Modern radiotherapy increasingly uses photon beams with widths approaching that of the detectors used to characterise them, therefore perturbations of the dose distribution caused by the presence of non-water detectors become increasingly significant. Using Monte Carlo calculations we determined the dose that would be recorded by detectors of different sizes and compositions and compared these to the “true” dose-to-water. The ratio of the “true” dose to the dose that would be measured, Fdetector, was calculated under a wide range of conditions and shown to vary rapidly for small field sizes. By creating virtual voxels with a water-like atomic composition but a density equivalent to detector materials (silicon, diamond, air) we showed that this field-size dependent variation of Fdetector is due to the non-water-like densities of detectors, rather than differences in atomic number. Therefore ideal small-field relative dosimeters should have small active volumes and water-like density.

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