Abstract
Percentage depth dose (PDD) distributions measured with three commercial p‐type silicon diode detectors in a water phantom show significant discrepancies from ionization‐chamber‐measured data for megavoltage (4‐ and 18‐MV) photon beams for large square field sizes ranging from 10×10 to . For a given large photon field, PDD data measured with a standard and energy‐shielded photon‐field diode (PFD) shows the least disagreement with the reference data, while the PDD data measured with a small and unshielded stereotactic‐field diode (SFD) shows the largest. The over‐responses of all diodes shown in the measured PDD distributions for the large photon square fields as well as the systematic errors observed among the three different diode detector types suggest: (1) the photon beam softening on the beam's central axis increases with field sizes and depths in water, and (2) the presence of energy‐shielding mixture (for filtering out low‐energy, backscattered photons in reaching the silicon chip of the diode) as well as the lateral dimension of the diode's sensitive area may be critical factors affecting the accuracy of the PDDs measured with the diodes for these fields. Monte Carlo simulations will be used to investigate these potential effects and correct for them when confirmed.
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