arrangements

Table 10.2 shows typical values for commonly used energies). Low energy beams, e.g. 4–6 MV are suited to more superficial volumes, e.g. head and neck PTVs, while deep pelvic PTVs will require 10–20 MV. Beam energies above 15 MV do not give a great benefit to the planner, but their frequent use on a linac can introduce radiation protection problems due to neutron production.

The build-up region is a significant feature of the MV beam in that it gives ‘skin sparing’ allowing doses that would severely damage the skin to be delivered deep into the patient. However, a PTV which extends from near the surface to a depth requiring MV photons may require bolus to be added to the skin. By placing this tissue equivalent material on the skin, the build-up region is shifted into the bolus and the maximum dose of the beam is delivered to on or near the skin. Unless the PTV is superficial, the planner should consider the impact of patient immobilization devices acting as build up and increasing the surface dose.

Standard field arrangements may be modified for specific patient situations such as metal hip implants. Here, it is preferable to avoid beams entering through the implant so a three-field arrangement must be modified such as shown in Figure evolve 10.17.

Single fields

Most plans are based on the intersecting of multiple field paths to create a high dose region, but single fields may sometimes be the ideal choice especially for superficial PTVs. The spine can be treated with a single posterior field, although this will usually be done as part of a multifield arrangement to treat the whole central nervous system. To cover a PTV length of greater than 40 cm, the field may need to be treated at extended SSD, i.e. the isocentre will be positioned off the skin surface (see Figure evolve 10.20). To produce a uniform dose to the cord lying at varying depths, a compensator may be employed, but the most efficient technique is to use top up fields all centred on a single isocentre, delivering a few MU to otherwise underdosed sections of the PTV.

Turned wedges

Wedges are usually most beneficial with the wedge in the plane of the plan (usually the transverse patient plane). However, if the patient contour varies out of this plane, a turned, or longitudinal wedge may be required to produce an even dose in the superior-inferior direction. This may be accomplished by overlying two fields with identical dimensions, but with the collimator rotated 90°. For multibeam plans, it may be possible to achieve uniform dose with a single turned wedge on one field only.

Parallel-opposed fields

The typical isodose distribution arising from a parallel-opposed field is shown in Figure evolve 10.18. If the separation is not too great for the available beam energy, then a dose range of +7 to −5% is perhaps just achievable. The high dose regions may present a problem at wide separations and also the hour-glass shaped contour at about the 90–95% level may result in unacceptably low doses at the midplane of the volume. Wedges may be employed where the contour slopes or internal heterogeneities are present and the use of shielding can produce irregular dose distributions conforming to complex PTV shapes. Clinical applications include large pelvis volumes and whole brain treatments.

Beams weighted in 2:1 ratio

The standard parallel-opposed distribution can be modified to deliver a higher dose to one side of the PTV without resorting to a more complex plan. It must be clear what the plan request means by 2:1 weighting. For simple manually calculated plans, it would usually mean that the ratio of MU set is 2:1. However, with computerized plans, a dose reference point may be positioned somewhere other than at the midplane depth so it must be clear to which point the 2:1 ratio applies. The presence of heterogeneities further complicates the issue as the TPS may adjust the set MUs to deliver equal dose from each field to a point by compensating for the different densities each beam has traversed. This demonstrates the potential hazards arising from changing from basic manual calculations to CT-based dosimetry where a seemingly simple request for 2:1 weightings can take on several different interpretations (see Figure evolve 10.18).

Breast treatments

The breast can be treated with a parallel-opposed pair, but the technique is generally refined to minimize lung dose by creating a non-diverging edge, either by fully asymmetric fields with the central axis along the field edge, or by slightly angling the fields so that the central axes are not parallel to one another but the back edges are (Figure evolve 10.19).

Wedges will usually be required to produce a uniform PTV dose by compensating for the ‘missing tissue’ and the presence of lung. Due to time constraints, the PTV is not outlined if treating the entire breast and the PTV is normally defined during simulation or virtual simulation and will not be specifically contoured and the gantry angles determined by the accepted amount of lung in the field. For post-mastectomy ‘chest wall’ treatments, the contour shape combined with the significant presence of lung may result in open fields or even a wedge orientated with the thin end at the apex of the breast.

A beam of typically 5 MV is adequate for most breast treatments but, as the separation between the medial and lateral field edges increases beyond about 22 cm, a higher energy beam is required. The increased energy results in reduced breast tissue coverage superficially due to the increased build-up depth so should be avoided unless essential.

Segmented fields can produce a more uniform dose especially in larger or irregular outlines (see Figure evolve 10.19A–C). Usually, a single segment on one field is sufficient to bring the dose range within ICRU [1, 2] requirements. For left sided breast treatments it may be necessary to shield the heart.