Shaped fields Shaped fields

This section considers the effects of shaped radiation fields. To understand how shaping a treatment field can affect the dosimetric characteristics of the beam it is helpful to con- sider the different components that contribute to the radiation dose for a radiation field.

8.6.1

Primary dosePrimary dose

This is the dose due to radiation incident on the phantom or patient (not scattered radiation from within the patient). It has two components: direct primary radiation

Wedge angle Central axis Field size (F) 50% 100% 80% Wedge angle Central

axis Standard depth (eg 10cm) Field size (F)

F/4 F/4

(a) (b)

Fig. 8.9

SHAPED FIELDS 99

and radiation scattered from within the treatment head (head scatter or sometimes termed collimator scatter).

8.6.1.1 Direct primary radiation

This component is radiation that has srcinated in the X-ray target and passes through the flattening filter without interacting. It is not dependent on field size.

8.6.1.2 Head scatter

This component is predominantly from radiation scattered from the flattening filter and also from the collimator jaws, primary collimator, monitor chamber and mirror. It is usually modelled as an extra focal source located at a scatter plane within the treat- ment head as shown in Fig. 8.10a. This component is field size dependent; as the jaw size increases more of the scatter source is visible and hence more radiation emerges from the treatment head.

8.6.1.3 Collimator exchange

The head scatter component is responsible for the effect in which the output for an elongated field (e.g. 30cm by 4cm) is different from the output for the same elongated field with the longer field dimension changed between the jaws e.g. 4cm by 30cm. This effect can be several percent and is due to the different jaws exposing different amounts of the scatter plane (Fig. 8.10b). Even though the area of the extra focal source exposed by the two elongated fields remains the same, the shape is changed resulting in the different amounts of head scattered radiation.

8.6.2

Scatter in the patientScatter in the patient

In the patient, radiation is scattered from the irradiated volume. This scattered radia- tion component is called the ‘phantom scatter’ component. The larger the surface field size the more scattered radiation there will be (Fig. 8.11). The amount and pro- portion of scattered radiation will increase with increasing depth. Generally there is more scattered radiation at lower energies.

Inner Outer Scatter plane S S Isocentre SS 100cm Scatter plane Isocentre Collimator jaw S (a) (b) 100-X_J Extended source XJ Fig. 8.10

Fig. 8.10 Schematic illustrating how a) the head scatter component is represented by an extended source and b) the location of the jaws within the treatment head affects the size of the extended source visible from the isocentre.

X-RAY BEAM PHYSICS 100

8.6.3

Methods to account for scattered radiationMethods to account for scattered radiation

There are a number of ways in which changes in output with field size and shape due to scattered radiation are considered. The following deals with some more common methods.

8.6.3.1 Scatter factor (SF)

Scatter factor (SF) is defined as the ratio of the total absorbed dose at a point to the primary dose at that point:

=

Primary dose

SF depends on beam energy, field size and depth. SF goes to 1 as the field size goes to zero.

8.6.3.2 Peak scatter factor (PSF)

This is a special case of the scatter factor where the reference point is on the beam axis at the depth of maximum dose.

8.6.3.3 Backscatter factor (BSF)

This is a special case of the scatter factor where the point of interest is at the surface of the phantom. It is used at energies below 400kV.

8.6.3.4 Output factor

The output factor is the dose at d_maxfor a set field size, normalized to a reference field size (usually a 10cm by 10cm field).

8.6.3.5 Tissue phantom ratio (TPR), Tissue maximum ration (TMR), tissue air ratio (TAR)

These quantities all give ‘tissue dose’ as a ratio of some reference dose which has been measured under reference conditions. To calculate the tissue dose you multiply the

Scatter component Primary component Field size 10 100% ~85% ~110% Fig. 8.11

Fig. 8.11 Schematic diagram showing the increase in output with increasing field size and how the relative contributions vary. Nominal values are shown which are indicative only.

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reference dose by the appropriate ratio. All these ratios refer to dose at the same point in the beam (i.e. at a fixed distance from the source), usually the isocentre. Hence there is no divergence and no SSD dependence. These ratios depend on depth (d), energy (E) and size and shape of the field. The depth dependence is due to attenuation and scattering and does not incorporate a divergence effect.

8.6.3.5.1 Tissue phantom ratio (Fig. 8.2)

The TPR is defined as the ratio of the absorbed dose at a point on the central axis at any given depth to the absorbed dose on the central axis at the same distance from the source but with the surface of the phantom moved so that the point is at a specified reference depth. The collimator settings remain unchanged.

8.6.3.5.2 Tissue maximum ratio

TMR is a special case of the TPR in which the reference depth is the depth of maximum dose.

8.6.3.5.3 Tissue air ratio

A TAR is defined as the product of the tissue maximum ratio (TMR) and the peak scatter factor (PSF). TARs were used for Co-60 units but have been replaced by TPRs for megavoltage calculations. TAR was srcinally defined as the ratio of the absorbed dose at a point on the central axis at a depth in tissue, to the tissue dose, in air at the same point in the beam. This leads to problems at higher energies where significant thicknesses of build up material are required for electronic equilibrium and hence dose in air is not being measured. At high energies large build up thickness, as are required, give rise to attenuation and scattering; therefore the true primary dose is not being measured.

8.6.3.6 Equivalent square field

The depth dose characteristics of rectangular and circular fields can be represented by calculating an equivalent square field size with the same characteristics. This is not a square field of the same area but of a field whose dimensions can be represented by:

= ( )

2ab a b

where a and b are the two sides of the rectangle andσ is the dimension of one of the sides of the equivalent square. This is only an approximation. The BJR Supplement 25 has extensive tables of equivalent field sizes.

8.6.3.7 Scatter from irregularly shaped fields

For irregularly shaped fields, the Clarkson sector integration technique is often employed. In this the irregular field is divided into a series of sectors of circular fields. These sectors can then be summated to determine the output from the irregular field (see section 10.3.10). Dose calculations for irregularly shaped fields are usually under- taken within the treatment planning system.

Chapter 9