Treatment Planning for EBT

Rena Widita

The aims of treatment planning To localize the tumour volume in patient To define the target volume for treatment To measure the outline of the patient and to place

within it the target volume and other anatomical structures which may affect the dose distribution

To determine the optimum treatment configuration required to irradiate the target volume

To calculate the resultant dose distribution in the patient

To prepare an unambiguous set of treatment instructions for the radiographers.

Imaging/diagnostic

In treatment planning consideration must not only be given to the accuracy and precision of dose to a specified point/s, but also to the geometric tolerance of dose delivery

It has been estimated that there is a 4 mm positional error in treatment and this should be taken into account when the target volume is defined

Optimisation of treatment may be constrained by several factors

Physical factors: Fundamental nature of the radiation used The limitation of treatment equipment

Clinical factors, which may make it inappropriate to give the most sophisticated treatment when the patient will not gain any significant benefit.

SINGLE FIELDS

In photon therapy, treatments with a single field are generally used only for palliative purposes

It is only with low energy x-rays that single-photon beams are used curatively

Calculation Two data charts are required for the

calculation of the monitor units or a linac or the time to be set on a cobalt unit to deliver the prescribed dose to the target An output chart A depth dose table

The data are normally tabulated for square fields

Output Chart for 6 MV X-rays beam

Giving the required number of monitor units to deliver a dose of100cGy to the position of the dose maxon the central axis at 100 cm SSD

Field Size (cm2) Output (mu/100cGy)6 x 6 104.38 x 8 102

10 x 10 10012 x 12 98.515 x 15 96.720 x 20 94.8

Depth dose chart for 6 MV (SSD = 100 cm)

Depth (cm) Field Size (cm)6 x 6 8 x 8 10 x 10

1.5 100 100 1002 98.1 98.2 98.34 89.3 89.8 90.26 80.6 81.8 82.68 71.6 72.9 74

10 63.7 65.1 66.2

If the field is rectangular, an equivalent square field size which would have the same output and depth dose characteristics is used

s = the side of the equivalent square a,b = the sides of the rectangular field

Example Prescribed dose : 1500 cGy No. Of fractions : 5 Target depth : 6 cm Field size : 12 cm x 7 cm SSD : 100 cm Equivalent square : 8.8 cm PDD : 82.1% Output Factor : 101.2 mu/100cGy

Daily monitor units = (1500/5)*(101.2/100)*(100/82.1)= 370 mu

Output and depth dose data have been interpolated

Non-standard treatment distance

When the field size needed is larger than can be obtained at the standard SSD, and so an extended SSD is required

Shortened SSD can be used to increase the dose rate and therefore to decrease treatment time

Altering SSD causes a change in output factor and to a lesser, in the depth dose

The variation in output factor with SSD can be assumed to depend on the inverse square law (ISL) provided that the deviation from the standard SSD is not large (less than about 10 cm)

The correction for the output factor

f = the treatment distance fo = the nominal SSD dmax = the depth of dose max

The variation of dose with depth depends on both the attenuating properties of tissue and on the ISL

For a point at a depth d in a beam with its dose max at dmax, the correction to the depth dose measured at an SSD of fo is:

Opposed Coaxial Fields

Can be used to irradiate volume of tissue in which the tumour cannot be accurately defined

Or because the intention is to give a relatively low dose of palliation

For isocentric opposed field: separations of 12-24 cm would produce an error of 0.5% or less

Example for opposed coaxial fields

Prescribed dose : 3000 cGy No. Of fractions : 10 Separation : 18 cm Field size : 14 cm x 8 cm SSD : 100 cm Equivalent square : 10.2 cm Central depth dose : 70.2 * 2 = 140.4% Maximum percentage dose : 100 + 46.5 = 146.5% Output Factor : 99.8 mu/100cGy

Daily monitor units = (3000/10)*(99.8/100)*(100/140.4)= 213 mu

Maximum dose = 3000*(146.5/140.4) = 3130 cGy

Output and depth dose data have been interpolated

MULTIPLE FIELDS

Curative treatments generally require a higher dose

> 3 fields are used (except head & neck, 2 fields)

TP is concerned with the selection of the parameters required to produce the optimum dose distribution: Number of fields Orientation of fields Field sizes Wedges Weights

Beam Weighting A method of achieving uniformity in the

target volume to adjust the contribution from each beam

A

B D

C

O

I

IIIII

Point I II IIIA 70 48 44

B 58 62 40C 52 56 50D 58 42 56

O 60 52 48

Depth dose data for the field arrangement

Make sure that the doses to B and D from field I are equal. If not use a wedge on this field to make them equal

Dose to B = 62 w2 + 40 w3 and Dose to D = 42 w2 + 56 w3

20 w2 = 16 w3 (if w2 = 1, w3 = 1.25) Determine the weight for field I so that the total doses to point A and C

are equal Dose to A = 70 w1+ 48 w2 + 44 w3 and Dose to C = 52 w1+ 56 w2

+ 50 w3 18 w1 = 8w2 + 6w3 w1 = 0.86 Use the weights to calculate the doses to each point: Dose A = dose C = 163, dose B = dose D = 162, dose O = 164

A three field treatment technique. The dose to 5 points within the target volume is to be balanced by altering the relative weights of the three fields

Dose calculation Dose calculation for multiple fields involves the sumation of the dose

distributions of the individual fields. The total relative dose Dp at a point P for an irradiation with n fields

is given by

DDi,p = the depth dose at point P from the ith fieldwi = the weight for that field

Dose calculation within the patient

In the previous section, in the calculation of the dose at a point it was assumed that the radiation beam was normally incident on a unit density medium in practise, a patient differs from this homogeneous situation both in shape and composition these differences must be taken into account when calculating the depth dose.

For routine treatment planning by computer it is not necessary to know how these algorithms work.

However, the physicist responsible for the purchase of new planning software should understand the limitations and accuracy of the methods used

Algorithms for dose calculation in an inhomogeneous medium can be divided into 4 types that are essentially a compromise between speed and accuracy.

Type Algorithm Account for

1 [1] manual methods Effective depth2 [2] power law TAR Distance between inhomogeneity and

point of calculation3 [3] equivalent TAR Scatter correction Volume

integration of differential scatter-air ratiosPosition and shape of inhomogeneity

4 [3] convolution methods Monte Carlo As type 3 but includes electron equilibrium at interfaces

[1] manual methods Can be used manually but occasionally

are used in computer calculations

Only correct for the effective depth of a point, considering only changes in the primary component of dose

[2] power law TAR Takes account of the position of the

calculation point with respect to the inhomogeneity, but considers the inhomogeneity to be of uniform thickness over the beam width

This is known as the tissue-air-ratio (TAR) power law method

[3] equivalent TAR Scatter correction Volume integration of differential

scatter-air ratios

Takes into account the [3] shape of the inhomogeneity

These algorithms are the most complex used on commercial planning systems

[3] convolution methods Monte Carlo

Not considered to be practical owing to the lengthy calculation times involves

Monte carlo calculations are not practical but the technique has been used to provide the input data for convolution methods.