penumbra, which is more pronounced at depth (seen with higher electron energies). Electrons dose falls off rapidly with depth, mitigating some of the problems seen with photon fields.
When junctioned on the skin surface, a hot spot will develop within the deeper structures. This is more of an issue for higher energy electrons as the hot spot may spread very deeply. For more superficial treatments, the hot spot often occurs in subcutaneous tissue and is less of a concern.
When junctioned deeply, cold spots may develop at skin surface. This is usually not desirable but may be of use when the treatment site is located deep to the skin.
Combination of electron fields with different energies is a particular problem. Lower energy electrons will tend to bulge into the higher energy electron field, creating a hot spot at the surface. The higher energy electrons will then spread laterally at deeper depths, possibly leading to a cold spot forming once the lower energy field has petered out.

Methods to deal with electron/electron junctions

Electron-electron junctions between beams of the same energy are relatively straightforward. Frequently, the penumbra of one beam will compensate the penumbra of the other, and give a uniform dose. Extreme care must be taken with patient setup to prevent overdosage or underdosage of critical structures.
Beams of different energies are more difficult to junction. This is because lateral scatter of the beams occurs as different depths - superficial for the lower energy beams and deeply for the high energy beams. These situations should be planned and an appropriate place for the junction should be determined to prevent untoward outcomes.

Photon-electron junctions

Given the different characteristics of photon and electron isodose distributions, it can be estimated that combination of these two fields would create problems. In general, electrons will bulge laterally into the photon field, creating a hot spot on the photon side of the junction. A corresponding cold spot may form on the electron side of the field. This effect is pronounced when an extended SSD is used for the electron field, as the penumbra becomes more blurred and electrons scatter even further into the photon beam. These issues arise particularly in head and neck treatments where the shoulders may limit direct application of electrons to the neck surface.