Beam energy

The energy of the beam can be influenced by the frequency of the RF waves. This affects the ‘ears’ or ‘horns’ at the edge of the beam because the effect of the flattening filter is changed. Beam energy can be determined with the tissue phantom ratio between 20 cm and 10 cm deep (TPR20/10). This is measured directly by measuring the dose delivered at 10 cm deep in a water phantom for a fixed number of monitor units, say 200MU, and then placing a further 10 cm of water equivalent material on top of the phantom without moving the position of the ionisation chamber. In practice it is convenient to measure the 5/15 ratio since this can be done immediately after a calibration at 5 cm deep by simply adding 10 cm of water on to the phantom. (Instead of the water, solid water can also be used.) A change of 5/15 ratio greater than 1% which corresponds to a change in energy of 0.5 MV should be investigated. Some quick check devices (Section 11.2) have one detector which can be placed under a steel absorber to make energy measurement at the same time as flatness and constancy.

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