Flatness and symmetry

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11.1. Flatness scans

 
The establishment of an appropriate standard for beam profile measurements is problematic. On the other hand the beam symmetry is easily defined and is not very dependent on the depth of measurement. However, the flatness of the beam depends on the size and shape of the measurement phantom. At commissioning of an accelerator it is necessary to state that the beam profile suits with the accelerator’s specification. This must be carried out in a water phantom making measurements at the depth or depths (usually at the maximum and at 10 cm deep) at which the flatness is given. Subsequently it is important to check that the profile has not changed significantly since the beam data were measured. This may be satisfactorily done using an ionisation chamber contained in a small block of PMMA. The profiles obtained in this way should not be taken into account as absolute measurements of flatness and should be compared to a similar measurement made at the time of commissioning. The dose at the periphery of the beam should be higher than at the centre in these ‘in air’ scans. This is influenced by the energy of the beam and by the focal spot size on the target. A change in the beam profile implies to there is something wrong with the setting way of the accelerator and it is not really appropriate to apply the same criteria as at commissioning. The monitor ion chambers which are monitoring the whole of the beam a change in the profile may affect the dose calibration of the accelerator and this is often the cause of differences between the dual dosimetry channels. Therefore the profile should not be differ more from its shape at commissioning than 2%. The asymmetry of the beam is related to the beam steering.
Flatness (IEC 2007) and symmetry are measured only within the ‘flattened area’ which is defined by IEC. Symmetry is the ratio of the dose rates at symmetrical points on either side of the beam axis and flatness is the ratio of maximum to minimum dose anywhere in the beam. In X-ray beams the minimum is usually close to the central axis of the beam. Although the IEC specification allows an asymmetry of 3%, it should be easier to achieve better beam symmetry with a modern accelerator. However, 3% is sufficient measurement accuracy for a quick check. A very convenient measuring of the beam profile is with a multi-element array of diodes or ionisation chambers. Not only their use is fast, they also give some information about the temporal stability of the beam and are particularly convenient for setting up beam steering. It may be useful to perform a scan with the flattening filter removed when assessing the cause of a beam asymmetry. (With a computer controlled accelerator, removing the flattening filter can often be achieved by typing a command.) The resulting scan show clearly whether the peaked dose distribution is correctly centred and it is symmetrical.

11.2. Quick checks of beam flatness

 
For a quick check of flatness it is appropriate to measure the dose at the centre and two symmetric points on the beam profile. For this purpose a number of rapid check devices are available. Film can also be used, but the decreasing of the machine time should be compared with the cost of film and the time taken to read it out. An electronic portal imaging device (EPID) can provide a very rapid constancy check.