Beam modifying devices Beam modifying devices

The treatment beam can be both shaped geometrically and can have its intensity dis- tribution modified. These can be achieved using intensity modulated radiation therapy (IMRT) techniques. Another widely used method for achieving simple changes in intensity is to use wedges wedges , while field shaping can be achieved using shielding blocks placed on an accessory tray.

8.5.1

Principles of wedgesPrinciples of wedges

Wedged beams are used for three main purposes: i) Combining beams from non-orthogonal angles, ii) Compensating for changes in surface shape,

iii) Compensating for changes in depth dose fall off for beams incident perpendicular to the wedged beam.

0 10 20 30 40 50 60 70 80 90 100 110 −20 −15 −10 −5 0 5 10 15 20 Distance from 20% to 80% penumbra Field size Fig. 8.7

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There are three types of systems that produce wedged beams: i) Manual fixed physical wedges,

ii) Universal physical wedge, iii) Dynamic wedges.

8.5.1.1 Manual or physical wedges

These are wedge shaped pieces of aluminium, brass or steel. A series of different wedges is usually in use e.g. 15, 30, 45, 60 degrees. These will have different physical dimensions and may be constructed of different materials. Physical wedges are less commonly used nowadays. This is because they need to be physically inserted in the position of the accessory tray. Inserting and removing the wedge can be difficult especially at non-zero gantry and collimator angles. Also carrying and storing them requires careful ergonomic design within the treatment room. They also block the light field used for setting up the patient, so the wedge is often inserted after the patient is set up, exacerbating the manual handling problems.

8.5.1.2 Universal physical wedge

A single physical wedge can be used to create a range of wedge angles by combining the wedge field with a plain or open field irradiation. This design of wedge is used in Elekta accelerators. The wedge is automatically positioned in the beam within the treatment head above the position of the mirror and below the monitor chamber. However, wedging can only occur in one direction which becomes a problem if the wedge is combined with a MLC, when being able to wedge both in the direction of the leaf movement and perpendicular to this may be useful.

8.5.1.3 Dynamic or virtual wedges

Dynamic or virtual wedges are created by moving a secondary collimator jaw across the treatment field while the beam is on. The amount of wedging is determined by the length of time the jaw is in the treatment field. Wedging in different directions can be achieved by movement of different jaws. For Varian dynamic wedges, the wedge factor is strongly dependent on field size and this effect needs to be carefully modelled within the treatment planning system. Off-axis and half-blocked wedged fields are created in the same way. Again care is needed in modelling the wedge factor.

8.5.2

Wedge factor (WF)Wedge factor (WF)

To deliver the same radiation dose to a point within a wedged field as for a plain field the number of monitor units set on the accelerator needs to be increased. This is achieved by use of a WF. The WF is defined as:

Wedge Factor Wedged Field P ain ie

(

) =

The reciprocal of the WF indicates the increase in monitor units (MUs) required to deliver the same dose as for an identical plain field. The WF is a function of wedge type, wedge angle, beam energy, field size and shape, off-axis position and depth.

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8.5.2.1 WF variation with field size

For physical wedges, the WF increases with field size. This is due to the amount of scattered radiation from the wedge increasing as the field size increases, and is typi- cally of the order of 5–10 % for clinically useful field sizes. For dynamic wedges, the variation with field size can be much larger, up to 50% with increasing field size.

8.5.2.2 WF variation off-axis

Off-axis, the WF for physical wedges tends to follow the profile of the wedge i.e. they increase or decrease in the wedged direction and remain approximately constant in the non-wedged direction. For dynamic wedges, the WF approximately matches the on-axis factor for the same field size with only small differences being observed.

8.5.2.3 Effect on %dd

For physical wedges, the wedge hardens the beam i.e. increases the mean energy of the beam making the % dd for a wedged field more penetrating than for a plain field because the lower energy components of the photon spectrum are absorbed in the wedge. This is more pronounced at lower energies (4–6MV) where Compton scatter- ing is the predominant. At higher energies ( >15MV), where pair production becomes more important, beam softening (a decrease of the depth dose with respect to the open field) is also possible. For dynamic wedges, there is no hardening of the beam and hence no change in the %dd or WF with changes in depth. The moving jaw atten- uates the beams almost completely so there is no transmitted beam to harden.

8.5.3

Wedge angleWedge angle

The wedge angle is defined as the slope of the line joining two points equidistant from the central axis and half the width of the field apart on the isodose curve which passes

0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0 5 10 15 20 25

Square field size (cm) W e d g e f a c t o r Fig. 8.8

Fig. 8.8 Wedge factor variation with square field size for a manual wedge (dashed line) and a Varian dynamic wedge (solid line) at 15MV.

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through the central axis at a reference depth (usually 10cm). Alternate definitions include the angle between the tangent to a nominal isodose curve at the central axis, such as the 80% isodose curve, and the line perpendicular to the central axis (Fig. 8.9). Wedge angles between 10 and 60 degrees are clinically used.

8.5.4

Accessory traysAccessory trays

Even with the widespread use of MLCs it is sometimes necessary to use shielding blocks to shape the treatment field. These shielding blocks are located externally to the treatment head on accessory trays. The blocks may be fixed or free standing. The trays are usually constructed of Perspex. Double trays, where the shielding block is inserted between two layers can also be used. The single or double tray will attenuate the radiation beam and this effect needs to be accounted for in the monitor unit calculations by use of a tray factor defined as:

Tra actor

=

ith ra Wit out Tray

Lower energy megavoltage beams will be attenuated more by the presence of the tray. The tray factor increases a little with increasing field size, is due to increased scatter radiation from the tray. The scattered radiation also contains contamination electrons which increase the surface dose and hence reduce skin sparing when the tray is in use.